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681

Chapter 13

Design of Nonstructural Systems and Components

John D. Gillengerten, S.E.Senio Structural Engineer, Office of Statewide Health Planning and Development, State of California.

Key words: Nonstructural Components, Bracing, Seismic Restraint, Architectural Components, Mechanical AndElectrical Component Bracing, Bracing Of Pipes, Ducts, Conduits, Nonstructural Performance Objectives

Abstract: For the majority of buildings, the nonstructural components represent a high percentage of the total capitalinvestment. Failure of these components in an earthquake can disrupt the function of a building as surely asstructural damage, and can pose a significant safety risk to building occupants as well. Past earthquakes havedramatically illustrated the vulnerabilities of the nonstructural components. Apart from the falling hazardposed by the light fixtures, non-structural failures can create debris that can block egress from the building,and hamper rescue efforts. In this Chapter, we deal chiefly with those components and systems that areinstalled in the structure during construction or remodel, for which design details are provided on theconstruction documents. We will touch briefly on the contents and equipment items that the owner oroccupants may place in the building. The failure of these items may pose a significant risk to the occupantsof the structure. However, these items are diverse, and the designer should address their anchorage andbracing on a case-by-case basis. Nonstructural elements can generally be divided into architectural,mechanical, and electrical systems and components. Architectural components include items such as exteriorcurtain walls and cladding, non-load bearing partitions, ceiling systems, and ornaments such as marqueesand signs. Mechanical components and systems include boilers, fans, air conditioning equipment, elevatorsand escalators, tanks and pumps, as well as distributed systems such as HVAC (Heating, Ventilation, andAir Conditioning) ductwork and piping systems. Electrical components include transformers, panels,switchgear, conduit, and cable tray systems. Components may be mounted at grade (on the ground floor orbasement of a building) or installed on the upper levels or roof of the structure. Our focus is on“nonstructural components” as opposed to “nonbuilding structures”. Nonstructural components consist ofequipment and systems that are supported vertically and laterally by a structural framework independent ofthe component itself -- a piece of equipment supported by a building frame, for example. In addition, we willconsider the anchorage and bracing of moderately sized components at or below grade, such as chillers,pumps, and fans.

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13. Design of Nonstructural Systems and Components 683

13.1 INTRODUCTION

For the majority of buildings, thenonstructural components represent a highpercentage of the total capital investment.Failure of these components in an earthquakecan disrupt the function of a building as surelyas structural damage, and can pose a significantsafety risk to building occupants as well. Pastearthquakes have dramatically illustrated thevulnerabilities of the nonstructural components.Figure 13-1 illustrates the collapse of asuspending ceiling system in the 1971 SanFernando Earthquake. Apart from the fallinghazard posed by the light fixtures, failures ofthis nature create debris that can block egressfrom the building, and hamper rescue efforts.Figure 13-2 shows a heavy rooftop tank that fellfrom its saddle mounts in 1994 NorthridgeEarthquake. Failure of this tank flooded thelower levels of the building.

In this Chapter, we deal chiefly with thosecomponents and systems that are installed in thestructure during construction or remodel, forwhich design details are provided on theconstruction documents. We will touch brieflyon the contents and equipment items that theowner or occupants may place in the building.The failure of these items may pose asignificant risk to the occupants of the structure,as illustrated in Figure 13-3. However, theseitems are diverse, and the designer shouldaddress their anchorage and bracing on a case-by-case basis.

Nonstructural elements can generally bedivided into architectural, mechanical, andelectrical systems and components.Architectural components include items such asexterior curtain walls and cladding, non-loadbearing partitions, ceiling systems, andornaments such as marquees and signs.Mechanical components and systems include

Figure 13-1. Damaged suspended ceiling and light fixtures, Olive View Hospital, San Fernando Valley Earthquake of 1971(Steinbrugge Collection, Earthquake Engineering Research Center, University of California, Berkeley.)

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boilers, fans, air conditioning equipment,elevators and escalators, tanks and pumps, aswell as distributed systems such as HVAC(Heating, Ventilation, and Air Conditioning)ductwork and piping systems. Electricalcomponents include transformers, panels,switchgear, conduit, and cable tray systems.Components may be mounted at grade (on theground floor or basement of a building) orinstalled on the upper levels or roof of thestructure.

Our focus is on “nonstructural components”as opposed to “nonbuilding structures”.Nonstructural components consist of equipmentand systems that are supported vertically andlaterally by a structural framework independentof the component itself -- a piece of equipmentsupported by a building frame, for example. Inaddition, we will consider the anchorage andbracing of moderately sized components at orbelow grade, such as chillers, pumps, and fans.

Figure 13-3. Overturned library shelves

Nonbuilding structures are supported on orbelow grade, and do not rely on anotherstructure for vertical and lateral stability.Examples of nonbuilding structures includelarge industrial boilers and machinery, cooling

Figure 13-2. Rooftop tank failure, 1994 Northridge Earthquake


towers, industrial storage rack systems, pressurevessels, and tanks. There are wide variations inthe construction and dynamic properties ofnonbuilding structures. Components such aspressure vessels, boilers, and chillers may berigid structures, massively constructed withlittle inherent ductility. Seismic response ofthese components is often characterized bysliding or overturning at the level of connectionto the ground. When damage occurs to thesecomponents, it is often concentrated in theconnections or anchor bolts. At the oppositeend of the spectrum are structures such ascooling towers, which are often flexible andhighly redundant, with behavior quite similar tothat for buildings.

The development of seismic designprovisions for nonstructural components haslagged behind that of primary structural system.Until the advent of seismic codes, there was noclear distinction between structural andnonstructural components. Buildings had nodedicated lateral force resisting system, relyingon plaster or brick walls and partitions forlateral strength. Earthquakes in the early part ofthe 20th century demonstrated the vulnerabilityof architectural features such as unreinforcedbrick parapets and exterior walls. Fewobservations were made regarding the seismicperformance of mechanical and electricalsystems, which existed in rudimentary forms.

In the 1933 Long Beach Earthquake, failureof fire sprinkler piping led to some of theearliest seismic provisions for piping systems.Lateral bracing provisions were added to the1961 Uniform Building Code, dealing chieflywith the design and attachment of architecturalcomponents. However, it was not until the 1964Alaska and 1971 San Fernando Earthquakesthat the vulnerabilities of nonstructuralcomponents and systems in modern buildingswere exposed. Earthquake reconnaissancereports from these and subsequent earthquakesidentified many conditions and practices thatcaused extensive property damage and putbuilding occupants at risk during strong groundshaking. Building code provisions haveundergone continual development,

incorporating lessons learned in theseearthquakes. For example, the 1964 AlaskaEarthquake demonstrated the vulnerabilities ofprecast concrete cladding systems. There werewidespread failures of ceiling systems andmechanical equipment in the 1971 SanFernando Earthquake, and failures in pipingsystems in the 1994 Northridge Earthquake.After each of these events, building codes havebeen modified in an effort to address thesevulnerabilities.

Structures that must continue inuninterrupted operation during and after anearthquake will require nonstructuralcomponent designs that exceed the levels inmost building codes. In general, building codestreat equipment and systems as “black boxes”,in that while the seismic design for the item islimited to anchorage and bracing, the integrityof the component itself is not expresslyconsidered. For example, seismic design of anelectrical transformer typically consists ofdesign of the anchor bolts connecting the unit tothe structure, and perhaps a check of themounting brackets on the transformerenclosure. However, checks of the integrity ofthe internal components of the unit are muchless common, and are not required by buildingcodes, even though the internal componentsmay be acceleration sensitive and vulnerable todamage at acceleration levels significantlylower than the design anchorage force. Forpiping systems, bracing designed to prevent acollapse of the piping system may not besufficient to prevent leaks or occasional breaks.The next generation of building codes willapply performance-based design to theanchorage and bracing of nonstructuralcomponents. In performance based-design, thedesign of a components or system is controlledby the level of seismic performance desired bythe owner of the structure, or mandated by thegoverning building official.

Section 13.2 discusses performanceobjectives for different nonstructuralcomponents and systems. Section 13.3examines different aspects of the seismicbehavior of nonstructural components. Section

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13.4 reviews the analytical approaches indifferent design standards. Sections 13-5 and13-6 discuss some of the design characteristicsof architectural and mechanical componentsand systems that have performed well in pastearthquakes.

13.2 PERFORMANCEOBJECTIVES

The basic objective of seismic design is toprovide an adequate level of safety, supplyingprotection that is appropriate for the seismichazard and the importance of the component orsystem. Beyond this basic level of safety, whichprotects occupants from life threatening injuryor death, higher levels of performance may bedemanded, to limit damage or protect againstloss of function. Tables 13-1 and 13-2 fromFEMA 274 provide, for a range of architectural,mechanical, electrical, and plumbing systemsand components, descriptions of damage statesat different performance objectives. Thesedescriptions depict the condition of thecomponent or system following a design levelearthquake.

For new construction, the minimum designobjective should be Life Safety. Nonstructuralcomponents and systems in buildingsconstructed to this performance objective do notpose a significant threat to life, although thebuilding may close for repairs following astrong earthquake. The emphasis is onelimination of falling hazards, but thenonstructural elements may not be functional orrepairable following a strong earthquake.

Essential facilities, such as hospitals, policeand fire stations, and emergency commandcenters may be designed with the intent thatthey meet the Immediate Occupancy orOperational performance objectives. Structuresdesigned to these performance objectives areexpected to be functional during or shortly afteran earthquake. Interruption of lifeline services(public utilities such as electricity, water, andsewer) may disrupt the function of buildingsdesigned to the Immediate Occupancyobjective. Structures designed to the

Operational performance objective generallyhave independent or back-up lifeline systems,and are not dependent on public utilities. Whenrehabilitating an existing structure, financial orphysical constraints may limit the designer tothe Hazards Reduced performance level, anobjective somewhat below Life Safety.

Acceptance criteria for nonstructuralcomponents depend on the consequences offailure, and the performance level desired. Forexample, a water piping system may meetacceptance criteria for Life Safety if anchoragefailures do not result in collapse of the system.The same installation will not meet the criteriafor Immediate Occupancy, if the piping systemdevelops leaks that will render the systeminoperable. Components or systems containingsignificant amounts of hazardous materialsrequire special care, since a breach may havecatastrophic consequences.

Current building codes approachperformance objectives indirectly. Buildingsconstructed to the minimum code provisions areexpected to meet the Life Safety objective.Essential facilities are designed to morestringent standards. Component anchorage isdesigned for higher force levels, and a broaderrange of components may be subject toanchorage and bracing requirements. However,the desired performance objectives for essentialfacilities are sometimes unclear, and therelationship between the code provisions andperformance objectives may not be defined.

Seismic design of nonstructural componentsis a balance between the potential losses versusthe cost of damage mitigation measures. Thereare many cases where significant damage canbe prevented by simply anchoring componentsto the floor or walls, at little cost. However,limiting damage to low levels in somecomponents can be extremely costly. With theexception of essential facilities, economicsshould drive the selection of the designperformance objective. An economic analysisshould consider not only the direct cost ofearthquake damage, but also indirect lossessuch as business interruption.


Table 13-1. Nonstructural Performance Levels and Damage – Architectural Components(13-2)

Nonstructural Performance LevelsComponent

Hazards Reduced Level Life SafetyImmediateOccupancy

Operational

Cladding

Severe damage toconnections andcladding. Many panelsloosened.

Severe distortion inconnections. Distributedcracking, bending,crushing, and spalling ofcladding elements. Somefracturing of cladding,but panels do not fall.

Connections yield;minor cracks (< 1/16”width) or bending incladding.

Connections yield;minor cracks (<1/16” width) orbending incladding.

Glazing

General shattered glassand distorted frames.Widespread fallinghazards.

Extensive cracked glass;little broken glass.

Some cracked panes;none broken.

Some crackedpanes; none broken.

PartitionsSevere racking anddamage in many cases.

Distributed damage;some severe cracking,crushing, and racking insome areas.

Cracking to about1/16” width atopenings. Minorcrushing and crackingat corners.

