Canadian Seismic Detailing

CANADIAN REQUIREMENTS FOR SEISMIC DESIGNOF DUCTILE STEEL STRUCTURES

M. I. Gilmor

M. I. Gilmor graduated fromthe University of Toronto in1966 and was subsequentlyemployed by the Toronto con-sulting structural engineeringfirm of Robert Halsall andAssociates. He began hiscareer designing institutional,commercial and governmentbuildings. In 1969 he obtaineda masters degree in structuralengineering from theUniversity of Toronto andlater was a lecturer in theDepartment of CivilEngineering.

In 1970, he joined the Canadian Institute of SteelConstruction (CISC) and held various positions within theInstitute. He is currently the Vice President of Operations.He is the editor of the Canadian Institute of SteelConstruction Handbook of Steel Construction and co-authorof the Canadian textbook, Limit States Design in StructuralSteel.

Mr. Gilmor is secretary of the Canadian StandardsAssociation Technical Committees S16, Steel Structures forBuildings, and S473, Steel Offshore Structures. He is thepast chairman of the Research Council on StructuralConnection and has represented Canada on the ISO TC 167SCI and SC2 committees for steel structures, materials,design and fabrication, and fabrication during the past tenyears.

Mr. Gilmor is a registered professional engineer in theProvince of Ontario and, in 1992, was elected a Fellow ofthe Canadian Society for Civil Engineers for his excellencein engineering and for services rendered to his professionand to Canada. For his work on standards, in 2001 hereceived the Canadian Standard Association's Award ofMerit.

ABSTRACT

The Canadian Standards Association formally approvedCSA Standard S16-01 in October of 2001. This newestCanadian standard for the limit states design of steel struc-tures continues to provide seismic requirements in itsClause 27. However, due to the significant amount of newinformation from research, such as the SAC project, thework in Canada on steel plate shear walls, bracing systemsand connections, and that from other sources, Clause 27underwent a complete overhaul. The basis of the new seis-mic requirements is capacity design principles. Clause 27provides design and detailing requirements for eight lateral-load-resisting systems over a broad range of ductile per-formance expectations. This paper provides an overview ofthese new seismic design requirements.

2002 NASCC Proceedings Session 30 • Page 1

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CANADIAN REQUIREMENTS FOR SEISMIC DESIGNOF DUCTILE STEEL STRUCTURESM. I. GILMOR

INTRODUCTION

On February 26, 1998 the Canadian StandardsAssociation's Technical Committee for Structural SteelBuildings, S16, meet in Toronto to begin what was tobecome the biggest overhaul of the Canadian Standard forthe design of steel structures since the introduction of thefirst limit states edition, CSA-S16.1-1974.

Formal approval of the revised Standard, CSA-S 16-01,was obtained in October, 2001, after 30 days of fullTechnical Committee meetings during the intervening timeand many more days of Task Group meetings of over adozen Task Groups charged with drafting the proposedtechnical changes to be discussed by the main TechnicalCommittee.

Seismic provisions for design of steel structures havebeen an integral part of the Canadian Standard for morethan a decade as Clause 27. However, since the draftingand adoption of the 1994 edition, two significant seismicevents, Northridge and Kobe, have had a lasting effect onhow the design of all structures is to be approached.

In Canada, about one half of the research grants from theSteel Structures Education Foundation have been directedat issues of the design and performance of steel structuresdue to earthquakes. These research projects have concen-trated their focus on lateral load resisting systems such asductile brace frames and steel plate shear walls as the SACventure had ductile moment frames well covered.

While most people generally do not associate Canadawith earthquakes, perhaps a greater percentage of Canada'spopulation lives in a seismic zone 1 or greater, with theareas of highest risk being Canada's west coast, than that ofthe population of the USA. Seismic zones in Canada'sNBCC range from 0 to 6 for both velocity and for acceler-ation.

Seismic load requirements are given in section 4.1.9 ofPart 4 of the National Building Code of Canada 1995. Part4 also provides the requirements for loads and load combi-nations, load factors, importance factors, and live loads dueto snow, wind, rain and ice. In short, Part 4 of the NBCC

M. I. Gilmor is a registered professional engineer in theProvince of Ontario, Canada.

1995 provides Canadian engineers what ASCE 7 providesfor American engineers. It is to the requirements of theearthquake requirements of the 1995 NBCC that Clause 27of CSA- SI6-01 responds.

