Seismic Design Steel Structures 2005

7/28/2019 Seismic Design Steel Structures 2005

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19/04/2013 Canadian Seismic Design of SteelStructures - by Alfredo Bohl 1

Canadian Seismic Design of Steel Structures

An Organized Overview

By: Alfredo Bohl

University of British Columbia

Department of Civil Engineering

March, 2005


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Parameters:

V: Design shear force.

S(Ta): Design spectral response

acceleration.

Mv: Factor for the higher mode

effects on the shear base.

IE: Earthquake importance factor of

the structure.

W: Expected weight of the structure.

Rd: Ductility-related force

modification factor.

Ro: Overstrength-related forcemodification factor.

od

Eva

RR

WIMTSV

Lateral seismic force at the baseaccording to the 2005 NBCC

Introduction

The overview given in this report is

based on the provisions contained

in:

The upcoming 2005 Edition of the

National Building Code of Canada.

Clause 27 of the 8th. Edition of theHandbook of Steel Construction.


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Overstrength-related force

modification factor (Ro):

This factor is related to the

calibration factor U used in the

previous code.

It takes into account in a more

explicit way the overstrength in

structures, by identifying the sources

of it and assigning factors that

consider each of these sources, like

the actual strength of the material,

rounding up of dimensions of the

elements, and redistribution ofinternal forces.

Ductility-related force modification

factor (Rd):

This factor corresponds to the R

factor used in the previous 1995

edition.

For steel structures, these have

been increased for ductile and

moderately ductile systems to 3.5

and 5.0, compared to 3.0 and 4.0 in

the previous code.

The design forces for these systems

are now lower; however, the details

requirements to ensure adequateductility according to these factors

are more demanding.

New force reduction factors in the 2005 NBCC


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Steel seismic force resisting systems (SFRS)

Classification of SFRS according tothere ductile behavior:

Ductile or Type D: They can sustainsevere inelastic deformations. Theyhave a force reduction factorbetween 4.0 and 5.0.

Moderately ductile or Type MD:Inelastic deformations are morelimited, members are designed toresist greater loads. They have aforce reduction factor between 3.0and 3.5.

Limited ductile or Type LD: These

are newly introduced types offrames. Inelastic deformations areeven more limited and design loadsare greater than in type MDelements. They have a forcereduction factor of 2.0.

Characteristics of SFRS:

The 2005 NBCC recognizes different

types of SFRS, their corresponding

Rd and Ro factors, and the design

and detail requirements for each of

them according to the CSA standard

CSA-S16-01.

In each of these SFRS, there are

certain structural elements which are

designed to dissipate energy by

inelastic deformation; these must be

able to sustain various cycles of

inelastic loading with a minimumreduction of strength and stiffness.

The other elements and connections

must respond elastically to loads

induced by yielding elements.


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SFRS with Rd = 5.0

Ductile moment-resisting frames:

The energy dissipating elements are

the beams.

Beams must be capable of plastic

hinging without connection failures.

Plastic hinges in columns is only

permitted at their base, except for

single-storey buildings.

Maximum axial load in columns

limited to 0.3AFy for all load

combinations, since their flexural

resistance deteriorates fast when

high axial loads are applied.

Ductile members must be class 1

and capable to undergo inelastic

response without stability failures.


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SFRS with Rd = 5.0

Ductile moment-resisting frames:

Limited inelastic deformations are

permitted in column joint panel

zones if they are properly detailed.

The beam-to-column connections

must be capable to develop an inter-

storey drift angle of 0.04 rad undercyclic loading.

Advantages: They absorb less shear

forces due to their flexibility and

have high energy dissipation

capacity.

Disadvantages: Large inter-storeydrifts may cause severe P-delta

effects and non-structural damage.


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SFRS with Rd = 5.0

Ductile plate walls:

Newly introduced system in the

CAN/CSA S16-01.

The main energy dissipating element

is the web plate, framing elements

also dissipate energy once the plate

has yielded.

Same requirements for beams,

columns, panel zones and

connections; except that columns

must always be class 1.

Elements are proportioned so that

yielding occurs first in the web plate(principle of capacity design).

The top and bottom web plates must

also be anchored to stiff elements.