Cracking to about1/16” width atopenings. Minorcrushing andcracking at corners.

Ceilings

Most ceilings damaged.Light suspendedceilings dropped.Severe cracking in hardceilings.

Extensive damage.Dropped suspendedceiling tiles. Moderatecracking in hard ceilings.

Minor damage. Somesuspended ceilingtiles distrupted. A fewpanels dropped.Minor cracking inhard ceilings.

Generallynegligible damage.Isolated suspendedpanel dislocations,or cracks in hardceilings.

Parapets andOrnamentation

Extensive damage;some fall innonoccupied areas.

Extensive damage; somefall in nonoccupied areas.

Minor damage. Minor damage.

Canopies &Marquees

Extensive distortion. Moderate distortion. Minor damage. Minor damage.

Chimneys &Stacks

Extensive damage. Nocollapse.

Extensive damage. Nocollapse.

Minor cracking. Negligible damage.

Stairs & FireEscapes

Extensive racking. Lossof use.

Some racking andcracking of slabs, usable.

Minor damage. Negligible damage

Light FixturesExtensive damage.Falling hazards occur.

Many broken lightfixtures. Falling hazardsgenerally avoided inheavier fixtures (> 20pounds)

Minor damage. Somependant lights broken.

Negligible damage

DoorsDistributed damage.Many racked andjammed doors.

Distributed damage.Some racked andjammed doors.

Minor damage. Doorsoperable.

Minor damage.Doors operable.

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Table 13-2. Nonstrucutral Performance Levels and Damage, Mechincal, Electrical, and Plumbing Systems/Components(13-2)

Nonstructural Performance Levels

System/Component

Elevators out of service;counterweights off rails.

Elevators out ofservice;counterweights do nodislodge.

Elevators operable;can be started whenpower avilable.

Elevators operate.

HVACEquipment

Most units do not operate;many slide or overturn;some suspended units fall.

Units shirt onsupports, rupturingattached ducting,piping and conduit, butdo not fall.

Units are secure andmost operate if powerand other requiredutilities are available.

Negligible damage.

Ducts

Ducts break loose ofequipment and louvers;some supports fail; someducts fall.

Minor damage atjoints, with someleakage. Somesupports damaged, butsystems remainsuspended

Minor damage atjoints, but ductsremain serviceable.

Negligible damage.

PipingSome lines rupture. Somesupports fail. Some pipingfalls.

Minor damage atjoints, with someleakage. Somesupports damaged, butsystems remainsuspended.

Minor leaks developat a few joints.

Negligible damage.

Fire SprinklerSystems

Many sprinkler headsdamaged by collapsingceilings. Leaks develop atcouplings. Some branchlines fail.

Some sprinkler headsdamaged by swayingceilings. Leaksdevelop at somecouplings.

Minor leakage at afew heads or pipejoints. Systemremains operable.

Negligible damage.

Fire AlarmSystems

Ceiling mounted sensorsdamaged. Systemnonfunctional

May not function. System is functional System is functional

EmergencyLighting

Some lights fall. Powermay not be available.

System is functional System is functional System is functional

ElectricalDistributionEquipment

Units slide and/oroverturn, rupturingattached conduit. UPSsystems short out. Dieselgenerators do not start.

Units shift on supportsand may not operate.Generators providedfor emergency powerstart; utility servicelost.

Units are secure andgenerally operable.Emergencygenerators start, butmay not be adequateto service all powerrequirements.

Units are functional.Emergency power isprovided, as needed.

PlumbingSome fixtures broken;lines broken mainsdisrupted at source.

Some fixtures broken;lines broken mainsdisrupted at source.

Fixtures and linesserviceable; however,utility service maynot be available.

System is functional.On-site water supplyprovided, if required.


13.3 NONSTRUCTURALCOMPONENT BEHAVIOR

Nonstructural components can be classifiedas deformation or acceleration sensitive. If theperformance of a component is controlled bythe supporting structure’s deformation(typically measured by inter-story drift), it isdeformation sensitive. Examples of deformationsensitive components include partitions, curtainwalls, and piping systems running floor to floor.These components are often rigidly connectedto the structure and span from floor to floor.Since they are vulnerable to racking anddamage due to story drift, they are deformationsensitive.

When a component is not vulnerable todamage from inter-story displacements, such asa mechanical unit anchored to the floor of astructure, the component is accelerationsensitive. Acceleration sensitive componentsare vulnerable to shifting or overturning, if their

anchorage or bracing is inadequate. The forceprovisions of building codes generally producedesign forces high enough to prevent sliding,toppling, or collapse of acceleration sensitivecomponents. Many components are bothdeformation and acceleration sensitive,although a primary mode of behavior cangenerally be identified. Table 13-3, taken fromFEMA 274, identifies typical nonstructuralcomponents and whether they are accelerationor deformation sensitive.

Good seismic performance of deformationsensitive components can be obtained in twoways, by limiting the inter-story drift of thesupporting structure, or by designing thecomponent or system to accommodate theexpected lateral displacements without damage.For higher structural performance objectives,the drift limit criteria for deformation sensitivecomponents may govern the design of theprimary lateral force-resisting system. Inaddition to considering the effects of lateral

Table 13-3. Nonstructural Components: Response Sensitivity (13-2)

Sensitivity SensitivityComponent

Acc. Def.Component

Acc. Def.A. Architectural B. Mechanical Equipment

Exterior Skin Mechanical EquipmentAdhered Veneer S P Boilers and Furnaces PAnchored Veneer S P General Mfg. And Process Machinery PGlass Blocks S P HVAC Equipment, Vibration Isolated PPrefabricated Panels S P

1.

Glazing Systems S PHVAC Equipment. NonvibrationIsolated

P

PartitionsHeavy S P

1

HVAC Equipment, Mounted In-linewith Ductwork

P2.

Light S P 2. Storage Vessels and Water HeatersInterior VeneersStone, Including Marble S P

Structural Supported Vessels(Category 1)

P3.

Ceramic Tile S P Flat Bottom Vessels (Category 2) PCeilings 3. Pressure Piping P Sa. Directly Applied to Structure P 4. Fire Suppression Piping P Sb. Dropped, Furred, Gypsum Board P 5. Fluid Piping, not Fire Suppressionc. Suspended Lath and Plaster S P Hazardous Materials P S

4.

d. Suspended Integrated Ceiling S P Nonhazardous Materials P S5. Parapets and Appendages P 6. Ductwork P S6. Canopies and Marquees P7. Chimneys and Stacks P8. Stairs P SAcc. = Acceleration-Sensitive P = Primary ResponseDef. = Deformation-Sensitive S = Secondary Response

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displacement of the primary structure, caremust be taken that components and systems donot impact each other during the earthquake.Impact has been the source of widespreaddamage in past earthquakes. Interactionbetween components can be avoided bymaintaining adequate clearances betweenflexibly supported equipment and systems. Inaddition, flexible couplings should be providedbetween rigid or braced components and thosethat are flexibly mounted or free to displace.Finally, displacement sensitive components andtheir connections must be designed to withstandtheir own inertial forces, generated by theearthquake.

Approximate median drift values that can betolerated by different components aresummarized in Table 13-4. The values given aremedian values based on recommendations inFEMA 273. These drift values are expected togenerate severe damage to the nonstructuralcomponent at the Life Safety PerformanceLevel, and moderate damage at the ImmediateOccupancy Performance Level. The drifts areactual expected (unreduced) values. Thesedeformations can be accommodated throughflexible couplings, sliding joints, or throughdeformation of ductile elements in thecomponent or system. The proximity ofcomponents to structural members and othersystems must be considered. Distributionsystems, such ducts, pipes, and conduits will“swing” between bracing points during groundshaking. Impacts between systems should beavoided, since they can cause support failures,and in piping systems, loss of contents.

Table 13-4. Drift Limits for Deformation SensitiveComponents(13-1)

Performance ObjectiveComponent Life Safety Immediate

OccupancyAdhered Veneer 0.03 0.01Anchored Veneer 0.02 0.01

Nonstructural Masonry 0.02 0.01Prefabricated Wall Panels 0.02 0.01

Glazing Systems 0.02 0.01Heavy Partitions 0.01 0.005Light Partitions Not required 0.01Interior Veneers 0.02 0.01

The amount of separation betweencomponents needed to prevent interactionshould be determined. When determining theamount of separation needed, both thedeformations of the bracing system and thedeformations of the component betweenbracing points need to be considered. Forexample, the total displacement in a pipingsystem should consider the deformation of thepipe braces and the deformations of the pipeitself between support points under the designseismic loading.

Design standards treat various types ofnonstructural components individually, butinterrelationships exist between componentsthat must be considered. Failure of onecomponent can precipitate failure of entiresystems. For example, a pipe suffering asupport failure may drop on to a suspendedceiling system, which in turn could fall into anexit corridor. Conversely, a well-braced piperigidly attached to a flexibly mounted pump canproduce undesirable performance (Figure 13-4).

Acceleration sensitive components shouldbe anchored or braced to the structure toprevent movement under the design loading.Care must be taken that these components arenot anchored in such a way as to inadvertentlyaffect the structural system. For example, if thebase of a component with significant strengthand stiffness is anchored to the floor and the topof the component is rigidly braced to the floorabove, it can have the unintended effect ofaltering the response of the structural system.An example of this type of unintendedinteraction between a nonstructural componentand the structural system is illustrated in Figure13-5. The nonstructural masonry partition actsas a shear wall, which can lead to an unintendedredistribution of lateral load. This conditioncould be avoided by providing isolation jointsbetween the masonry wall and the structuralcolumns wide enough to prevent interactionbetween the two elements, while providing asliding connection at the top of the wall, whichprovides out-of-plane support but allows in-plane movement.


Figure 13-4. Failure of a flexibly mounted pumpconnected to a braced piping system, 1994 Northridge

Earthquake

Components and systems mounted at orbelow grade respond to ground shaking in afashion similar to buildings. The dynamicproperties of the component (mass and

stiffness) and the characteristics of the groundmotion (frequency content, duration, etc.)govern their response. Behavior of componentson the upper floors of buildings is complicatedby the interaction of the dynamic characteristicsof the structure and component. In cases wherethe mass of the nonstructural component islarge in comparison with the overall mass of thestructure, the techniques presented in thisChapter should be used with great caution.Large components may have a significant effecton the overall response of the structure.Depending on the dynamic properties of thecomponent and the supporting structure, theirdynamic response may be closely coupled. Ingeneral, if the component weight exceeds 20%of the total dead weight of the floor, or exceeds10% of the total weight of the structure, theprocedures discussed in this section should notbe used. In such cases, the component and the

Figure 13-5. Nonstructural partition acting as a shear wall, 1964 Alaskan Earthquake. (Steinbrugge Collection, EarthquakeEngineering Research Center, University of California, Berkeley.)

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structure should be analyzed together, includingproper representation of the flexibility of thecomponent and it's supports.

Mechanical components with rotating orreciprocating components are often isolatedfrom the structure with vibration isolationmounts. The isolation mounts may use eitherrubber-in-shear, springs, or air cushions toprevent transmission of vibrations to thestructure. Vibration isolation mounts candramatically alter the dynamic properties ofcomponents, by increasing their flexibility.Seismic inertial forces on isolated componentsare amplified. Improperly designed vibrationisolation installations can fail under theincreased dynamic and impact loads. Isolationmounts must be specifically designed to resistthese effects. Housekeeping pads used tosupport equipment should be castmonolithically with the structural slab, or beadequately reinforced and doweled to thestructural slab.

The ability to survive the earthquakephysically intact does not guarantee theperformance objective for the component orsystem has been met. As noted in Tables 13-1and 13-2, a component or system may need tobe functional following an earthquake in orderto meet higher performance objectives. Thisrequires seismic design of the operating parts ofmechanical and electrical components, eitherthrough dynamic testing or through analysis.Systems relying on lifelines may require on-siteback-up sources of water, emergency electricalpower, and waste water storage to meet theOperational objective.

