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2012 Jim Dunlop Solar

Chapter 10

Mechanical Integration

Design Considerations ● Array Mounting Configurations ● Structural Loads ● Installation

PresenterPresentation NotesThe mechanical integration of photovoltaic arrays requires an understanding of the site conditions, the physical and electrical characteristics of PV modules chosen, the desired electrical output for the array, and the mounting system and structural attachments. It also involves considerations for the installation, maintenance and accessibility of equipment, and architectural integration. The objective is to produce the least-cost mechanical installation that is safe, secure, appealing and appropriate for the application.

References: Photovoltaic Systems, Chap. 10Minimum Design Loads for Buildings and Other Structures, ASCE 7Wind Load Calculations for PV Arrays; Stephen Barkaszi, FSEC & Colleen O’Brien, BEW Engineering: www.solarabcs.org/wind/

Mounting hardware manufacturers websites:Unirac: www.unirac.comProfessional Solar Products: www.prosolar.comIron Ridge: www.ironridge.comDirect Power & Water: www.dpwsolar.com

2012 Jim Dunlop Solar Mechanical Integration: 10 - 2

Overview

Identifying design objectives and key considerations for the

mechanical integration of PV arrays.

Recognizing the common types of mounting methods and materials used to install PV arrays on the ground and to building rooftops.

Defining desirable materials properties consistent with the operating life expectancies of PV systems.

Analyzing structural loads on PV arrays and their attachments.

PresenterPresentation NotesReference: Photovoltaic Systems, p. 255


Mechanical Considerations

The mechanical integration of PV arrays and associated equipment involves the consideration of many factors, including: Module physical and electrical characteristics. Array electrical design and output requirements. Mounting location, orientation and shading. Type of mounting surface (roof or ground mount) Access and pathways for installation, maintenance and fire codes. Structural loads on modules, mounting structures and attachments. Thermal characteristics of modules and effects of mounting system. Weathersealing of building penetrations and attachments. Materials and hardware compatibilities with the application environment. Aesthetics and appearance.

PresenterPresentation NotesPV arrays are constructed from building blocks of individual PV modules, panels and subarrays that form a mechanically and electrically integrated DC power generation unit. The mechanical and electrical layout and installation of PV arrays involves many interrelated considerations and tradeoffs that are affected by the system design, the equipment used and the site conditions.

Reference: Photovoltaic Systems, p. 255-260


Optimizing Array Performance

The electrical performance of PV arrays is affected by a number

of mechanical integration issues, including: Orienting PV arrays for maximum solar energy gain and avoiding shading. Maximizing air flow around the array to minimize operating temperatures. Installing all modules in series source circuits in the same plane. Facilitating access to the array for maintenance and cleaning.

PresenterPresentation NotesReference: Photovoltaic Systems, p. 255-260


Array Temperatures

Operating temperatures for PV arrays are strongly dependent on

the mounting system design and airflow around the array. Rack mounted arrays have the greatest passive cooling and lowest

operating temperatures.

Direct mounts have the highest operating temperatures.

Standoff mounts have moderate operating temperatures, depending on the standoff height. Maximum passive cooling gains are generally achieved with the tops of PV modules 3 to 6 inches above the roof surface, as long as air flow is not impeded underneath the array.

PresenterPresentation NotesHigher operating temperature reduce array voltage, power output and energy production, and accelerate degradation of modules and their performance over many years. Mounting system designs have a strong effect on average and peak array operating temperatures.

Rack mounted arrays have the greatest passive cooling and lowest operating temperatures, with temperature rise coefficients from 15 to 25 °C/kW/m2. Direct mounts have the highest operating temperatures, with temperature rise coefficients of 35 to 40 °C/kW/m2. Standoff mounts have moderate operating temperatures, depending on the standoff height.



Electrical Configurations

The required electrical configuration and roof (or other area)

dimensions will often dictate the best mechanical layout for PV arrays.

Source circuits are usually grouped together with series-connected modules adjacent to one another on a common support.

Source circuit combiner boxes are strategically located to minimize the length of conductors and trip hazards around the array.

PresenterPresentation NotesPreferably, PV modules in source circuits are installed in a single row or rack, with each module adjacent to another, with the module junction boxes aligned on the same sides to facilitate wiring connections. Note that PV module connector leads are only a certain length and additional cabling and connectors may be required for non-standard installations.



Material Selection

Use materials and components that are corrosion and UV resistant, and have compatible life expectancies with PV systems:

Structural members Hot dip galvanized or aluminum

Fasteners

Stainless steel or galvanized

Weather sealants Resistant to UV and temperature extremes Flexibility and long life

PresenterPresentation NotesReference: Photovoltaic Systems, p. 268, 279-280


Weathersealing Materials

Any penetration or attachments through roofs and other building surfaces must be properly flashed and weathersealed.

Use elastomeric, butyl rubber, polyurethane or other long-life, flexible sealants suitable for the materials being sealed.