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SFRS with Rd = 5.0

Ductile plate walls:

Columns must be stiffened at the

base, so that the plastic hinges form

at a certain distance above the base

plate.

Advantages: They have very large

stiffness, reducing the amount ofnon-structural damage during an

earthquake.

Disadvantages: They may be more

expensive; and calculating the

tension fields in the plate web and

determining the yielding sequence ofthe plate and the framing system is

still a problem, due to the limitations

of the strip model.


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SFRS with Rd = 4.0

Ductile eccentrically braced frames:


the links, which are the beam

segments between the brace

connections and the beam.

Links must be class 1.

Link rotation limits depend on if it

yields in shear or flexure.

Full-depth stiffeners at both ends of

the link and intermediate stiffeners

are required to make sure that it will

have a ductile behavior.

Beams outside the link, braces andcolumns must be stronger than the

link.


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SFRS with Rd = 4.0

Ductile eccentrically braced frames:

If the link is directly connected to the

column, the link beam-to-column

connection must be able to develop

anticipated plastic deformation.

The columns must be designed for

secondary moment effects due tothe frame drift.

Advantages: Combines the ductile

behavior of the moment-resisting

frame and the stiffness of the

concentrically braced frame.

Disadvantages: Since all the energydissipation is restricted to the link,

the collapse mechanism forms once

this element has yielded; while other

SFRS are more redundant.


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SFRS with Rd = 3.5

Moderately ductile moment-resisting

frames:

Requirements are the same as for

ductile moment-resisting frames,

except for the following:

Beams must be class 1 or 2.

Maximum axial load in columns

limited to 0.5AFy for all load

combinations, since their flexural

resistance deteriorates fast when

high axial loads are applied.


must be capable to develop an inter-storey drift angle of 0.03 rad under

cyclic loading.


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SFRS with Rd = 3.0

Moderately ductile concentrically

braced frames:


the diagonal braces.

Only configurations that allow

inelastic response without losing

stability are permitted, like tension-

compression, chevron or tension-

only bracing systems.

Because ground motions may occur

in any direction, the dimensions of

the diagonal braces must be such

that the shear resistance in eachstorey provided by the tension forces

developed in these elements is

similar for storey shears acting in

opposite directions.


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SFRS with Rd = 3.0

Moderately ductile concentrically

braced frames:

In tension-compression systems,

braces must be class 2 to delay local

buckling, but they must yield before

the other elements.

In chevron systems, the beams must

be strong enough to resist yielding

and buckling forces from the braces

together with gravity loads, without

considering the support from the

braces.

In tension-only systems, the energydissipation capacity is very limited,

the braces must be able to carry all

the seismic loads.


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SFRS with Rd = 3.0

Moderately ductile concentricallybraced frames:

Brace connections must have aductile rotational performance if highinelastic response is expected.

Beams, columns and connections

must resist forces induced byyielding of the braces.

The columns must be designed forsecondary moment effects due tothe frame drift.

Advantages: They have very largestiffness.

Disadvantages: They tend to have asoft-storey response, so heightrestrictions apply depending on thebracing system used.


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19/04/2013Canadian Seismic Design of Steel

Structures - by Alfredo Bohl 15

SFRS with Rd = 2.0

Moment-resisting frames with limited

ductility:


moderately ductile moment-resisting

frames, except for the following:

Beams must be class 1 or 2, while

columns must be class 1 and I-

shaped.

In high seismic areas, buildings with

this system cannot exceed 12

storeys.


must be capable to develop an inter-storey drift angle of 0.02 rad under

cyclic loading.


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SFRS with Rd = 2.0

Limited ductility plate walls:


ductile plate walls, except for the

following:

The energy dissipating element is

the web plate, not the framing

elements.

Beams, columns and their

connections do not have any special

requirements, since they are not

expected to yield.

Buildings with this type of system

cannot exceed 12 storeys.

Limited ductility concentrically

braced frames:


moderately ductile concentrically

braced frames, except for the

following:

Height restrictions are relaxed, since

elements are designed for higher

forces.

Diagonal braces can be class 2 or

lower in low seismic areas.

Brace connections do not need to

have a ductile rotationalperformance in low seismic areas if

braces are slender.