13.4 DESIGN STANDARDS

The development of analytical techniquesfor nonstructural components has mirrored thatfor the primary structure of buildings. Most ofthese techniques use equivalent lateral forcemethods, where the component is designed fora lateral seismic force that is expressed as afraction of the component weight. Deformationsensitive components are designed toaccommodate the design story drifts, amplified

to the levels expected in the design earthquake.The objective of these approaches is to producean anchorage or bracing scheme for thecomponents that can withstand the accelerationsgenerated by the earthquake, without allowingthe component to shift or topple. In addition,the component must be able to tolerate theactual deformations of the primary structurewithout becoming dislodged, or adverselyaffecting the primary structure.

In this section we will examine theprovisions of four design standards, the 1994and 1997 editions of the Uniform BuildingCode (UBC), the Tri-Services Manual, and the1997 National Earthquake Hazards ReductionProgram (NEHRP) Recommended Provisionsfor Seismic Regulations for New Buildings andOther Structures, FEMA 302. These provisionsprovide the designer with guidance on typicalnonstructural seismic issues, and may be usedas resources on more complex or unusualprojects.

Building codes may exempt componentsfrom anchorage and bracing requirements,depending on the level of seismic risk at thesite, the occupancy of the structure, and theimportance of the components. In regions oflow seismicity, all components are typicallyexempt from seismic bracing requirements. Inregions of moderate seismicity, bracingrequirements are often limited to criticalsystems or hazardous components, such ascantilever parapets. In areas of high seismicity,furniture and components that are floormounted and weigh less than 400 pounds aregenerally exempt from anchorage and bracingrequirements. Items that or are suspended fromthe wall or ceiling and weigh less then 20pounds are also typically exempt. However,exempt unanchored components may pose arisk, and consideration should be given torestraining items that could shift or topple, bothfor safety reasons and to limit property loss.

All components requiring anchorage shouldbe designed for a minimum seismic force.Seismic forces are dependent on the followingfactors: component weight; flexibility orstiffness of the component and/or supports;


input acceleration at the point of attachment tothe structure; an importance factor based onfunctionality requirements or the hazard posedby the item; and the ductility, redundancy andenergy absorption capability of the componentand its attachments to the structure. Positiverestraints must be provided, and friction forcesthat are induced by gravity should be ignored,because vertical ground motions may reduce theeffects of gravity. The effects of prying actionand connection eccentricities on anchor loadsshould be accounted for in the design.

13.4.1 1994 UBC/Tri-Services ManualStatic Analysis

Both the 1994 UBC and the Tri-ServicesManual are based on procedures presented inthe 1990 edition of the Structural EngineersAssociation of California SeismologyCommittee Recommendations. In the 1994UBC, the design lateral force for components isgiven by the basic formula:

F ZI C Wp p p p= (13-1)

where:Fp = lateral force applied to the center of

mass of the componentZ = seismic coefficient that varies

depending on the seismic zone in which thestructure is located, and varies from 0.075 and0.4

I p = component importance factor, which

depends on the occupancy of the structure andvaries from 1.0 to 1.5

Cp = horizontal force factor, typically

equal to 0.75 for most components, 2.0 forcantilever parapets and appendages,

Wp = weight of the component.

Components are classified as flexible orrigid, depending upon their dynamiccharacteristics. Rigid components are thosewith a fundamental period of vibration less than0.06 seconds. Flexible components are thosewith higher fundamental periods. The values for

Cp are amplified by a factor of 2 for flexiblecomponents, and may be reduced by 2/3 forcomponents mounted at or below grade.

The response of components located on theupper levels is complicated by the dynamicresponse the structure in the ground shaking.Seismic input motion to the nonstructuralcomponent is filtered and amplified by thestructure. This can produce dramaticamplifications of lateral force demands on thecomponent, especially if the fundamental periodof vibration of the component approaches apredominant mode of vibration of thesupporting structure. In the Tri-ServicesManual, an effort is made to more preciselyconsider the amplification of the seismicresponse experienced by flexible equipment onthe upper levels of structures.

In the Tri-Services Manual, the forceequation is modified to:

F ZI A C Wp p p p p= (13-2)

whereAp = magnification factor, dependent upon

the ratio of the fundamental period of thecomponent, Ta , and the period of the buildingT.

The component period may be determinedby

TW

ka

p= 0 32. (13-3)

wherek = Stiffness of the equipment and/or the

component supports, measured as kips per inchdeflection of the center of gravity of thecomponent, and

Wp = weight of the component, in kips.

The values of Ap vary from 1.0 to 5.0,depending on the relationship of the dynamiccharacteristics of the component and thesupporting structure. If the dynamic propertiesof either the equipment or the structure areunknown, then a default value of Ap = 5.0 isused. For rigid components (Ta ≤ 0.06 seconds),Ap = 1.0. When the period of non-rigid or

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flexibly mounted equipment is not known, butthe fundamental period of the building isknown, estimated values of Ap may be takenfrom Table 13-5, taken from Freeman, (1998).

Table 13-5. Estimated Amplification Factors, Ap Non-rigid and Flexibly Supported Equipment (Reference 13-3)

Builing PeriodT (seconds)

< 0.5 0.75 1.0 2.0 > 3.0

Ap 5.0 4.75 4 3.3 2.7

Where the dynamic properties of thestructure and the equipment are known, then thevalue of Ap may be computed by firstdetermining the fundamental period of thecomponent, Ta using Equation 13-3. Then theratio of Ta /T is determined, and theamplification factor Ap found from theappropriate curves from Figure 13-6, takenfrom the Tri-Services Manual.

Figure 13-6 shows the relationship betweenAp and the ratio of the component to structureperiod. For a given component, the computationof the Ap factor can be somewhat involved,since higher modes of vibration of the structuremust be considered. For structures withfundamental periods less than 2 seconds,Freeman recommends that Ap factors for thefirst, second, and third modes of vibration becomputed. For structures with periods of greaterthan 2 seconds, the fourth and fifth modesshould also be considered. The largest value ofAp governs. The product of IpApCp need notexceed 3.75.

13.4.2 1997 UBC Analysis

The 1997 UBC provisions introducedsignificant changes in the design procedures fornonstructural components. These changes weredriven by analysis of instrument recordsobtained from buildings that have experiencedearthquake shaking. An examination of theserecords indicated that buildings experience atrapezoidal distribution of floor accelerations,varying linearly from the ground acceleration atthe base to 3 or 4 times the ground accelerationat the roof. Figures 13-7 and 13-8, taken fromFEMA 303, plot the amplification of peakacceleration versus height in the building basedon data obtained from 405 building strongmotion instrument records. Figure 13-7 showsthe variation of the ratio of peak structuralacceleration A to peak ground acceleration Ag

versus height in the building for all records.Figure 13-8 shows the variation of the ratio ofpeak structural acceleration A to peak groundacceleration Ag versus height in the building forrecords where Ag exceeded 0.10g. Theaccelerations in both figures are mean valuesplus one standard deviation. The amplificationof shaking as a function of height in thebuilding is clearly shown. Other conceptsintroduced in the 1997 UBC includeconsideration of "near fault" and soils effects,use of Strength Design level loads, andintroduction of an in-structure amplificationfactor, ap, which accounts for the forceamplification effects experienced by flexiblecomponents.

(b ) W h e n th e fu n d am e n ta l p e rio d o f th e b u ild in g is g re a ter th a n 0 .5se c o n d s (T > 0 .5 ). (N o te : If Ta / T < 1 .2 . A p is e q u al to v a lu e o b ta in e d f ro m Tab le 1 3 .5

5 .05 .0

4 .04 .0

3 .03 .0

2 .01 .000

0 0 .8

1 .01 .0

1 .2 1 .2 2 .0

2 .02 .0

T / Ta

Ap

T / Ta

Ap

T = 0 .5 0 sec . T = 0 .7 5 sec T = 1 .0 se c

T = 2 .0 se cT = > 3 .0 se c

4 .04 .7 5

3 .32 .7

Figure 13-6. Amplification factor, Ap for nonrigid and flexibly supported equipment(13-5)


Figure 13-7. Amplification of peak ground acceleration(mean + 1σ) vs. building height(13-7)

Figure 13-8. Amplification of peak ground acceleration(mean + 1σ) vs. building height, Ag > 0.10g(13-7)

The design lateral force for nonstructuralcomponents in the 1997 UBC is given by

Fa C I

R

h

hWp

p a p

p

x

rp= +

1 3 (13-4)

whereFp = lateral force applied to the center of

mass of the componentap = in-structure amplification factor, that

varies from 1.0 to 2.5Ca = seismic coefficient that varies

depending on the seismic zone in which thestructure is located and the proximity to activeearthquake faults. Ca varies from 0.075 to 0.66

Ip = component importance factor, whichdepends on the occupancy of the structure andvaries from 1.0 to 1.5

Rp = component response modificationfactor, which varies from 1.5 to 3.0

hx = element or attachment elevation withrespect to grade, hx shall not be taken as lessthan 0.

hr = the structure roof elevation, withrespect to grade

Fp shall not be less than 0.7CaIpWp, and neednot exceed 4CaIpWp.

The ap factor accounts for the dynamicamplification of force levels for flexibleequipment. Rigid components, defined ascomponents including attachments which havea period less than 0.06 seconds, are assigned anap = 1.0. Flexible components, which aredefined as components including attachmentswhich have a period greater than 0.06 seconds,are assigned an ap = 2.5. Values of Rp areassigned based on the nature of the connectionsto the structure, as well as the properties of thecomponent. Components fabricated of ductilematerials and attachments may be assigned a Rp

of 3. Components fabricated of nonductilematerials or attachments are assigned a Rp of1.5. Where connection of the component toconcrete or masonry is made with shallowexpansion, chemical, or cast-in-place anchors,Rp is taken as 1.5. Shallow anchors are definedas those anchors with an embedment length todiameter ratio of less than 8. If the anchors areconstructed of brittle materials (such as ceramicelements in electrical components), or whenanchorage is provided by adhesive, Rp is takenas 1.0. The term "adhesive" in this case refers toconnections made using surface application of abonding agent, and not anchor bolts embeddedusing epoxy or other adhesives. An example ofanchorage made with adhesive would be postbase plates glued to the surface of the structuralfloor in a raised access floor system. The designforces for equipment mounted on vibrationisolation mounts must be computed using an ap

of 2.5 and a Rp of 1.5. If the isolation mount isattached to the structure using shallow or

696 Chapter 13

expansion-type anchors, the design forces forthe anchors must be doubled.

In addition to lateral force requirements, the1997 UBC specifies that for essential orhazardous facilities components must bedesigned for the effects of relative motion, ifthe component is attached to the structure atseveral points. An example would be a verticalriser in a piping system that runs from floor tofloor. The component must accommodate theMaximum Inelastic Response Displacement,∆ M , defined as:

∆ ∆M SR= 0 7. (13-5)

13.4.3 1997 NEHRP Analysis

The 1997 NEHRP provisions are similar inform to those in the 1997 UBC, although thereare several significant differences. Many ofthese differences arise from the way groundshaking intensity is expressed. Rather thanexpressing ground shaking intensity throughcoefficients which are related to earthquakezones, the 1997 NEHRP expresses shakingintensity through peak spectral accelerations,which are mapped for long and short periodstructures. Because contour maps are used topresent spectral accelerations, the increase inground shaking intensity due to near-faulteffects are directly accounted for, without theneed for an additional factor in the forceequation. As with the 1997 UBC, design loadsare expressed at Strength Design (or Load andResistance Factor Design) levels. Based onstudy of the records of instrumented buildingsin areas of higher ground shaking intensity(Figure 13-8), the amplification of motion fromthe ground to roof levels was reduced from 4 to3.