PresenterPresentation NotesBuilding codes require all roofing system and building penetrations to be properly flashed and weathersealed. There are a variety of weathersealing methods used, depending on the type of penetrations or attachments used, with some requiring sealants and others not. Weather sealants, where required, should maintain flexibility over 30-50 year life and expected temperature extremes, readily adhere to roofing and construction materials, and have UV inhibitors to resist degradation from sunlight. Butyl rubber, elastomeric and polyurethane based sealants have long life and readily adhere to dry roofing materials, including metal, wood and asphalt shingles. Silicone, latex and acrylic sealants are generally not suitable for the application.



Appearance

The appearance of PV arrays and their mounting systems reflects on the quality of the overall installation.

Practices that result in improved aesthetics for PV arrays include: Mounting array parallel to sloped roofs Use array layouts and aspect ratios that are consistent with the

dimensions of roofs and buildings. Minimize spacing between modules in array other than required for

access, pathways and fire safety. Neatly conceal wiring and mounting hardware to make it as inconspicuous

as possible.



Minimizing Costs

Practices to minimize the cost of installing PV arrays include:

Using array layouts that are consistent with the electrical design

requirements and provide the shortest possible routing for conductors.

Using plug/receptacle connectors to wire modules together.

Minimizing the number of attachment points and roof penetrations.

Using standardized mounting hardware.

Creating process-oriented installation approach.



Types of Mounting Systems

Orientation type Fixed-tilt, adjustable and sun-tracking arrays Determines available solar resource

Ground-mounted arrays

Racks, poles and sun-tracking mounts

Roof-mounted arrays Racks for flat roofs Standoff mounts for sloped roofs Direct mounts

Building-integrated PV (BIPV) arrays

Replace conventional building material or an architectural feature

PresenterPresentation NotesPV arrays can be mounted on the ground or attached to buildings or other structures using a variety of methods. PV array mounting orientations can also be classified as fixed-tilt, adjustable or sun-tracking mounts. Ground-mounted designs include racks, poles mounts and sun-tracking arrays. Common building mounts include standoff mounts and rack mounts that can be retrofitted to existing rooftops. Building-integrated PV (BIPV) arrays, including direct mounts and integral mounts are integrated with building components and cladding materials such as windows, awnings and roofing tiles. Reference: Photovoltaic Systems, p. 260-267


Fixed-Tilt Arrays

Fixed-tilt PV arrays are installed on non-moving structures in a constant orientation toward the sun, and are the most common design for mounting PV arrays.

PresenterPresentation NotesFixed-tilt PV arrays are non-movable structures that position the PV array in a constant orientation. Fixed PV arrays installed in northern latitudes are tilted up from the horizontal and oriented toward the south, and away from shading obstructions to maximize the solar energy received. Most PV arrays installed on buildings and ground mounts are fixed-tilt PV arrays.



Adjustable-Tilt Arrays

Adjustable-tilt PV arrays permit adjustment of array tilt angle to optimize seasonal solar energy gain.

NREL, Steve Wilcox

PresenterPresentation NotesAdjustable-tilt PV arrays use mounting structures with removable fasteners, telescoping legs or other manual means to allow for seasonal adjustments of the array tilt angle. Adjusting the tilt angle of PV arrays twice per year, around the time of the equinoxes in the spring and fall, can marginally improve system output, but is generally not practiced for most installations.

Suggested Exercise: Compare the differences for your location for the average peak sun hours on a fixed south-facing latitude tilt surface compared to an adjustable south-facing surface adjusted to latitude - 15° tilt on April 1st, and adjusted to latitude + 15° tilt on October 1st.

Reference: Photovoltaic Systems, p. 261


Sun-Tracking Arrays

North

South

West

East

Vertical-Axis Tracking

Optional seasonal adjustment

Two-Axis Tracking East-West Tracking

Optional seasonal adjustment

PresenterPresentation NotesSun-tracking arrays use mounting structures that automatically and continually move the array surface array to follow the sun’s position throughout the day. Sun-tracking arrays are characterized by their tracking mode and whether they track the sun on one or two axes. Most single-axis trackers are designed to move the array surface east to west on a north-south tracking axis that is tilted from the horizontal.

Sun-tracking arrays can receive up to 20-30% more solar radiation than fixed south-facing arrays. Single-axis trackers do not point exactly to the sun at all times, but generally receive 20% or more solar radiation than received on south-facing fixed-tilt surfaces. Marginal benefits are achieved in going from single to two-axis tracking. Point-focus concentrating PV modules require two-axis sun-tracking to capture the direct beam solar radiation component, while linear-focus concentrators can use single-axis tracking.



Sun-Tracking Arrays

PresenterPresentation NotesSun-tracking arrays can be controlled by passive of active means. Passive means use solar heating of working fluids in the tracker internal structure to create weigh shift and move the tracker, or to pressurize piston actuators to move the structure. Active tracking methods use electromechanical drives or stepper motors that are controlled by smaller PV modules attached to the array, or by external power sources and microprocessors.



Sun-Tracking Arrays

NREL, Warren Gretz

PresenterPresentation NotesThe tradeoff for sun-tracking arrays involves considering the initial costs and recurring maintenance, versus the cost of additional modules on a fixed array to achieve similar energy performance. Tracking PV arrays are usually installed on the ground as opposed to on top of buildings due the large structural loads at the foundations. Sun tracking arrays also require larger surface areas and sufficient spacing between individual trackers to avoid one tracker shading an adjacent one. Some tracker designs use a backtracking approach to limit the tracker movement early in the morning and late in the afternoon and sacrifice some solar energy gain, in order to permit closer spacing of individual trackers and avoid shading.