Load combinations follow a companion action approachas earthquakes are rare events but of high intensity. Thus theload combinations are given as:(a) and(b) either

(i) for storage and assemblyoccupancies; or

(ii) for all other occupancies,where D is the Dead load; E is the Earthquake load; andis the Importance factor. Note that the load factors are 1.0or less when combined with the earthquake loads.

TYPES OF LATERAL-LOAD-RESISTING SYSTEMS

The 1995 NBCC assigns force modification factors, R, tovarious lateral-load resisting systems in relation to theircapacity to dissipate energy. This energy dissipation isassumed to occur as the structural elements undergo inelas-tic deformations. The greater the ability of the structure todissipate energy, the higher is the assigned value of R

The force modification factor, R, is then used to reducethe magnitude of the seismic base shear and to increasedeflections under seismic loading.

While the requirements of section 4.1.9 of the 1995NBCC are normally required, the designer may, alternative-ly, determine the maximum anticipated seismic loads fromnon-linear time-history analyses using appropriate structur-al models and ground motions.

Clause 27 provides requirements for members, elementsand connection details of the lateral-load resisting systemthat will exhibit ductility consistent with the R valuesassumed in the analysis and applies to all steel structures inCanada for which energy dissipation capability is required.In previous editions of S16.1, this has included all struc-tures for which In this newest edition, structures forwhich R = 1.5 some minimum requirements are introducedin Clause 27.10 to achieve the traditionally assumed energydissipation properties of these framing systems to requirethat brittle failure is avoided in higher seismic zones.


Eight classes of lateral load resisting systems, all withR > 1.5, are described in Clause 27:

• ductile moment resisting frames (Type D, with R =5.0)

• moderately ductile moment resisting frames (TypeMD, with R = 3.5)

• moment resisting frames with limited ductility (TypeLD with R = 2.0)

• moderately ductile concentrically braced frames(Type MD, with R - 3.0)

• limited ductility concentrically braced frames (TypeLD, with R = 2.0)

• eccentrically braced frames (R = 4.0)• ductile plate walls (Type D, with R = 5.0)• limited ductility plate walls (Type LD, with R = 2.0).In addition, other special framing systems, such as

frames that incorporate special bracing, ductile truss seg-ments, seismic isolation, or other energy-dissipatingdevices are permitted under Clause 27.11.

The lateral-load-resisting systems are thus designedaccording to capacity design principles to resist the maxi-mum anticipated seismic loads.

In capacity design(a) specific elements or mechanisms are designed to dis-

sipate energy;(b) all other elements are sufficiently strong for this

energy dissipation to be achieved;(c) structural integrity is maintained;(d) elements and connections in the horizontal and ver-

tical load paths are designed to resist these seismicloads;

(e) diaphragms and collector elements are capable oftransmitting the loads developed at each level to thevertical lateral-load-resisting system; and

(f) these loads are transmitted to the foundation.Structural members and their connections that are not

part of the lateral-load-resisting system must also be capa-ble of supporting gravity loads when subjected to seismi-cally induced deformations.

STRUCTURAL STEEL AND WELDREQUIREMENTS

Structural steels used must limit the ratio of yield to tensilestrengths to less than or equal to 0.85 and the limit the spec-ified minimum yield to 350 MPa. Under specific condi-tions, however, the yield strength of columns can be up to480 MPa. Thus, steels anticipated for use are CSA G40.21350W and ASTM A992. To determine the probable yieldstress for determining the developable capacity of a mem-ber, the factor R has been introduced in this edition. As theapplicable steels are limited, a value for R of 1.1 has beenselected. However, a minimum value of R F of 385 MPa is

prescribed in recognition that steel making practices haveachieved much higher than specified yields for some steelssuch as A36.

Toughness is a requirement for velocity- or acceleration-related seismic zones 4 or higher, for rolled shapes withflanges 40 mm or thicker, or plates and built-up shapes over51 mm in thickness, used in energy-dissipating elements orwelded parts. These shall have minimum average CharpyV-Notch impact test values of 27 J at 20°C.