The design lateral force for nonstructuralcomponents in the 1997 NEHRP is given by:

Fa S W

R

I

z

hp

p DS p

p

p

= +

0 41 2

.(13-6)

whereFp = lateral force applied to the center of

mass of the componentap = in-structure amplification factor, that

varies from 1.0 to 2.5SDS = spectral acceleration, short periodIp = component importance factor, which

depends on the component and occupancy ofthe structure and varies from 1.0 to 1.5

Rp = component response modificationfactor, which varies from 1.0 to 3.5.

z = element or attachment elevation withrespect to grade. z shall not be taken as less than0.

h = the average roof height of the structurerelative to grade

Fp need not be greater than 1.6SDSIpWp, andmay not be less than 0.3SDSIpWp. The ap factoris defined in the same manner as that found inthe 1997 UBC. Values of ap and Rp forarchitectural and mechanical components arepresented in Tables 13-6 and 13-7, respectively.When combining seismic and vertical loads, thereliability/redundancy factor, ρ, is taken as 1.0.

The component Importance Factor, Ip, istaken as 1.5 for life-safety components whichmust function after an earthquake, componentswith hazardous contents, storage racks inoccupancies open to the public, and allcomponents that could effect continuedoperation in essential (Seismic Use Group III )structures. For all other components, Ip, is takenas 1.0.

Values of Rp in the 1997 NEHRP areassigned based on the over-strength anddeformability of the component's structure andattachments. Deformability is defined as theratio of ultimate deformation to limitdeformation. Ultimate deformation is thedeformation at which failure occurs, and whichis deemed to occur if the sustainable loadreduces to 80 percent or less of the maximumstrength. Limit deformation is defined as twicethe initial deformation that occurs at a loadequal to 40 percent of the maximum strength.Low deformability components havedeformability of 1.5 or less, and are assigned aRp = 1.25. High Deformability components


have deformability greater than 3.5 whensubjected to four fully reversed cycles at thelimit deformation, and are assigned an Rp = 3.5.Limited deformability components, defined ascomponents that have neither high nor lowdeformability, are assigned a Rp = 2.5. Thedesign force Fp for vibration isolatedcomponents must be doubled.

Component anchorage to concrete andmasonry are subject to additional requirements.Anchors embedded in concrete or masonrymust be proportioned to carry the least of thefollowing:– The design strength of the connected part, or– Two times the force in the connected part

due to the prescribed forces, or

Table 13-6. Architectural Component Coefficients (FEMA 302)Architectural Component or Element ap

a Rpb

Interior Nonstructural Walls and PartitionsPlain (unreinforced) masonry walls 1.0 1.25All other walls and partitions 1.0 2.5

Cantilever Elements (unbraced or braced to structural frames below its center of mass)Parapets and cantilever interior nonstructural walls 2.5 2.5Chimneys and stacks where laterally supported by structures 2.5 2.5

Cantilever Elements (Braced to a structural frame above its center of massParapets 1.0 2.5Chimneys and Stacks 1.0 2.5Exterior nonstructural walls 1.0 2.5

Exerior Nonstructural Wall Elements and ConnectionsWall Element 1.0 2.5Body of wall panel connections 1.0 2.5Fasteners of the connecting system 1.25 1

VeneerHigh deformability elements and attachments 1.0 2.5Low deformability elements and attachments 1.0 1.25

Penthouses (except when framed by an extension of the building frame) 2.5 3.5Ceilings

All 1.0 2.5Cabinets

Storage cabinets and laboratory equipment 1.0 2.5Access floors

Special access floors 1.0 2.5All other 1.0 1.25

Appendages and Ornamentations 2.5 2.5Signs and Billboards 2.5 2.5Other Rigid Components

High deformability elements and attachments 1.0 3.5Limited deformability elements and attachments 1.0 2.5Low deformability elements and attachments 1.0 1.25

Other flexibile componentsHigh deformability elements and attachments 2.5 3.5Limited deformability elements and attachments 2.5 2.5Low deformability elements and attachments 2.5 1.25

aA lower value for ap may be justified by detailed dynamic analysis. The value for ap shall not be less than 1.00. The value of ap = 1 isfor equipment generally regarded as rigid and rigidly attached. The value of ap = 2.5 is for flexibile components of flexibly attachedcomponents and flexibile components including attachments.

bRp = 1.25 for anchorage design when component anchorage is provided by expansion anchor bolts, shallow chemical anchors, orshallow (nonductile) cast-in-place anchors or when the component is constructed of nonductile materials. Power-actuated fasteners (shotpins) shall not be used for component anchorage in tension applications in Seismic Design Categories D, E or F. Shallow anchors are thosewith an embedment length-to-diameter ratio of less than 8.

698 Chapter 13

– The maximum force that can be transferredto the connected part by the componentstructural system.Components must also meet requirements

for relative displacements. Seismic relativedisplacement, Dp, is defined as

Dp = δxA-δyA (13-7)

WhereδxA = deflection at building level x of the

structure, determined by elastic analysis andmultiplied by the Cd factor

δyA = deflection at building level y of thestructure, determined by elastic analysis andmultiplied by the Cd factor

Dp need not exceed

D X Yhp

aA

sx

= −( )∆

(13-8)

WhereX = height of upper support attachment at

level x as measured from the base.Y = height of upper support attachment at

level y as measured from the base.hsx = story height used in the definition of

the allowable drift∆aA = allowable story drift of for the

structure

Table 13-7. Mechanical and Electrical Component Coefficients (FEMA 302)Mechanical and Electrical Component or Elementc ap

a Rpb

General MechanicalBoilers and furnaces 1.0 2.5Pressure vessels on skirts and free-standing 2.5 2.5Stacks 2.5 2.5Cantilevered chimneys 2.5 2.5Other 1.0 2.5

Manufacturing and Process MachineryGeneral 1.0 2.5Conveyors (nonpersonnel) 2.5 2.5

Piping SystemsHigh deformability elements and attachments 1.0 3.5Limited deformability elements and attachments 1.0 2.5Low deformability elements and attachments 1.0 1.25

HVAC System EquipmentVibration isolated 2.5 2.5Nonvibration isolated 1.0 2.5Mounted in-line with ductwork 1.0 2.5Other 1.0 2.5

Elevator Components 1.0 2.5Escalator Components 1.0 2.5Trussed Towers (free-standing or guyed) 2.5 2.5General Electrical

Distributed systems (bus ducts, conduit, cable tray) 2.5 3.5Equipment 1.0 2.5

Lighting Fixtures 1.0 1.25aA lower value for ap is permitted provided a detailed dynamic analysis is performance which justifies a lower limit. The value for ap

shall not be less than 1.00. The value of ap = 1 is for equipment generally regarded as rigid or rigidly attached. The value of ap = 2.5 is forfexibile components or flexibly attached components.

bRp = 1.25 for anchorage design when component anchorage is provided by expansion anchor bolts, shallow chemical anchors, orshallow (nonductile) cast-in-place anchors or when the component is constructed of nonductile materials. Power-actuated fasteners (shotpins) shall not be used for component anchorage in tension applications in Seismic Design Categories D, E or F. Shallow anchors are thosewith an embedment length-to-diameter ratio of less than 8.

cComponents mounted on vibration isolation systems shall have a bumper restraint or snubber in each horizontal direction. The designforce shall be taken as 2Fp.


The provisions for cases where theconnection points are on two separate structuresare developed in a similar manner.

W p

4 .0 ’

F p

2 .5 ’φ

Figure 13-9. Boiler Example

Example 13-1A steam boiler will be installed in the

mechanical penthouse on the roof of a 4-storybuilding. The dimensions of the unit are shownin Figure 13-9. The fundamental period of theboiler is 0.04 seconds. There are 4 one-inchdiameter anchor bolts, one at each corner of theboiler, embedded in a concrete slab. The boltshave an embedment length of 6 inches. Thebuilding is in a region of high seismicity, UBCSeismic Zone 4. Per the 1997 UBC, the site iswithin 5 kilometers of a "Type B" seismicsource, and located on Soil Profile Type SD. Perthe 1997 NEHRP, the 0.2 second spectralresponse acceleration is Ss = 175% g.

1. Using the 1994 UBC provisions,determine the shear and tension demands on theanchor bolts.

Z = 0.4 (Seismic Zone 4)Ip = 1.0 (Standard occupancy structure)Cp = 0.75Wp = 20.0 kips

The period of the component is less than 0.06seconds, so the equipment is considered rigid.The design lateral force for the component,determined by Equation 13-1 is

F ZI C Wp p p p= = 0.4(1.0)(0.75)(20.0 kips) = 0.3(20.0 kips) = 6.0 kips

The shear per anchor bolt isV = Fp/4 = (6.0 kips)/4 = 1.5 kips per

anchor bolt.The overturning moment is

Mot = (6.0 kips)(4.0 ft) = 24.0 kip-ftand the resisting moment is

Mr = (0.85)(20.0 kips)(1.25 ft) = 21.3 kip-ft.

Note that the resisting moment is reduced15%, to take into account the effects of verticalacceleration. Taking the sum of the momentsabout a corner of the base, the uplift force Ft inthe anchors equals

Ft =−

=24 0 213

2 5

. .

( . )(2 anchor bolts / side) 0.54 kips

Note that the Tri-Services Manual willproduce identical results, since for this case Ap

= 1.0.2. Using the 1997 UBC provisions,

determine the shear and tension demands on theanchor bolts, assuming the bolts will bedesigned using Allowable Stress procedures.

hx = hr = 40 feet (roof top installation)Ip = 1.0 (Standard occupancy structure)ap = 1.0 (rigid component)Wp = 20.0 kipsle/db = 6.0/1.0 = 6 (the ratio of anchor boltembedment length/bolt diameter)For Soil Profile Type SD, Ca = 0.44Na,

where Na is the near source factor. Our site iswithin 5 kilometers of a "Type B" seismicsource, so Na = 1.0. Therefor Ca = 0.44. Rp=1.5,because the ratio of anchor bolt embedmentdepth to diameter of 6 is less than 8.

The design lateral force for the componentdetermined by Equation 13-4 is

Fa C I

R

h

hWp

p a p

p

x

rp= +

1 3

700 Chapter 13

F Wp p= +

( . )( . )( . )

.( )

10 0 44 10

151 3

40

40

= 1.17(20.0 kips) = 23.4 kipsThe shear per anchor bolt, is

V = Fp/4 = (23.4 kips)/4 = 5.9 kips peranchor bolt.

The overturning moment isMot = (23.4 kips)(4.0 ft) = 93.6 kip-ft

and the resisting moment isMr = (0.9)(20.0 kips)(1.25 ft) = 22.5 kip-ft.

Note that in this case, the resisting momentis reduced 10%, to take into account verticalaccelerations. Taking the sum of the momentsabout a corner of the base, the uplift force Ft inthe anchors equals

Ft =−

=936 22 5

2 5 2

. .

( . )( anchors / side) 14.2 kips

To convert these shear and tension forces toAllowable Stress Design levels, we divide by afactor of 1.4, to obtain

V = 5.9 kips/1.4 = 4.2 kipsFt = 14.2 kips/1.4 = 10.1 kips

3. Using the 1997 NEHRP, determine theshear and tension demands on the anchor bolts,assuming the bolts will be designed usingAllowable Stress procedures.

z = h = 40 feet (roof top installation)Ip = 1.0 (Standard occupancy structure)ap = 1.0 (rigid component)Rp=1.25, because the ratio of anchor bolt

embedment depth to diameter is less than 8. ForSoil Profile Type SD, SMS = 1.0SS, and SDS =(2/3)SMS. Therefore,

SDS = (2/3)(1.75 g) = 1.17 gThe design lateral force for the component

determined by Equation 13-6 is

Fa S W

R

I

z

hp

p DS p

p

p

= +

0 41 2

.

F Wp p= +

( . )( . )( . ).

.

( )0 4 10 117

125

10

1 240

40

= 1.12(20.0 kips) = 22.5 kipsThe shear per anchor bolt is

V = Fp/4 = (22.5 kips)/4 = 5.6 kips peranchor bolt.


and the tension per bolt from overturning is

Ft = =90 0

2 5 2

.


and the dead load tributary to each anchor boltis

FD = 20.0 kips/4 bolts = 5.0 kipsThe gravity load is reduced by 0.2SDSD to

account for the effects of vertical seismicaccelerations. The net tension per bolt is

T = 18.0 - [5.0-(0.2)(1.17)(5.0)] = 14.2 kipsper bolt.To convert these shear and tension forces to

Allowable Stress Design levels, we divide by afactor of 1.4, to obtain


The design shear and tension demands onthe bolt must be doubled unless the designstrength of the connected part or the maximumforce that can be delivered by the componentstructural system limits the load to anchor bolts.An example of a mechanism that could limit theforce to the bolt would be yielding of a steelbase plate or bracket. For the purposes ofcomparison, we assume that the base plateyields at the design load.