Sun-Tracking Arrays

Dennis Whalen/IBEW 68 Dennis Whalen/IBEW 68

PresenterPresentation NotesActive sun-tracking arrays use hydraulic pistons or motors to drive the tracking mechanism.



Ground-Mounted Arrays

Ground-mounted PV arrays are commonly used for larger

systems. Arrays can be optimally oriented and offer more flexibility in design than

do roof-mounted arrays. Need for restricted access usually requires fencing or elevating arrays. Safer access for installation and maintenance.

Common types of ground-mounted arrays include: Rack mounts Pole mounts Sun-tracking mounts

PresenterPresentation NotesGround-mounted arrays are detached from buildings, and usually permit the greatest flexibility in mounting and orienting the array. Types of ground mounts include racks, poles and sun-tracking mounts. Ground mounts require anchoring to foundations such as concrete, setting poles directly in the soil, or by self-ballasting. The site conditions and the methods and materials specified by the mounting system designer manufacturer dictate the best installation practices.

Since ground-mounted arrays are typically at lower elevations, shading from nearby trees, fences, buildings and other obstructions may be a concern. Ground-mounted PV arrays generally require restricted access by fencing or elevating the array to reduce safety hazards.



Ground-Mounted Arrays

Ground-mounted arrays need protection from physical harm and for safety purposes, by fencing or elevating the array.



Rack-Mounted Arrays

Rack mounts are flexible designs commonly used for ground mounts and flat roof mounts.

PresenterPresentation NotesRack-mounted arrays are commonly used on the ground, buildings and other structures, and offer the greatest flexibility in mounting the array at specific tilt angles. Small rack-mounted arrays can be installed on poles, and larger racks can be installed in multiple rows for larger arrays.



Rack-Mounted Arrays

Rack-mounted PV arrays can be installed on the ground using concrete footers, on roofs, or on the sides of buildings.



Rack-Mounted Arrays

Large array structures may be installed as a monolithic unit and require heavy lifting equipment.

PresenterPresentation NotesSome array structures may be installed as a monolithic unit by a crane.



Rack/Pole Mounted Arrays

PresenterPresentation NotesRack-mounted arrays can be elevated on poles above harms way for safety and protection.



Pole-Mounted Arrays

Pole-mounted arrays are used for many applications. Larger arrays require substantial foundations.

PresenterPresentation NotesPole-mounted arrays employ either fixed, adjustable, or sun-tracking arrays installed on a rigid metal pipe or wooden pole. Pole-mounted designs allow the arrays to be elevated to protect from harm and to avoid shading, and most allow the array azimuth angle to be rotated for optimal orientation. Due to the large foundation loads, pole mounts are usually installed on the ground and not on buildings, and have limitations on the size of array they can support based on he size of the pole and foundation.



Pole-Mounted Arrays

Small PV arrays are commonly installed on different types of poles.



Pole-Mounted Arrays

Some pole-mounts may be directly buried in the earth.



Pole-Mounted Arrays

Most pole-mounted arrays use reinforced concrete foundations.

PresenterPresentation NotesDepending on the site soil conditions, poles may be set directly into holes and compacted, or may require reinforced concrete foundations. Many types of standard utility and light poles are rated for wind loads, and their ability to support heavy equipment. Foundations used for pole mounts must have sufficient mass and any attachments must have appropriate strength to counter the forces attempting to blow the pole over during high wind load conditions.



Roof-Mounted PV Arrays

Rooftops are a popular location for installing PV arrays for a

number of reasons: Large areas of unused space with good solar orientation. Elevation provides limited access for safety and security, and helps avoid

shading.

Common types of roof-mounted PV arrays include: Standoff mounts Rack mounts Direct mounts

PresenterPresentation NotesRoof-mounted PV arrays may use standoff mounts, rack mounts and building-integrated PV arrays. Rooftops often have large areas of unused space, and are popular locations for installing PV arrays. Rooftop locations provide higher elevations that help avoid shading and offer additional protection and safety for the array. Most roof-mounted PV arrays use fixed-tilt support structures that are retrofitted to existing rooftops. Roof mounts may also be classified according to the type of roof structure or roof covering the array attaches to, such as sloped or flat roofs, or asphalt shingle, metal, tiles or composition roofing materials. For practical and structural considerations, roof-mounts generally do not use movable sun tracking arrays or pole mounts.



Standoff-Mounted Arrays

Standoff mounts are the most common way PV arrays are attached to sloped rooftops.

Gary Lee Sharp Solar

PresenterPresentation NotesStandoff-mounted arrays are the most common way PV arrays are attached to sloped rooftops. Standoff mounts typically locate the PV modules 3 to 5 inches above and parallel to the roof plane. They are not usually tilted at a different angle than the roof surface. This is because the added complexity and costs required to install mounting structures obliquely with respect to the roof surface do not usually justify the marginal increases in solar energy gain and system performance. Several manufacturers provide standard mounting hardware for standoff arrays that meet the structural loads for most applications.