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Characteristics:

Connections in type D and MD

frames must be tested to ensure that

they satisfy certain deformation

criteria under cyclic loading.

Testing procedures are described in

the FEMA 350 document.

The test assemblies must represent

the prototype characteristics, and the

test loading the deformation

magnitude and cyclic nature.

For each given combination of beam

and column size, tests of at least twospecimens must be performed. The

results obtained must be able to

predict the mean value of the drift

angle capacity of the connection.

Physical tests for connections in moment-resisting frames


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Characteristics:

The size of the beam used in the

specimen must be at least the

largest depth and heaviest weight

used in the structure.

The column must provide a flexural

strength consistent with therequirements of strong-column-

weak-beam connections, and must

have a height similar to the real

column, so that the drift angles

obtained are representative of the

real structure. The mean drift angle capacity must

not be less than a certain limit.

Tests results must be supported with

analytical design procedures.

Physical tests for connections in moment-resisting frames


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Bolted unstiffened end plateconnection:

The beam is welded to an end plate,

extended above and below the

flanges. The beam flange-to-plate

joints have complete-penetration-

groove welds, and the beam web isconnected to the plate with fillet or

complete-joint-penetration-groove

welds. Then, the end plate is bolted

to the column using eight bolts.

Design principle: Member sizes are

selected to preclude brittle failuremodes, and for yielding to occur as a

combination of beam flexure and

panel zone yielding. This applies

also for the other connections.

Moment-resisting connections for seismic applications


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Bolted stiffened end plateconnection:

The beam is welded to an end plate.

The beam flange-to-plate joints have

complete-penetration-groove welds,

and the beam web is connected to

the plate with fillet or complete-joint-penetration-groove welds. The end

plate extensions at the top and

bottom of the beam are stiffened

with vertical stiffeners that extend

outward from beam flanges. Then,

the end plate is bolted to the column

using 16 bolts.

Elements must have similar sizes

and details to those that were tested,

to predict their behavior. This applies

also for the other connections.



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Reduced beam section connection:

The flexural resistance of the beam

is reduced at a certain distance from

the connection, so that yielding and

plastic hinging occurs in the beam.

The top and bottom beam flanges

have circular radius cuts for thispurpose. The flanges of the beam

are connected to the columns only

with complete joint penetration

groove welds. A shear tab, that can

be bolted or welded, is used for the

web connection.

This type of connection cannot be

used for type LD frames, but the

previous two connections may be

used for these cases.



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Special truss moment frames:

This system is designed in such a

way that the inelastic deformation is

moved to some segments of the

truss that are specially designed.

This truss has several diagonal

members in a segment at themidspan designed for this purpose,

they absorb most of the energy and

dissipate it by yielding. After the

earthquake, the diagonal members

that were damaged can easily be

repaired or replaced.

This system has the advantage that

it weighs less than common framing

systems. Also, it provides substantial

cost and time savings compared to

these systems.

Special seismic steel framing systems


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Friction-damped steel frames:

Friction dampers are designed in

such a way they have moving parts

that will slide over each other during

a strong earthquake. Friction is

created between these sliding

elements, which dissipates energybuilt up in the structure. Examples of

these are the basic sliding joint, the

rotation sliding joint, the dual level

joint, the Pall friction device and the

Sumitomo friction device.

They have the advantage that theirbehavior is not seriously affected by

repeated cycles of displacement, the

friction force between surfaces can

be controlled, and they are not

affected by fatigue.

Special seismic steel framing systems

Basic sliding joint

Pall friction device

Rotating sliding joint


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Conclusions

An overall overview of the seismicdesign of steel structures in Canada

has been carried out. The design

procedures for SFRS, moment-

resisting connections, and some

special framing systems, which are

spread in various documents andpublications, have all been organized

in this report.

Each of the SFRS presented have

their own advantages and

disadvantages.

There may be cases in which itmight not be possible to use the

prequalified connections, and

physical tests are usually very

expensive and cannot be afforded by

small engineering companies. More

research is needed to developdesign procedures for various types

of connections with different element

sections.

Systems like the special truss

moment frame and the friction-

damped steel frame provide safe

and economic solutions compared toconventional framing systems, and

may be implemented in future codes.


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