The results obtained from the four methodsare summarized in Table 13-8. Clearly, thedesign of rigid, acceleration sensitivecomponents has become significantly moreconservative in the 1997 UBC and NEHRPprovisions. Design bolt shears in our exampleincrease by 126% and 180% respectively, usingthe 1997 NEHRP and 1997 UBC. Increases in


the design uplift demands on the anchor boltsincrease even more dramatically, over 18 timesthe 1994 UBC provisions using the 1997 UBC.

A portion of these increases can beattributed to changes in the characterization ofground shaking in regions of high seismic risk.In addition, the 1997 UBC and NEHRPprovisions include factors to account foramplification of ground motion in the upperportions of structures. Finally, the 1997provisions attempt to refine and rationalize thereduction factors (Rp). Individually, each ofthese changes can be justified, but collectively,the produce very conservative results, that aredifficult to justify in the light of experience inrecent earthquakes.

Table 13-8. Summary, Example 1 ResultsMethod Bolt Shear Bolt Tension

1994 UBC 1.5 kips 0.54 kipsTri-Services Manual 1.5 kips 0.54 kips

1997 UBC 4.2 kips 10.1 kips1997 NEHRP 4.0 kips 10.1 kips

W p

4 .0 ’

F p

3 .5 ’

Figure 13-10. Emergency Generator Example

Example 13-2An electrical generator is installed on the

third floor of a 5-story emergency commandcenter. The dimensions of the unit are shown inFigure 13-10. The generator is mounted on fourvibration isolation mounts (one at each cornerof the unit), with a lateral stiffness of 3kips/inch each. The building floor-to-floorheight is 12 feet, and the fundamental period ofthe building is 0.5 seconds. The building is in

UBC Seismic Zone 3, a region of moderatelyhigh seismicity, and is not in the proximity ofan active fault.. Per the 1997 NEHRP, the 0.2second spectral response acceleration is Ss =100% g. The site has been identified as SoilProfile Type SD

1. Using the 1994 UBC provisions,determine the shear and tension demands on thevibration isolation mounts.

Z = 0.3 (Seismic Zone 3)Ip = 1.5 (essential occupancy structure)Cp = 0.75Wp = 15.0 kips

The period of the equipment can beestimated using equation (13-3):

TW

ka

p= 0 32.

Ta = 0 32150

30.

.

( .

kips

kips / inch)(4 mounts)

Ta = 0.36 seconds

The period of the component is greater than0.06 seconds, so the equipment is consideredflexible and the value of Cp must be multipliedby a factor of 2. The design lateral force for thecomponent using Equation 13-1 is

F ZI C Wp p p p= = 0.3(1.5)(2 x 0.75)(15.0 kips) = 10.2 kips

The shear per vibration isolation mount isV = Fp/4 = (10.2 kips)/4 = 2.6 kips permount.


and the resisting moment isMr = (0.85)(15.0 kips)(2.0 ft) = 25.5 kip-ft.

Taking the sum of the moments about a cornerof the base, the uplift force Ft in the vibrationisolation mount equals

mountper kips 1.3

e)mounts/sid 2)(0.4(

5.257.35

=

−=Ft

702 Chapter 13

2. Using the provisions of the Tri-ServicesManual, determine the shear and tensiondemands on the vibration isolation mounts.

Since Ta = 0.36 seconds, the equipment isconsidered flexibly mounted. The ratio ofcomponent period to fundamental period of thestructure is

T

Ta = =

0 36

0 50 72

.

..

Entering the graph in Figure 13-6, we obtaina value for Ap = 4.5. It is unlikely that thehigher modes of vibration of the building willproduce a greater value of Ap. Then the designlateral force using Equation 13-2 is

F ZI A C Wp p p p p=

pW)75.0)(5.4)(5.1)(3.0(= = 1.52 Wp

However, IpApCp need not exceed 3.75,which in this example governs. Substitutingthese values into Equation 13-2, we find

Fp = =( . )( . )0 3 3 75 Wp 1.13 Wp = 16.9 kips

The shear per vibration isolation mount isV = Fp/4 = (16.9 kips)/4 = 4.2 kips permount.


and the resisting moment isMr = (0.85)(15.0 kips)(2.0 ft) = 25.5 kip-ft.Summing moments about a corner of the

base, the uplift force Ft in the vibration isolationmount equals

mountper kips 4.2

e)mounts/sid 2)(0.4(

5.252.59

=

−=Ft

3. Using the 1997 UBC provisions,determine the shear and tension demands on thevibration isolation mounts, assuming they willbe designed using Allowable Stress procedures.

hx = (3 floors)(12 feet/floor) =36 feethr = (5 floors)(12 feet/floor) = 60 feetIp = 1.5 (essential occupancy structure)ap = 2.5 (flexible component)Rp = 1.5 (vibration isolated component)Wp = 15.0 kips

Na = 1.0 (no nearby faults)For Soil Profile Type SD, Ca = 0.36Na = 0.36.Substituting these variables into Equation (13-4), the design lateral force for the componentusing Equation 13-4 is

Fa C I

R

h

hWp

p a p

p

x

rp= +

1 3

F Wp p= +

( . )( . )( . )

.( )

2 5 0 36 15

151 3

36

60

= 2.52 Wp

However, Fp need not exceed 4CaIpWp, soFp= 4(0.36)(1.5) Wp = 2.16 Wp = 32.4 kips

The shear per isolation mount isV = Fp/4 = (32.4 kips)/4 = 8.1 kips perisolation mount.


and the resisting moment isMr = (0.9)(15.0 kips)(2.0 ft) = 27.0 kip-ft.Summing the moments about a corner of the

base, the uplift force Ft in the vibration isolationmount equals

Ft =−

=1134 27 0

4 0 2

. .


To convert these shear and tension forces toAllowable Stress Design levels, we divide by afactor of 1.4, to obtain


4. Using the 1997 NEHRP/FEMA 273,determine the shear and tension demands on thevibration isolation mounts, assuming themounts will be designed using Allowable Stressprocedures.

z = 36 feeth = 60 feetIp = 1.5 (essential component)ap = 2.5 (flexible component)Rp = 2.5Wp = 15.0 kipsFor Soil Profile Type SD, SMS = 1.1SS, andSDS = (2/3)SMS. Therefore,


SDS = (2/3)(1.1)(1.00 g) = 0.73 gThe design lateral force for the component

from Equation 13-6 is

Fa S W

R

I

z

hp

p DS p

p

p

= +

0 41 2

.

F Wp p= +

( . )( . )( . ).

.

( )0 4 2 5 0 73

2 5

15

1 236

60

= 0.96(15.0 kips) = 14.4 kipsSince the component is mounted on

vibration isolators, the design force is doubled,so

Fp = (2)(14.4 kips) = 28.8 kipsThe shear per isolation mount is

V = Fp/4 = (28.8 kips)/4 = 7.2 kips perisolation mount.


and the tension per mount from overturning is

kips 12.8 e)mounts/sid 2)(feet 0.4(

8.100 ==Ft

The dead load tributary to each isolation mountis

FD = 15.0 kips/4 mounts = 3.8 kipsThe gravity load is reduced by 0.2SDSD, and thenet tension per isolation mount is

T = 12.6 - [3.8-(0.2)(0.73)(3.8)] = 9.4 kipsper mount.To convert these shear and tension forces to

Allowable Stress Design levels, we divide by afactor of 1.4, to obtain


The results obtained from the four methodsfor this example are summarized in Table 13-9.Again, this example shows the four methodscan produce results that differ significantly. The1994 UBC is by far the simplest method, andyields the lowest design forces. The other threemethods add significant complexity, andproduce higher design forces. In this example,

the higher design forces may be justified, sincethere have been a number of failures ofvibration isolated equipment in recentearthquakes. As with Example 13-1, a portionof the increase in design force using the 1997UBC and the 1997 NEHRP can be attributed tochanges in the design ground shakingintensities. The amplification of design forcesin the upper levels of the structure is in keepingwith strong motion data obtained from recentearthquakes. Figure 13-8 shows that a linearamplification of ground acceleration by a factorof three from the ground to roof levels (as usedin the 1997 NEHRP) bounds instrument recordswell, while the amplification factor of 4 used inthe 1997 UBC is conservative. The 1994 UBCand Tri-Services approaches ignore thisphenomenon.

Table 13-9. Summary, Example 2 ResultsMethod Bolt Shear Bolt Tension

1994 UBC 2.6 kips 1.3 kipsTri-Services Manual 4.2 kips 4.2 kips

1997 UBC 5.8 kips 7.7 kips1997 NEHRP 5.1 kips 6.6 kips

13.5 DESIGNCONSIDERATIONS FORARCHITECTURALCOMPONENTS

13.5.1 General

Architectural nonstructural componentsinclude items such as exterior curtain walls andcladding; non-load bearing partitions; ceilingsystems; and ornaments such as marquees andsigns. In addition, they can include a wide arrayof shelving, cabinets, workstations, andequipment that are installed by the buildingoccupant.

For life safety, the objective of the designshould be to limit the severity of damage to thearchitectural components so that they do nottopple, or detach themselves from the structureand fall. For higher performance objectives, itmay be necessary to control damage to the

704 Chapter 13

components so that functionality is notimpaired. For example, a curtain wall systemthat does not fall from the building or blockegress may be considered to have met a lifesafety performance objective. For immediateoccupancy, it may be necessary to limit damageof the system so that it continues to be weather-tight.

Much of the information in the followingsections has been adopted from the excellentdiscussion of nonstructural components foundin FEMA 273 and FEMA 274.

Some architectural components areinherently vulnerable to earthquake damage.For example, cracking will occur in stucco andplaster at relatively low levels of groundshaking. Limiting building drift, or providingcomponent anchorage to protect these materialsfrom damage, is generally not cost-effective.Much damage can be minimized throughcareful detailing of the components. Theobjective is to minimize the amount ofdistortion experienced by the element due storydrift, and for acceleration-sensitive items,provide adequate anchorage to prevent shiftingor toppling. With proper attention to detailing,damage in moderate ground shaking can belimited to a level that is easily andinexpensively repaired.

Damage to building contents outside thescope of the designer, such as furniture,countertop items (for example, computers),cabinets, and shelving can be limited byproviding adequate anchorage for these items.The contents of cabinets and shelving can berestrained. However, most items that areportable are difficult to anchor effectively forseismic forces. People using these items willoften prefer not to employ the seismic latches,tethers, or other restraint devices provided,since they generally make the use of the itemless convenient.

13.5.2 Architectural Finishes

Plaster and stucco are common finishmaterials that are very brittle. At relatively lowdisplacements, plaster begins to crack. As

displacements increase and the finish is furtherdistorted, the material spalls, and can separatefrom the supporting lath. Plaster directlyapplied over structural elements that form partof the lateral force-resisting system is especiallyvulnerable. In a large earthquake, the structuralelements are expected to experience inelasticbehavior, and the distortions of the elementsassociated with this behavior will usually causesignificant damage to the plaster finish.Generally, repairs to the plaster finishes areinexpensive, and the damage does not representa significant hazard. However, failure of a largeplaster or gypboard surface, such as a ceiling,can pose both a falling hazard and block thepath of egress. Ceiling systems should bedesigned to accommodate the expecteddistortions of the supporting structure withoutcollapse. Where significant diaphragmdistortions are expected, consideration shouldbe given to isolating the furring for plasterceilings from the diaphragm.

Shear cracking of surface finishes neardoors and windows is a common form ofearthquake damage, and is probablyunavoidable. Although this type of damage ismost common in plaster surfaces, other wallfinishing materials are vulnerable. Post-earthquake repairs are relatively inexpensive,provided matching materials are available. Fortile finishes, finding a suitable matching tile forrepairs can be difficult.