Self-Ballasted Arrays

A self-ballasted array is a type of rack-mounted array that uses the weight of the assembly and additional concrete blocks to secure the array.

Ascension Technology

University of Wyoming

PresenterPresentation NotesA self-ballasted PV array is a type of rack mount that relies on the weight of a the PV modules, support structure and additional ballast material to secure the array. Self-ballasted arrays are intended to reduce or eliminate direct structural connections to a building or foundation, thereby avoiding additional labor and weathersealing concerns. Typical ballast materials include sand and concrete blocks installed in trays at the bottom of the racks. Self-ballasted arrays usually require additional restraints in seismic and high wind load regions.



Building-Integrated PV Arrays

Building-integrated PV (BIPV) arrays substitute for building cladding materials such as roofing or glazing systems.

Sharp Solar

Lawrence Berkley Labs

Spire Solar Chicago

PresenterPresentation NotesBuilding-integrated PV (BIPV) arrays replace conventional building cladding, where PV modules are integrated into roofing materials, glazing, awnings and other architectural features. Integral mounts and direct mounts are types of BIPV arrays. The advantage of BIPV arrays is that the PV array replaces conventional building materials, saving on materials and construction costs. Most BIPV arrays use custom designed modules and require special installation procedures.



Direct-Mounted Arrays

Direct mounts attach PV modules directly to a roofing surface, and use special PV modules and custom designs.

United Solar

Davis Energy Group

PresenterPresentation NotesA direct mount is a fixed array mounting system where the PV modules are attached flush to an existing roof surface or decking. Special PV modules and custom array designs are typically required for direct mount applications. Direct-mounted arrays also experience high operating temperatures.



Integral-Mounted Arrays

Integral mounts use PV arrays to replace conventional roofing systems.

Solar Design Associates

Steven Strong

PresenterPresentation NotesIntegral-mounted arrays are a type of BIPV array where PV modules replace conventional building cladding, such as roofing and window systems. Integral mounts are custom designs using special installation and weathersealing procedures.



Mobile PV Arrays

Some PV arrays are mounted to trailers and other vehicles along with other equipment for portability.

SolarWorld

PresenterPresentation NotesMobile PV arrays require custom designs and special attachments to protect the array from shock and vibrations.



Poor Installation Practice

John Hardwick

John Hardwick

PresenterPresentation NotesHow not to install PV arrays!


Structural Standards

The mechanical design of PV

arrays is governed by ASCE 7, Minimum Design Loads for Buildings and Other Structures.

Adopted in the International Building Code and local codes.

PresenterPresentation NotesThe attachment of PV arrays to buildings and other structures is governed by the standard ASCE 7, Minimum Design Loads for Buildings and Other Structures, which is adopted into most building codes throughout the U.S. However, it does not specifically address the installation of roof-mounted arrays. PV system installers and designers are responsible for ensuring that the design and structural attachments of PV arrays meet all anticipated loads, and that allowable loads on existing structures and mounting systems are not exceeded.

References: Photovoltaic Systems, p. 268-279Minimum Design Loads for Buildings and Other Structures, ASCE 7


Types of Mechanical Loads

Dead Loads (D) Static loads due to the weight of the array and mounting hardware, typically about 5 pounds per

square foot (psf). Higher for self-ballasted designs.

Live Loads (L) Experienced during maintenance activities, generally on the order of 3 psf.

Wind Loads (W)

Typically the highest of all the types of mechanical loads experienced by PV arrays, can be 40-60 psf or higher in hurricane zones.

Snow Loads (S)

Can be higher than 20 psf in northern climates.

Hydrostatic Loads (H) Due to lateral pressure of the earth (soil) or ground water pressure.

Earthquake (Seismic) Loads (E)

Based on seismic design category, requires special mounting for heavy equipment over 400 lbs.

PresenterPresentation NotesPV arrays must be designed and secured to withstand the maximum possible mechanical loads.

Dead loads (D) are static loads due to the weight of the array and mounting hardware, typically about 5 pounds per square foot (psf) for most PV arrays. Self-ballasted arrays can have substantially higher dead loads.Live loads (L) are loads from temporary equipment and personnel during maintenance activities. Generally these loads are small for PV installations, on the order of 3 psf. Typical flat and pitched roofs must be designed for a minimum uniformly distributed live load of 20 psf. All roofs subject to maintenance workers must be designed for a minimum concentrated point load of 300 lbs.Wind loads (W) are typically the highest of all loads experienced by PV arrays, and the only uplifting force. Snow loads (S) can be up to and greater than 20 psf in northern climates.Hydrostatic Loads (H) are due to the lateral pressure of the earth (soil) or ground water pressure on a structure.Seismic (earthquake) loads (E) are based on region and seismic design category.

References: Photovoltaic Systems, p. 268-279Minimum Design Loads for Buildings and Other Structures, ASCE 7


Allowable Stress Design

Allowable Stress Design determines the design loads for structural materials based on not exceeding elastic stresses.

Combination loads from PV arrays having the most adverse

effects of the structure are considered.

Examples of combined loads acting on rooftop arrays that are used for allowable stress design include the following:

D + S (combined downward force) D + W (combined downward force) D + 0.75W + 0.75S (combined downward force) 0.6D + W (combined uplift force)

PresenterPresentation NotesAllowable Stress Design is method to determine the design loads for structural materials based on the maximum allowable elastic stress limits for the structural materials used. Consequently, it includes a factor of safety for unfactored loads. Allowable stress design considers various load combinations, and the most unfavorable loading condition is used for structural design.

Reference: Minimum Design Loads for Buildings and Other Structures, ASCE 7, Chap. 2


Structural Evaluation

Trusses or beams

Point attachments to structure

Roof surface

PV Modules

Module support rails

Module attachments

PresenterPresentation NotesFor common standoff-mounted PV arrays installed just above and parallel to sloped roofs, the mounting methods are quite similar among different PV modules and mounting system suppliers.

Key points of the structural evaluation include:PV module allowable loads and required position of deflection support and attachments.PV module attachments to underlying beams or rails (machine screws or clamps).Allowable deflections in beams or railsPoint attachments to structure



Wind Loads

Wind loads are generally the largest loads experienced by PV arrays, and address two principal components of the structural design:

Main Wind-Force Resisting System (MWFRS): Structural elements that provide support and stability for the structure. PV array attachments to a structure may be considered part of the

MWFRS.

Components and Cladding: Building envelope materials, such as roofing systems and glazing, that are

not part of the MWFRS. PV modules and attachments to the immediate support structure are

considered components and cladding.

PresenterPresentation NotesThe Main Wind-Force Resisting System (MWFRS) consists of structural elements that provide support and stability for the structure, like walls. beams, trusses, roof deck, etc. The PV array attachments to a structure may be considered part of the MWFRS.

Components and cladding are building envelope materials such as roof coverings and glazing that are not part of the MWFRS. PV modules and attachments to the immediate support structure are considered components and cladding. Different calculations also apply to wind load calculations for the MWFRS and for components and cladding. Higher wind loads generally apply to components and cladding where the load is distributed over a smaller area than for the MWFRS. A conservative approach evaluates all PV array elements as components and cladding.

Reference: Minimum Design Loads for Buildings and Other Structures, ASCE 7, Chap. 6


Wind Loads

Three methods are permitted to determine design wind loads:

Method 1: Simplified Procedure

• Applies to most standard enclosed building types less than 60 ft high.

Method 2: Analytical Procedure Method 3: Wind Tunnel Procedure

PresenterPresentation NotesWind loads are by far the most significant concern for mounting PV arrays for most applications. Most PV modules are listed to handle wind loads of 2400 Pa (50 psf), and some are tested for loads up to 5400 Pa (112 psf). Generally, PV modules must be supported in certain positions to achieve maximum load capability. Refer to specific PV module manufacturer’s installation instruction for allowable mounting configurations and maximum loads.

Three methods can be used to determine the design wind loads for buildings and other structures; a simplified procedure, an analytical procedure, and a wind tunnel procedure. The simplified procedure applies to many residential and commercial buildings.

References:Photovoltaic Systems, p. 268-279ASCE 7, Chap. 6


Simplified Procedure

The simplified procedure for calculating design wind loads for

components and cladding involves determining the following:

Basic wind speed (V) Structure classification and importance factor (I) Exposure category (B, C or D) Height and exposure adjustment coefficient (λ) Topographic factor (Kzt) Roof type, slope and pressure coefficient zones (a) Effective wind area (A) and mean roof height (h) Net design wind pressures for h=30ft and I=1.0 (Pnet30) Net design wind pressures (Pnet)

PresenterPresentation NotesThe design loads for components and cladding can be computed using the simplified method if certain conditions are met. The building must be enclosed and regular-shaped, and must have a flat roof or a gable roof with slope no more than 45 degrees, or a hip roof sloped no more than 27 degrees. The mean building height must be no more than 60 ft and the building or site must not have unusual characteristics or wind response.

References: Photovoltaic Systems, p. 268-279ASCE 7, Chap. 6


Design Wind Pressures

For components and cladding, net design wind pressures (Pnet)

are applied normal (perpendicular) to each building surface.

Pnet = λ x Kzt x I x Pnet30 λ = height and exposure adjustment factor at mean roof height

• (1.0 for h ≤ 30 ft in exposure B Kzt = topographic factor

• (1.0 for normal terrain; no escarpments) I = importance factor

• (1.0 for category II structures) Pnet30 = net design pressure for exposure B, h = 30 ft and I = 1.0

For many applications, λ, Kzt and I all equal 1.0, then:

Pnet = Pnet30

PresenterPresentation NotesReference: ASCE 7, Chap. 6


Basic Wind Speed Maps

Source: www.floridabuilding.org

Source: ASCE 7-05 www.asce.org/sei

Source: Brevard County, FL

PresenterPresentation NotesBasic wind speed maps show the maximum design wind speed by location.