Adhered veneer refers to thin surfacematerials, such as tile, thin set brick, or stone,which rely on adhesive attachment to a backingor substrate for support. This includes tile,masonry, stone, terra cotta and similar materialsnot over 1 inch in thickness, as well as ceramictile and exterior plaster (stucco). Thesematerials are supported by adhesive (notmechanical) attachment to a supportingsubstrate, which may be masonry, concrete,cement plaster, or a structural framework.Adhered veneers are deformation sensitive, andtheir seismic performance depends on theperformance of the supporting substrate.Adhered veneer materials are often inherentlybrittle. Deformation of the substrate leads to


cracking, which can result in the veneerseparating from the substrate. The key to goodseismic performance is to detail the substrate soas to isolate it from the effects of story drift.The materials are most vulnerable atdiscontinuities, such as corners and openings.

The threat to life safety posed by adheredveneers depends on the height of the veneer, thesize and weight of the fragments likely tobecome dislodged, and the nature of theoccupancy. It is important to distinguishbetween falling of individual units such as tiles,which typically would not be considered a life-safety issue, and large areas of the veneerseparating from the substrate and falling.

Anchored veneer consists of masonry unitsthat are attached to the supporting structure bymechanical means. This type of veneer is bothacceleration and deformation sensitive. Themasonry units can be dislodged byaccelerations which distorts or fail themechanical connectors. Deformations of thesupporting structure may displace or dislodgethe units by racking. Damage to anchoredveneers can be controlled by limiting the driftratios of the supporting structure, isolating unitsfrom story drift through slip connections orjoints, and by anchoring the veneers for anadequate force level that includes considerationof the vertical component of ground shaking.Special attention should be paid at locationslikely to experience large deformations,especially at corners and around openings.

Masonry veneer facades on steel framebuildings should be avoided unless the veneer issecurely tied to a separate wall or frameworkthat is independent of the primary (gravity andlateral load carrying) steel frame. Otherwise,adequate provisions for the large expectedlateral deformation of the steel frame must bemade. Wire or straight rod ties should not beused to anchor face brick to a wall, especiallywhen a layer of insulation or an air gapseparates the two elements. Large masonryfacades may be designed as part of thestructural system.

13.5.3 Exterior Ornaments andAppendages

Exterior ornaments and appendages arenonstructural components that project above oraway from the building. They includemarquees, canopies, signs, sculptures, andornaments, as well as concrete and masonryparapets. These components are accelerationsensitive, and if not properly braced oranchored can become disengaged from thestructure and topple. Building codes requireconsideration of vertical accelerations forcantilever components. Features such asbalconies are typically an extension of the floorstructure, and should be designed as part of thestructure. Parapets and cornices, unless wellbraced, are flexible components and designforces for these components should beamplified accordingly.

Heavy roof tiles pose a significant fallinghazard, unless the tiles are securely attached tothe roof diaphragm. One method of method ofsecuring mission tiles is shown in Figure 13-11.The tie wires used to secure the tiles should becorrosion resistant.

13.5.4 Partitions

Partitions are vertical non-load bearingelements that are used to divide spaces. Theymay span vertically floor to floor orhorizontally between cross walls. In somecases, partitions span to a hard ceiling (plasteror gypboard), or may extend to the ceiling, andstop, with lateral bracing extending to the flooror roof structure above. Partitions mayclassified as heavy or light. Heavy partitions aregenerally constructed of masonry materialsincluding glass block masonry. They are self-supporting for gravity, isolated from thestructural framework, and weigh in excess of 10pounds per square foot (note that if thesepartitions are not isolated from the structuralframework, they may behave as part of thebuildings lateral force resisting system). Lightpartitions consist of wood or metal studscovered with gypboard, lath and plaster, orwood. Light partitions typically weigh less than10 pounds per square foot.

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Partitions are acceleration and deformationsensitive. Partitions spanning floor to floor willsuffer shear cracking and distortion due to storydrift, unless detailed to accommodate driftswithout racking. If the partitions undergosignificant distortions, adhered veneers can falloff. In the out-of-plane direction, highaccelerations can cause flexural cracking and ifthe top or bottom connections to the structurefail, collapse. If partitions are isolated from thesupporting structure or are free standing, theybecame acceleration sensitive.

Seismic performance of partitions iscontrolled by attachment of the finish materials,and the support conditions at the floor or roofstructure and ceiling system. The topconnection should allow for vertical movementof the floor or roof structure and horizontal in-plane motion, but resist out of plane forces.Partitions in buildings with flexible structuralframes should be anchored to only onestructural element, such as a floor slab, andseparated by a physical gap from all other

structural elements. Reinforced masonrypartitions tied to more than one structuralelement should be considered part of thestructural system. Unreinforced masonry shouldnot used for partitions or filler walls.

Connections at the top of the partitionshould accommodate in plane movement, butprovide out of plane support. A gap, with anadequately sized resilient filler (if necessary forsound or fire separation), should isolate thestructural frame from the nonstructural partitionwalls. Figure 13-12 illustrates one method ofproviding this separation for heavy partitions,while at the same time bracing the wall againstout-of-plane motion. Figures 13-13 and 13-14illustrate methods of bracing full and partialheight light partitions. Partial height partitionsshould never be laterally supported bysuspended T-bar ceiling systems. Partitions thatcross building seismic joints are particularlysusceptible to damage due to differentialstructural movement across the joints.

Figure 13-11. Roof tile anchor details


Figure 13-12. Nonbearing masonry wall details

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Figure 13-13. Seismic bracing for light partitions (13-10)

Figure 13-14. Seismic bracing for light partitions


Some modern interior planning approachesutilize nonanchored partition systems that relyupon the self-weight of the partitions, corners,or spread bases to supply stability. Whensubjected to seismic forces, these partitions aremore susceptible to overturning than anchoredsystems. Decisions on their use should take intoaccount the flexibility of such systems and theirease of installation, as opposed to the possibledanger of overturning during an earthquake. Ofparticular importance in this regard are thosesystems that utilize hanging furniture or storagesystems as part of the partition system. Somepartitions are lightweight screens and may notnecessarily cause injury or significant damage ifoverturned. However, other systems are moredense and heavier than full-height stud andgypsum board walls. The weight and stabilityof these systems must be given carefulconsideration in areas of high seismic risk.

13.5.5 Curtain Wall Systems

Curtain wall systems consist ofprefabricated wall units and a variety of glasswall systems. Prefabricated wall systemsinclude precast units (including units faced withan adhered or attached veneer), laminated metalfaced insulating panels, unitized curtain wallsystems, and steel framed panels withmechanically attached masonry, Glass FiberReinforced Concrete (GFRC), metal, or stonefacing. These units may span vertically, fromfloor-to-floor or horizontally. Glass curtain wallsystems include stick-framed systemsassembled on site, sloped glazing and skylights,storefront systems, and structural glazing.Curtain wall systems are both acceleration anddeformation sensitive, and can be dislodged bydirect acceleration or failure of connections dueto story drift.

Cladding units should have a minimum offour anchors per unit. Because of thermalmovement and shrinkage considerations, thecladding unit connection system is generallystatically determinate. Therefore, failure of asingle connection can result in the cladding unitbecoming unstable and falling from the

building. The consequences of an anchoragefailure are potentially grave, since large precastcladding units can weigh in excess of 20 kips.Therefore, building codes require that theconnections for prefabricated panels be designfor the unreduced expected story drift of thestructure, and be able to resist high inertialloads in such a manner as to preclude failure.For precast concrete panels, connections aredesigned to ensure ductile behavior. The bodyof the connection, made up of steel plate orshapes, is designed for 1.33 times the designforce for the panel skin. Elements of theconnection that may behave in a brittle manner,such as welds, bolts, and items embedded in theconcrete such as inserts and anchor bolts, aredesigned for 4 times the panel design force.

For panelized cladding systems, the unitsmust be detailed in such a manner so as topermit lateral story drift of the structural frame.Units that span from floor to floor mustaccommodate drift through sliding or bendingconnections, or by rocking. Sliding connectionsmay be detailed using bolts that slide in slottedholes. The length of the slot should equal twicethe expected story drift, plus the diameter of thefastener, plus an allowance for constructiontolerances. For sliding connections to beeffective, the fastener should be centered in theslot, because if the bolt “bottoms out” at the endof the slot in an earthquake, extremely highshearing forces will be developed in thefastener. Connections that rely on bending inductile elements can provide excellentperformance, provided that the bending elementis long enough to accommodate the expectedstory drifts without inelastic bending in thestrain hardening range. Connections usingthreaded rods should be carefully designed,since the rod may suffer a low-cycle fatiguefailure if subject to even moderate inelasticbending. The rocking mechanism permitscladding units to accommodate story drift byallowing vertical motions in the gravity load-bearing connections, through the use of verticalslots or oversize holes.

Special consideration should be given to thelayout of joints in prefabricated wall systems.

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At building corners, and when adjacent unitsutilize different methods for accommodatingstory drift, adjacent cladding units may notmove in a uniform manner. Joints betweencladding units may close causing adjacentpanels to come into contact, imposing highloads on the panels and their anchors.

Glass curtain wall systems are typicallyassemblies of structural subframes attached tothe main structure. They may be prefabricatedor assembled on site, and include stick framedcurtain walls assembled on site, prefabricatedunitized curtain wall systems, storefronts, andskylights. Glazing systems are predominantlydeformation sensitive, but can be damaged byhigh accelerations. Glazing in “dry”installations (where the glass is held in place byputty, a rubber/vinyl bead, or wood or metalstops) can shatter due to a combination ofracking of the frame due to story drift coupledwith out of plane forces. Failures of glazingsystems in past earthquakes have beenattributed to number of causes. Deficiencies inthe design of the supporting frame and thecutting and placement of the glass can result inpoor performance. A lack of sufficient supportaround the edges of the glass pane (edge bite),due to an oversized opening in the frame, or anundersized glass pane, may allow the glass tofall out. A lack of edge blocking can also allowthe glass panes to shift and fall from the frame.If the glass panes are cut large, there may beinsufficient clearance between pane and frameto permit racking. Frames that are attached tothe structure that are not detailed toaccommodate story drift will flex and twist.When frame racks due to story drift, the panecomes into contact with the frame and the glass,which cannot distort in-plane, will shatter. If thegasket around a glass pane loosens and fallsfrom the opening, it may allow the glass to fallout, or move and shatter in the frame.

The type of glass used also affects safety.Ordinary annealed glass produces sharp-edgedshards when broken. Safety (or tempered) glassis required when glass extends to within 18inches of the ground or floor. Tempered glassfractures into small round-edged pieces, which

pose a lower hazard. Laminated glass generallyremains intact, even if it cracks. Tempered orlaminated glass should be used in exits orwhere large glazed areas front public walks.

13.5.6 Ceiling systems

Ceiling systems are horizontal and slopingassemblies attached to or suspended from thestructure. At exterior locations, ceiling systemsmay be referred to as soffits. While there aremany different architectural treatments forceilings, structurally, they can be classified intotwo main categories of systems, those that areattached directly to the building structure(surface applied materials), and those that aresuspended from the structure by wires or othermeans.

Surface applied materials consist of wood,acoustical tile, gypboard, plaster, or metalpanels applied directly to wood or steel joists,concrete slabs, or metal deck. The surfacematerials may be attached with mechanicalfasteners or adhesive. This class of ceilingsystems also includes gypboard ceilingsattached to wood or steel furring supported by asupplemental framework, braced back to theprimary structure. Surface applied materialstypically perform well in earthquakes, providedthe structural elements supporting this systemperform reasonably well.

Suspended ceilings include T-bar systemswith integrated lighting and mechanicalcomponents, and suspended lath and plaster orgypboard systems. There are a variety ofsuspended T-bar systems available. Mostcommon are exposed spline systems (where thesupporting T-bar frame is visible), concealedspline systems (hidden supporting frame), andluminous systems (lighting diffused throughopaque panels). Suspended ceiling systems areacceleration and deformation sensitive. Seismicperformance of suspended ceiling systems iscontrolled by the behavior of the supportsystem, and historically concealed splinesystems have performed better than exposedspline systems.