References: Photovoltaic Systems, p. 272ASCE 7, Chap. 6


Structure Classification

Category I Buildings and structures that represent a low hazard to human life in the

event of failure, including agricultural, temporary and storage facilities.

Category II All buildings and other structures except those listed in Categories I, III,

and IV – applies to most residential and commercial facilities.

Category III Buildings and other structures that represent a substantial hazard to life in

the event of failure, including schools and congregation areas.

Category IV Buildings and other structures designated as essential facilities, including

hospitals, emergency services and shelters, public utilities and transportation centers. Source: ASCE 7-05

PresenterPresentation NotesBuildings and other structures are classified in categories based on their consequences of failure. Buildings and structures classified in higher categories must be designed for greater loads.

Reference: ASCE 7-10, Table 1-1


Importance Factor

The importance factor (I) considers the degree of the hazard, and

based on the structure category and whether the application is in a hurricane prone region with basic wind speed greater than 100 mph, or not.

For both hurricane and non-hurricane prone regions: For category II structures, the importance factor = 1.0 For category III and IV structures, the importance factor = 1.15



Exposure Category

The exposure category defines characteristics of the surrounding terrain for each wind direction considered. Exposure B applies to areas with obstructions such as tress and buildings

that prevails for at least 2630 ft (800 m) or 10 times the structure height, whichever is greater.

Exposure C applies to open terrain with obstruction heights 30 feet or less.

Exposure D applies to flat, unobstructed areas and water surfaces where the terrain prevails for at least 5000 ft (1524 m) or 10 times the structure height, whichever is greater.

• Extends inland from the shoreline for 660 ft (200 m), or 10 times the structure height, whichever is greater.

PresenterPresentation NotesMost buildings in urban and suburban areas fall under exposure B.

Reference: ASCE 7, Chap. 6


Height and Exposure Adjustment Coefficient

The height and exposure adjustment coefficient (λ) is a factor

used to adjust design wind pressures for mean roof height and exposure category.

For a mean roof height h = 30 ft or less in exposure B, λ = 1.0 For h = 30 ft in exposure C, λ = 1.4, and exposure D, λ = 1.66

For h = 60 ft:

For exposure B, λ = 1.22 For exposure C, λ = 1.62 For exposure D, λ = 1.87

PresenterPresentation NotesAdjustments factors increase wind loads for building heights above 30 feet and for exposure category.

Reference: ASCE 7, Figure 6-3


Topographic Factor

The topographic factor (Kzt) accounts for increased wind loads

due to hills, ridges and escarpments resulting in abrupt elevation changes near the site.

The topographic factor is considered whenever the terrain is unobstructed from similar features for 100 times the height of the feature, and the feature is two times or more the height of any other obstruction within a 2 mile radius. Where these conditions do not apply, Kzt = 1.0



Pressure Coefficient Zones

Pressure coefficient

zones “a” are defined for common roof types and slopes.

Zones define areas of a roof with higher wind pressures.

a = 3 feet minimum, or 0.4

times the mean building height or 10% of the least horizontal dimension, whichever is greater.

a a

a Gable Roof ≤ 7°

a

a

a

Flat Roof

a a

Hip Roof 7 - 27°

a a

a

Gable Roof 7 - 45°

Zone 1 – interior zones Zone 2 – end zones Zone 3 – corner zones

PresenterPresentation NotesPressure coefficient zones define areas of a roof with higher wind loads. The width of the pressure coefficient zone is a minimum of 3 feet, or 10% of the smallest horizontal dimension or 0.4h, whichever is greatest.



Net Design Wind Pressures

Net design wind pressures (Pnet30) increase from Zones 1 to 3, and are

always higher for a smaller effective wind area and for high basic wind speeds.

For a basic wind speed of 90 mph and roof slope of 0 to 7 degrees: For an effective area of 100 sf in Zone 1, Pnet30 = +4.7 and -13.3 psf. For an effective area of 100 sf in Zone 3, Pnet30 = +4.7 and -15.8 psf For an effective area of 10 sf in Zone 3, Pnet30 = +5.9 and -36.8 psf.

For a basic wind speed of 130 mph and roof slope of 7 to 27 degrees:

For an effective area of 100 sf in Zone 1, Pnet30 = +9.8 and -27.8 psf. For an effective area of 100 sf in Zone 3, Pnet30 = +9.8 and -33.0 psf For an effective area of 10 sf in Zone 3, Pnet30 = +12.4 and -76.8 psf.

Roof overhangs have even higher design pressures.

PresenterPresentation NotesNet Design Wind Pressures for components and cladding are determined from the basic wind speed, roof type and slope, pressure zone and effective wind area. These net pressures are defined for Exposure B, for a mean roof height of 30 feet, for importance factor = 1 and for topographic factor = 1. The effective wind area is the area tributary to a single set of supports. Higher wind pressures are given for smaller effective wind areas. The given pressure coefficients are applied as downforce (positive) and uplift (negative) forces perpendicular to the building surfaces. The positive and negative design wind pressures used for load calculations must not be less than +10 psf and -10 psf, respectively.



Calculating Design Loads: Example

Consider a standoff roof-mounted array to be installed on a single-story category II building in a non-seismic region. The basic wind speed is 130 mph, the exposure is B and the snow loads

are 10 psf. The PV modules will be mounted to rails sections and roof attachments

with an effective area greater than 100 sf. The array will be installed in the interior zones on a hip roof at 6 in 12 pitch, or about 26.5 degrees. The weight of the PV array and support structure is 4 psf.

From ASCE 7,Table 6-3:

Pnet30 = 9.8 psf (downforce) and -27.8 psf (uplift)

Pnet = λ x Kzt x I x Pnet30 Where λ, Kzt and I all equal 1.0: Pnet = Pnet30

PresenterPresentation NotesStandoff PV arrays attached close to and parallel with sloped roof surfaces can be considered components and cladding, and design loads can usually be evaluated using the simplified method.



Calculating Design Loads: Example (cont.)

Positive and negative wind pressures are then evaluated in the critical

load combinations to determine the design loads for the structure:

Downforce (positive pressure) D + S = 4 + 10 = 15 psf D + W = 4 + 9.8 = 13.8 psf D + 0.75W + 0.75S = 4 + 7.4 + 7.5 = 18.9

Uplift (negative pressure) 0.6D + W = 2.4 – 27.8 = -25.4 psf

PV modules and support structures must be able to withstand these

maximum positive and negative loads normal to the array surfaces.

PresenterPresentation NotesMost PV modules and pre-engineered mounting systems are designed to support the loads determined in the example.



The entire PV array mounting system and all components must

be designed to support the maximum expected load combinations, including: PV module laminates and frames Rails or beams Point attachments Foundations Fasteners

PV Array Structural Design

PresenterPresentation NotesPV array mounting system designs and all components must be able to withstand the maximum forces expected in any given application. Oftentimes, independent engineering or test results may be required to certify PV array structural designs for local building code compliance. The critical design area is usually the point attachments of the array mounting system to a structure.



PV Module Attachments

Manufacturer specifications give the maximum module loads and required supports and attachments points.

SolarWorld

PresenterPresentation NotesPV module specifications give the maximum mechanical loads that the module can support using specified supports and attachments.



PV Module Attachments

PV modules are commonly

attached to underlying rails or beams using bolted attachments or clamps to the module frame.

Bolted Attachments

Bottom Clamps Top Clamps

PresenterPresentation NotesMost standard flat-plate PV modules are glass laminates enclosed in an aluminum frame. The frame provides mechanical support for the laminate, and a means to structurally attach the module to a mounting system and for electrical grounding. PV modules are either bolted with fasteners or clamped to supporting rails or beams. Follow the PV module manufacturer’s installation instructions for the allowable mounting points to meet the maximum loads.

Suggested Exercise: Review PV module installation instructions for approved mounting and attachment methods.



PV Module Support

PV modules are usually

installed with the long module dimension perpendicular to the rails, to minimize the length of rail required.

Gary Lee

PresenterPresentation NotesCommonly, PV modules are installed across two aluminum rails that act as structural beams. The rails are then bolted to the underlying point attachments at specific locations along the rail that define the rail spans. PV arrays installed in higher wind regions require stronger rails, or smaller spacing between rail attachments to avoid excessive rail and module deflections. In common sloped rooftop applications, the rails are usually laid out with the length in an east-west direction across the roof, which permits variable width attachments to the underlying roof structural members, such as rafters or trusses. As the spacing between rafters or trusses is usually fixed, this may constrain the installation of rails up and down the roof slope (in a north-south direction). This is because PV modules require the support rails to be located at certain points on the module frame to support the specified mechanical loads. Refer to mounting hardware manufacturer’s data on maximum allowable loads and deflection on module support beams.

Suggested Exercise: Review some mounting system manufacturer’s websites and tools to determine the appropriate mounting hardware and support structures using any PV module for any size PV array for various applications.

References: Photovoltaic Systems, p. 268-279


Point Attachments

Point attachments connect the array assembly to a building or structure at distributed locations, and are usually the critical design point of the entire mounting system.

PresenterPresentation NotesPoint attachments connect the array assembly to the underlying structure (building or ground) at specified intervals. Point attachments produce concentrated loads on a structure or foundation.



Point Attachments

Increasing the number of point attachments decreases concentrated point loads, beam loading and deflections, while requiring more labor to install and weatherseal.

Gary Lee



Point Attachments

Additional blocking may be required for some installations to

adequately secure point attachments to the structure.

PresenterPresentation NotesPoint attachments are usually made directly to structural members like trusses, not to the roof decking. Additional blocking between trusses may be required for some installations.



Metal and Tile Roof Attachments

PV arrays mounted to metal and tile roofs use specially designed attachments.

Unirac

Gary Lee

Metal Roof Attachments

Tile Roof Attachments

PresenterPresentation NotesArray installations on metal roofs use special clamps that crimp to the metal roof seams, and rely on adequate attachment of the metal roofing to the underlying structure. These attachments do not penetrate the roof surface, and therefore avoid problems with leakage. Installations on slate or tile roofs use special attachments that direct the mechanical loads on the array directly to the roof structure, not on the tiles.