Suspended T-bar systems consist of alightweight grid, which supports ceiling panels,light fixtures, and HVAC diffusers. Thesesystems are highly vulnerable to damage unlessthe grid is securely braced with splay wires orother bracing devices and vertical compressionstruts (Figure 13-15). In an earthquake, theceiling is subject to forces from light fixturesand ceiling ventilation diffusers. Sprinklerheads projecting through the ceiling maydamage the panels and supports. Suspendedceiling systems in buildings with long spansand flexible structural systems are at greaterrisk.

Distortion of the grid can result in a loss ofpanels and may cause light fixtures anddiffusers to drop. However, light ceiling panelsthat weigh less than 2 pounds per square foot donot pose a life safety risk, and are generallymore of a nuisance than a hazard. Heavy itemssuch as light fixtures and HVAC diffusersshould have an independent supporting system,unless the ceiling suspension system is

designed to carry the added weight of thefixtures during an earthquake. Positive,mechanical connections should be provided tokeep the object attached to the grid. In addition,heavy items supported by the ceiling systemshould be provided with safety wires, to preventthe items from dropping should they becomedetached from the supporting grid.

Standards for seismic bracing of ceilingsystems have been developed(13-11). In general, aseismic bracing strategy for suspended ceilingsystem should provide bracing against lateraland vertical movements. The disposition ofbracing should account for concentrations ofmass, such as light fixtures and diffusers. Theceiling system should be rigidly attached to twoadjacent walls, and permitted to “float” alongthe walls directly opposite those where thesystem is attached. This permits the walls todistort and “rack” in plan without buckling thegrid or pulling it apart. Along the “floating”edges, a shelf angle provides vertical support tothe ceiling. The angle must be wide enough to

Figure 13-15. Seismic bracing for suspended T-bar ceiling systems (13-10)

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allow for differential movement betweenopposing walls. Care must be taken to ensurethat the grid is not inadvertently connected tothe walls at the “floating” edge. At theperimeter of the ceiling, the main and crossrunners of the ceiling grid should be supportedby hangers.

The connection between the main runnersand cross runners should positive, using lockingclips or screws, to prevent the ceiling grid fromcoming apart during an earthquake. Friction-type connections should be avoided.

Suspended lath and plaster and gypboardsystems can perform well, being inherentlyrigid in the plane of the ceiling. However, if theceiling system is heavy and large, carefulconsideration of the design and detailing isneeded, because the ceiling can pose significantrisk to life if it drops. Reference 13-12 outlinesthe seismic design requirements for rigidsuspended ceiling systems. Complexinstallations will require special engineering.

The hanger wires supporting the ceilingmust be securely attached to the structureabove, and the lath properly wired to the furringchannels. Proper installation of these wires iscrucial to satisfactory performance. Hangerwires may unwind, break, or fail at theconnection to the structure in a strongearthquake. Isolation joints must be provided atbuilding seismic separations. Without thesejoints, relative movements between thestructures will damage the ceiling, and in somecases, collapse of the ceiling.

Rigid ceiling systems should be bracedagainst vertical and lateral movement at regularintervals. Where gypsum board is used, seismicperformance can be enhanced by the use ofsteel nailing strips. If the ceiling is irregular(with changes in elevation, reentrant corners,etc.), the supporting channels should bemechanically connected with bolts, screws, orwelds. Corners should be rigidly braced. Thearrangement of lateral bracing should considerdiscontinuities in the ceiling created by rows oflight fixtures or HVAC diffusers. It may bedesirable to add bracing members at thesediscontinuities, to tie the ceiling together. Light

fixtures and diffusers should be securelyfastened to the ceiling supports. The use oftoggle bolts in the plaster or gypboard forattachment of these items should be avoided.

13.5.7 Exitways, Stair, and ElevatorEnclosures

Exitways, stair, and elevator enclosuresinclude treads, risers, landings, and surroundingshafts that make up the enclosures. Theseenclosures can be either acceleration ordeformation sensitive. If integral with thestructure, stairs and enclosures must beconsidered in the overall design and analysis,including their contribution to overall structuralstiffness and response due to bracing action.Failure of the enclosure can render the stairs orelevator unusable.

Following an earthquake, buildingoccupants will attempt to leave the buildingthrough the exitways. Care should be used, toensure that design features of the exitways donot impeded safe egress. The doors should bedesigned to accommodate seismic drift, so theywill not jam open or closed in an earthquake.The use of veneer or ceiling treatments thatcould become dislodged and fall should beavoided. The covers over seismic joints shouldbe designed to accommodate the expected storydrifts without significant damage. Light fixturesshould be adequately braced for seismic loads.Stone veneers should be properly anchored tothe supporting frames, and the frames should bedesigned to accommodate story drift withoutracking.

13.5.8 Building Contents

Building contents can pose a significant riskduring a strong earthquake. The followingsection provides general information onimproving the seismic performance of buildingcontents. However, since the contents aregenerally furnished and installed by the owner,anchorage of these items is typically outside thescope of the design professional.


Storage racks, such as those found inwarehouse stores, can pose a significant hazard.Storage racks installations should beengineered. Storage racks designed andinstalled in accordance with the standard of theRack Manufacturers Institute(13-15) have beenproportioned to withstand seismic forces.Special care should be taken to protect the legsof the racks, which are vulnerable to damagefrom forklifts. Adequate clearance between therack and structural elements, such as walls andcolumns, should be provided to preventinteraction between the rack and the buildingstructure.

Storage cabinets, bookshelves, filingcabinets, and display cases come in a myriad ofshapes and sizes. In general, items that are talland slender should be anchored to the wall toprevent tipping. Tall furniture, and items thathave glass shelves should not be placed in thepath of egress. Items that may shift or toppleshould not be placed where they could blockexit doors. Providing latches on cabinet doors,and shelf lips or face bars on open shelving, canprevent loss of contents during ground shaking.

13.6 MECHANICAL/ELECTRICALCOMPONENTS

13.6.1 General

Mechanical and electrical componentsconsist of equipment such as pumps, boilers,chillers, fans, transformers, and electricalswitchgear, as well as distribution systems suchas piping, ducts, conduits, and cable trays. Mostelectrical and mechanical equipment are pre-manufactured, “off the shelf” items. Thecharacteristics of each component aredeveloped based upon functional needs. Thesecharacteristics – such as the presence of internalspring isolators or ceramic components,determine damage potential.

When discussing seismic performance ofmechanical components, it is important to

differentiate between Life Safety andImmediate Occupancy performance objectives.Functionality of the component following anearthquake is not generally a life safety issue.For the Life Safety objective, it is usuallysufficient if the component does not shift ortopple during an earthquake. For ImmediateOccupancy, the component or system may berequired to function following the earthquake.For the higher performance objectives, thecomponent manufacturer must show throughanalysis or by shake-table test that thecomponent remains functional following theprescribed level of ground shaking.

Much of the information in the followingsections has been adopted from the discussionof mechanical and electrical components foundin FEMA 273 and FEMA 274.

Mechanical and electrical equipment isgenerally acceleration sensitive. Failure modesinclude sliding, overturning, or tilting of itemsmounted on the floor or roof. Items suspendedfrom or attached to walls or ceilings may sufferloss of support and fall. Distribution systems,such as piping, ducts or wiring connected to theunit can fail. Most equipment items are fairlyrobust, since they must survive the rigors oftransportation and installation at the job site.However, the internal components ofequipment may be blocked or restrained toprevent damage during transit. Upon removal ofthese restraints, the internal components of theitem may be much more vulnerable to shakingdamage.

Mechanical equipment and systems areeither rigidly anchored to the primary structure,or installed on flexible mounts (to controlvibration or permit thermal movements). Thelateral capacity of rigidly mounted equipment isoften governed by the capacity of the anchorbolt or fastener. Failures can also occur at theconnection between the component and pipes,ducts, or conduits that connect to thecomponent.

Vibrating mechanical equipment (typicallyequipment with rotating components, such aschillers, pumps or emergency generators) isoften installed on resilient mounting systems,

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particularly when the equipment is on the upperfloors of a structure. The most commonvibration isolation mounts rely on springs orelastomeric devices to limit the transmission ofvibration and sound to the rest of the structure.Unless specifically designed to resist seismicforces, isolated components are vulnerable todamage at low levels of ground shaking.

13.6.2 Rigidly Mounted Components

The primary aim of seismic design forrigidly mounted components is that they remainin place. The effects of shaking on internal partsare generally not considered. If functionality ofthe component is critical, then a specialevaluation is required. This may includeseismic qualification of the internal componentsof the equipment through shake table testing,detailed analysis, or experience data from past

earthquakes. In this section, our focus is onanchorage issues, and the design of structuralcomponents of the equipment, such as baseplates, anchor bolts, legs, braces, etc. Mostequipment is not a life safety threat unless it canoverturn or fall, or if failure of the componentresults in the interruption of a critical functionor the release of hazardous materials.

The lateral capacity of rigidly mountedcomponents can be governed by the capacity ofthe anchor bolts, the capacity of the unit frameor body, or by the capacity of a yieldingelement, such as a base plate or mounting tab.Installations with capacity governed by theanchor bolt capacity are the least desirable, butare often unavoidable. An example of a tankinstallation governed by anchor bolt capacity isillustrated in Figures 13-2 and 13-16. Thisrooftop saddle mounted tank displaced in theNorthridge Earthquake. The tank itself was

Figure 13-16. Tank in figure 13-2, expansion anchor failure


undamaged. Introduction of a yielding elementin the connection could have limited the loadsdelivered to the anchor bolts, precluding thisfailure. By introducing a yielding element, suchas a steel plate in weak axis bending, thedesigner can introduce a yielding element,providing a mechanism for the dissipation ofenergy and limiting the amount load that can bedelivered to the anchor bolts. Where this cannotbe done, the design forces for the anchor boltsshould be increased, to preclude a brittle failureof the installation.

The designer should consider the load pathfor the seismic forces of the component. Thedesign anchorage forces will generally besignificantly higher than the design lateralforces for the primary structure or framingsupporting the component. For example, aheavy air conditioning unit may be mounted ona roof diaphragm of untopped steel deck. At thepoint of attachment, the air conditioning unitmay deliver design lateral forces to the steeldeck that exceed the design shears for the roofdiaphragm. The designer should check the loadpath – in this case, the diaphragm capacity inthe immediate vicinity of the unit – for thecomponent vertical and lateral loads. Localelements of the supporting structure should bedesigned and constructed for the componentforces, where they control the design of theelements or their connections. When checkingthe supporting structure, the design forcesshould not be modified due to anchorageconditions. For example, using the 1997NEHRP provisions, it would not be necessaryto reduce the Rp factor due to shallow anchorbolt embedment.

In general, installations that rely on threadedpipe connections, for example, a vertical tanksupported on pipe legs, should be avoided.Threaded connections are subject to low-cyclefatigue failures. Saddle mounted tanks shouldbe restrained by straps or lugs to the supportingframe. Pipe and conduit connections toequipment should be designed to accommodatedifferential movement, through the use ofbraided or flexible connections.

13.6.3 Vibration Isolated Components

A vibration-isolated component canexperience much higher seismic accelerationsthan the same component, rigidly mounted.This is due to the amplification effects of thevibration mounts. The dynamic characteristicsof vibration-isolated equipment are dominatedby the properties of the isolation mount. Thefundamental period of isolated components canlengthen to the point were a resonancecondition with one or more modes of theprimary structure is possible. This can result inamplifications in lateral force by a factor of fiveor more. The key to controlling these effects isthrough the use of snubbers.