Lag Screw Attachments

Allowable withdrawal loads for lag screws in lumber depend on

the density and species of the wood, the diameter of the screw, and the thread penetration depth.

Lumber Type >

Douglas Fir

Southern Yellow

Pine

White Spruce

Screw Nominal Shank Diameter (in)

Specific Gravity

0.51 0.58 0.45

1/4 232 281 192

5/16 274 332 227

3/8 314 381 260

Allowable Withdrawal Loads for Lag Screws (lb/in)

PresenterPresentation NotesLag screws are commonly used to secure point attachments directly to the tops of trusses or other structural members. The screw diameter, thread embedment and species of lumber determine the allowable withdrawal loads. A proper size pilot hole should be drilled 60-70% of the screw shank diameter, unless self-drilling SPAX lag screws are used.

The allowable withdrawal loads in the table are calculated for lag screws in the side grain of seasoned wood, using the empirical formula: P = 1800G3/2D3/4, where G is the specific gravity, D is the screw diameter in inches, and P is the allowable withdrawal load in pounds per inches of thread penetration depth, and includes a factor of safety of 4.

References: Photovoltaic Systems, p. 274-275Marks’ Standard Handbook for Mechanical Engineers, 8th Ed.


Lag Screw Load Calculation: Example

Consider a PV array that is mounted above and parallel to a

sloped roof with a surface area of 200 square feet. Assume the maximum uplift design load is 40 psf, and there are a total of 20 point attachments using 5/16” diameter lag screws connecting the array structure to yellow pine trusses. What is the minimum length lag screw required?



Lag Screw Load Calculation: Example (cont.)

The maximum load on each point attachment is:

(200 sf x 40 psf) ÷ 20 attachments = 400 lbs per attachment.

The allowable withdrawal load for 5/16” lag screws in yellow pine is 332 lbs/in, therefore the screw embedment depth must be at least: 400 lbs ÷ 332 lbs/in = 1.2 inches

Since typical roofing composition and decking materials are

about 1-inch thick, the overall length of the screw must be at least 2-1/4” and must have at least 1-1/4” threaded portion. A nominal 3-in lag screw has 1-inch unthreaded portion, and would provide at least 2-inch embedment into the trusses.

PresenterPresentation NotesLag screw load calculations are based on point loads, diameter of the screw, thread embedment depth and type of lumber. Note that lag screws longer than 1 inch are not threaded the full length of the shank.



Typical Installation Detail

UniRac

PresenterPresentation NotesMany manufacturers offer universal hardware for installing all types and sizes of flat-plate PV modules on racks, poles, trackers, and on many types of roofs and other building surfaces. This greatly improves standardization, and reduces design, materials, and labor costs associated with installing PV arrays.

Reference: www.unirac.com


Typical Installation Detail

Professional Solar Products

Professional Solar Products

PresenterPresentation NotesReference: www.prosolar.com


Summary

Mechanical design considerations include module characteristics, locations for installing arrays, maximizing array performance, structural attachments, weathersealing and appearance.

PV arrays may be mounted to fixed or movable structures and installed on the ground, attached to rooftops or otherwise integrated in the building design.

Standardized mounting hardware facilitates the design and installation of PV arrays, and results in lower costs.

Use materials compatible with the environment and system life expectancy.

Wind loads are the principal design loads acting on PV arrays, and mounting structures must be designed accordingly.



Questions and Discussion


Chapter 10 ��Mechanical IntegrationOverviewMechanical ConsiderationsOptimizing Array PerformanceArray TemperaturesElectrical ConfigurationsMaterial SelectionWeathersealing MaterialsAppearanceMinimizing CostsTypes of Mounting SystemsFixed-Tilt ArraysAdjustable-Tilt ArraysSun-Tracking ArraysSun-Tracking ArraysSun-Tracking ArraysSun-Tracking ArraysGround-Mounted ArraysGround-Mounted ArraysRack-Mounted ArraysRack-Mounted ArraysRack-Mounted ArraysRack/Pole Mounted ArraysPole-Mounted ArraysPole-Mounted ArraysPole-Mounted ArraysPole-Mounted ArraysRoof-Mounted PV ArraysStandoff-Mounted ArraysSelf-Ballasted ArraysBuilding-Integrated PV ArraysDirect-Mounted ArraysIntegral-Mounted ArraysMobile PV ArraysPoor Installation PracticeStructural StandardsTypes of Mechanical LoadsAllowable Stress DesignStructural EvaluationWind LoadsWind LoadsSimplified ProcedureDesign Wind PressuresBasic Wind Speed MapsStructure ClassificationImportance FactorExposure CategoryHeight and Exposure Adjustment CoefficientTopographic FactorPressure Coefficient ZonesNet Design Wind PressuresCalculating Design Loads: ExampleCalculating Design Loads: Example (cont.)PV Array Structural DesignPV Module AttachmentsPV Module AttachmentsPV Module SupportPoint AttachmentsPoint AttachmentsPoint AttachmentsMetal and Tile Roof AttachmentsLag Screw AttachmentsLag Screw Load Calculation:�ExampleLag Screw Load Calculation:�Example (cont.)Typical Installation DetailTypical Installation DetailSummaryQuestions and Discussion

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