Isolated components can either be internallysnubbed (the snubber is an integral part of thevibration isolation device) or externallysnubbed (through the use of separate snubbers,installed independent of the isolation device).Regardless of the type of snubber used, it isvital that an elastomeric pad be provided toreduce the impact force generated when thecomponent strikes the snubber. Selection of theproper elastomeric pad can be crucial, and themanufacturers’ recommendations for thematerial should be closely followed. Researchand experience has shown that the degree offorce amplification due to impact can bereduced if the clearance between the componentand the snubber (air gap) is limited to ¼ inch orless. The use of inertia pads above the vibrationisolators should be carefully considered in thedesign of the system, since they can add a greatdeal of mass. As with fixed components, it isvital that a load path of adequate strength beprovided for vibration-isolated components.Special attention should be given to thereinforcement of housekeeping slabs, and totheir connection to the structural slab. Ideally,the housekeeping slabs should be castmonolithically with the structural slab. If this isnot possible, sufficient dowels should beprovided to transfer the lateral forces from thecomponent to the diaphragm. The design lateralforce should include any amplification effects

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due to the vibration isolators, and friction due togravity forces should be neglected.

When suspended components are mountedwith vibration isolation devices, care must betaken to ensure that the bracing elements do not“short out” the vibration isolators. For example,the benefits of vibration isolation may be lost ifthe unit is laterally braced with steel angles tothe structure. The hangers and braces must bedesigned for the amplified forces, and if hangerrods are used, the may need to be stiffened toprevent buckling under the compressive loadsgenerated by the vertical component of thebrace force. If the body of the component doesnot have sufficient strength and rigidity, asupplemental structural frame around the itemmay be necessary.

Flexible couplings should be providedwhere pipes, ducts, or conduits meet vibration-isolated systems. Figure 13-4 illustrates theresults of vibration-isolated component rigidlyattached to a braced pipe.

13.6.4 Piping Systems

Piping systems are predominantlyacceleration sensitive, but runs between floorsor buildings are deformation sensitive. Jointfailures caused by inadequate support orbracing, with accompanying loss of contentsunder pressure, are the most common failures.Most pressure piping systems (defined aspiping systems carrying fluids which, in theirvapor stage, exhibit a gage pressure greater than15 psi) are inherently ductile and havesufficient inherent flexibility to accommodateseismic motions. Attachments and braces forseismic loading are needed, particularly forlarge diameter pipes. Bracing in mostinstallations is performed to prescriptivestandards, such as the SMACNA and NFPA-13guidelines(13-13,13-14).

Flexible couplings to accommodate buildingmovements should be provided at structuralseparations, as well as at the base of thestructure where pipes pass from the ground intothe structure. Figure 13-17 illustrates such an

Figure 13-17. Piping details at a seismic gap


arrangement.Damage to fire suppression piping has

generally been the result of joint failures anddifferential movement between the piping andportions of structure. Failures have been causedby impact of branch lines and sprinkler headson adjacent elements, such as hard ceilings.Providing sway bracing at fire sprinkler branchlines and long sprinkler drops can reduce thistype of damage. Providing larger openings forthe sprinkler heads in hard ceiling surfaces canprevent the ceiling from fracturing the sprinklerheads due to movement of the piping.

Sway bracing requirements for piping arespecified in building codes and bracing isgenerally required at specified intervals, basedon the size of the pipe. A typical pipe brace

installation is shown in Figure 13-18.Additional bracing should be provided at bendsand elbows. Flexible couplings should beprovided where piping crosses seismicseparation joints, and where the piping isconnected to vibration isolated equipment.Small diameter pipes that are allowed to swayshould have flexible couplings installed atequipment connections. Where pipingpenetrates walls or floors, the pipe sleevesshould be large enough to accommodate anyanticipated relative movements.

The designer should note that in general, thesway bracing specified in the prescriptivestandard may not prevent local leaks. If thecontents of the piping system are hazardous, amore detailed analysis is warranted.

Figure 13-18. Seismic sway bracing for piping systems (13-10)

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13.6.5 Air Distribution Systems

Sheet metal ducts can tolerate largedistortions and generate low inertial loads, buthave little inherent strength. Ducts rarelycollapse, but the joints are particularlyvulnerable. Joint failures result in a loss of airpressure. In line equipment such as axial flowfans should be braced independent of the ductsystem.

Air distribution systems are predominantlyacceleration sensitive, but runs between floorsor buildings are deformation sensitive. Largeducts (over six square feet in area or 24 inchesin diameter) should be braced to the structure.Figure 13-19 illustrates a method of providingthis bracing. Flexible duct connections shouldbe installed with enough slack material to allowfor the expected differential movement betweenfans and the ductwork. Duct openings throughwalls or floors must be large enough toaccommodate the anticipated movement of theducts. Ceiling diffusers and registers should besecured to ductwork with sheet metal screws, toprevent them from falling should they becomedislodged. Diffusers connected to flexible ductsshould be provided with safety wires to thefloor or roof above, and should be securelyfastened to the ductwork.

Figure 13-19. Seismic sway bracing for HVAC ductsystems (13-10)

13.6.6 Elevators:

Elevator equipment includes the mechanicalequipment such as motor generators andsheaves, electrical controllers, as well as the carand counterweight frames and guide rails.Elevator machinery behaves in the samemanner as other heavy floor-mountedequipment. Mechanical and electricalcomponents should be anchored to resistinertial forces.

Experience in past earthquakes has shownthat the counterweight rails in elevators arevulnerable to damage. During the shaking, therails can bend, allowing the counterweights todisplace into the elevator shaft. Unless a carefulpost-earthquake survey of the shaft is made, itmay not be apparent that this has occurred. Thedanger is that the displaced counterweights maystrike the car if the elevator is operated. Recenteditions of the building codes have required thatthe heavier counterweight guide rails be used,to limit distortions in an earthquake. Elevatorsshould be equipped with a seismic switch, thatsenses significant ground shaking and shuts theelevator down, or forces it to operate in a “goslow” mode. The seismic switch should only bereset after an inspection by a qualifiedtechnician.

13.6.7 Electrical Equipment

Electrical equipment includes electrical andcommunication equipment, electrical panels,motor control centers, switch gear,transformers, emergency generators, batteryracks, light fixtures, and other fixedcomponents, as well as distribution systemssuch as conduit and cable trays. Thesecomponents are generally accelerationsensitive, except for conduit and cable trayscrossing building separations or running fromfloor to floor, which may deformation sensitive.

Electrical panels may be flush or surfacemounted. Flush mounted panels generallyperform well, providing the panel is attached tothe wall studs that frame the opening for thepanel. Surface mounted panels should be


screwed or bolted into the supporting wall studsor into a steel backing plate that spans betweenthe studs. Toggle bolts in plaster or gypboardshould be avoided.

Motor control centers and switchgear shouldbe anchored to the floor, and for tall units,braced or anchored at the top. If structuralbracing at the top is omitted at tall units,conduit running into the upper portions of theunit may be damaged while acting as bracing.Where relative movement between units couldoccur, flexible braided connections should beused in lieu of copper bus.

Emergency generators are generallyinstalled on vibration isolation mounts. In orderto function after an earthquake, all componentsof the system should be anchored for seismicforces. These components include the primemover and generator, starting equipmentincluding batteries, day tank, main fuel tank,radiators, exhaust silencers/mufflers, as well asthe motor control and switchgear. If the

generator is mounted on vibration isolators, theline from the main fuel tank to the generatorshould be flexible enough to accommodate theexpected lateral displacements of the isolatedcomponents. Emergency generators that relyupon the municipal water supply for enginecooling should be avoided, since utility lifelinesgenerally fail following a strong earthquake.Batteries should be restrained in adequatelyanchored racks or boxes.

Transformers come in a variety of sizes, andcan be floor mounted, or suspended from thewalls or ceiling. If wall or ceiling mounted, thetransformers should be adequately attached tothe supporting frame, and the frameproportioned to resist seismic forces. In somecases, floor mounted transformers are stacked,to conserve space. If the units are stacked, theupper unit should be bolted to the lower unit,and the adequacy of the lower unit frame orenclosure should be verified for the anchorageforces at the connection between the two units.

Figure 13-20. Seismic safety wires for HVAC diffusers, light fixtures

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Lighting fixtures come in a number of types,and may be recessed and surface mounted inceilings or walls, supported within a suspendedceiling, and suspended from the ceiling orstructure (pendant fixtures). Lighting fixturesupport failures are generally due to failure ofthe attachment of the fixture to the wall orceiling, or failure of the wall or ceiling.Distortions of T-bar system may allow fixturesto fall. Providing slack safety wires at opposingcorners of light fixtures in T-bar ceilings willprevent them from falling, should the griddistort and the fixtures become detached.Excessive swing of pendant fixtures should beavoided, since it may result in impact with otherbuilding components or the support attachmentmay pull out of the ceiling. Recessed lightingfixtures should be secured to the ceilingsuspension system. The suspension systemshould be of intermediate or heavy gradeconstruction, and be designed to carry theweight of the ceiling fixtures. In addition, thefixtures should be provided with independentsafety supports (Figure 13-20). Auxiliarysupport framing is required where thealignment of the lighting fixtures concentratessignificant mass in a portion of the ceiling grid.

Conduits and cable trays should be braced atregular intervals. Where conduit and bus ductspass across seismic joints, flexible connectionsthat can accommodate the expected relativedisplacements should be provided. Separateground connectors should be provided inconduit runs that pass across seismic joints.


REFERENCES

13-1 FEMA, 1997, NEHRP Guidelines for the SeismicRehabilitation of Buildings, Federal EmergencyManagement Agency (Report No. FEMA 273),Washington, D.C.

13-2 FEMA, 1997, NEHRP Commentary on theGuidelines for the Seismic Rehabilitation ofBuildings, Federal Emergency Management Agency(Report No. FEMA 274), Washington, D.C.

13-3 Freeman, S., “Design Criteria for NonstructuralComponents Based on the Tri-Services Manuals”,Proceedings of Seminar on Seismic Design, Retrofit,and Performance of Nonstructural Components, ATC29-1, 1998

13-4 International Conference of Building Officials, "Uniform Building Code," 1994 Edition, Whittier,California, 1994.

13-5 Departments of Navy, Army, and Air Force, 1992,"Tri-Services Manual: Seismic Design of Buildings,"Navy NAVFAC-355, Army TM 5-809-10, Air ForceAFM 88-3, Chap. 13, Washington, D.C.

13-6 International Conference of Building Officials, "Uniform Building Code," 1997 Edition, Whittier,California, 1997.

13-7 BSSC, 1997, "NEHRP Recommended Provisions forSeismic Regulations for New Buildings and OtherStructures, Part 1: Provisions and Part 2:Commentary," prepared by the Building SeismicSafety Council for the Federal EmergencyManagement Agency (Report No. FEMA 302 and303), Washington, D.C..

13-8 Singh, M.P., "Generation of Seismic Floor Spectra,"Journal of the Engineering Mechanics Division,ASCE, Vol. 101, No. EM5, October, 1975.

13-9 Biggs, J.M. and Roesset, J.M., "Seismic Analysis ofEquipment Mounted on a Massive Structure," InSeismic Design of Nuclear Power Plants, R.J.Hanson, editor, M.I.T. Press, Cambridge, MA, 1970.

13-10 FEMA, 1994, Reducing the Risks of NonstructuralEarthquake Damage, A Practical Guide, FederalEmergency Management Agency, (Report No.FEMA 74), Washington, D.C.

13-11 OSA, 1990, "Metal Suspension Systems for Lay-InPanel Ceilings, IR 47-4" California Office of theState Architect, Structural Safety Section, March1990.

13-12 OSA, 1990, "Drywall Ceiling Suspension -Conventional Construction - One Layer, IR 47-5"California Office of the State Architect, StructuralSafety Section, March 1990.

13-13 SMACNA, 1992, Guidelines for Seismic Restraintof Mechanical Systems and Plumbing PipingSystems, Sheet Metal Industry Fund of Los Angelesand Plumbing and Piping Industry Council, Sheet

Metal and Air Conditioning Contractors NationalAssociation, Chantilly, Virginia.

13-14 NFPA, 1996, Standard for the Installation ofSprinkler Systems, NFPA-13, National FireProtection Association, Quincy, Massachusetts.

13-15 RMI, 1990, Specification for the Design, Testing,and Utilization of Industrial Steel Storage Racks,Rack Manufacturers Institute.

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