Welds of primary members and connections in velocity-or acceleration-related seismic zones 2 or higher are to bemade using filler metals that have minimum averageCharpy V-Notch impact test values of 27 J at -30°C.However, this requirement may be waived in velocity- andacceleration-related seismic zones 3 or lower when thewelds are loaded primarily in shear.

BOLTED CONNECTIONS

Except where needed to conform to the details of a testedassembly, in order for bolted connections to ensure that fric-tion plays a role in load transfer, and that too rapid slip intobearing is avoided, they are:

(a) to use pretensioned high-strength bolts;(b) to have slip coefficient, ks of not less trfen 0.33,

when designed as bearing-type connections;(c) not to be considered to share load with welds;(d) not to have long slotted holes;(e) not to have short slotted holes unless the load is nor-

mal to the slot; and(f) to have end distances in the line of seismic force not

less than two bolt diameters when the bearing forcedue to seismic load exceeds 75% of the bearingresistance.

Second-order effects have been required in performinganalyses for moments and forces since the introduction oflimit states design. However, for seismic forces andmoments, the calculation involves using the deflectionsunder factor loads amplified by the R factor. Thus the U2

factor is taken as:

provided that the value of U2 does not exceed 1.4. This pro-vision ensures that the prescribed lateral resistance can bedeveloped from the anticipated inelastic seismic deforma-tions.

Page 4 • Session 30 2002 NASCC Proceedings

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MOMENT-RESISTING FRAMES

1. Type D (Ductile) Moment-Resisting Frames, R = 5.0

Type D, ductile, moment-resisting frames are those that aredesigned and detailed to develop significant inelastic defor-mation through plastic hinging in beams a short distancefrom the face of columns.

This is possible either by locally strengthening of thebeams near the columns (by haunches, cover plates or othermethods), by locally weakening of the beams at selectedplastic hinge locations some distance from the columns or,by using special detailing that ensures ductile response.

In order that beams develop plastic moments, beams sec-tions must be Class 1 and laterally supported such that:

Columns may be either Class 1 or 2 and also bracedwhen plastic hinging is expected at the column base. Inzones 4 or greater, the factored axial load must not exceed0.30AFy for all seismic load combinations.

Column panel zones are restricted as to the amount ofplasticity that can be permitted due to the detrimental effecton welds of beam flanges to column flanges when there istoo much deformation. For joints in which the beams arewelded to the columns, these curvatures may precipitatecracking of the beam weld at that location. Panel zoneyielding without considerable concurrent beam yielding isgenerally not desirable for these connections, and the cur-rent provisions limit this behavior except when using a con-nection detail for which panel zone yielding has been foundappropriate by testing.

Yielding in the panel zone is perceived by some as ben-eficial and a consensus opinion has not yet been reached.Thus, an upper limit of 0.2 is placed on the term

to ensure that the panel zone strength is notreached prior to development of the plastic momentenhanced by strain hardening in the adjacentbeams.

Extensive research (SAC) was initiated following theNorthridge earthquake to identify the reasons that led to thenumerous observed beam-to-column connection fractures,and to formulate new connection design requirements. Theresult of this large research endeavor is a database of con-nection types that have been experimentally proven able toprovide satisfactory seismic performance, with specificinformation regarding configurations, details, quality con-trol, and other requirements (FEMA 350).

The S16-01 requirement is that of demonstrated per-formance.

The beam-to-column joint shall maintain a

strength at the column face of at least the nominalplastic moment resistance of the beam, Mpb, througha minimum interstory drift angle of 0.04 radiansunder cyclic loading. Satisfaction of these criteriashall be demonstrated by physical testing.

When reduced beam sections are used, or whenlocal buckling limits the flexural strength of the beam,the beam need only achieve 0.8 Mpb at the columnface when an interstory drift angle of 0.04 radians isdeveloped under cyclic loading.Thus the designer must either:(a) use connections having identical configurations,

details, materials, procedures, and quality controls,and conforming to size and other limitations of thosealready proven satisfactory by tests, or

(b) conduct tests to demonstrate that under a number ofcycles of loading the required total drift specified inthis clause can be reached.

A new Appendix, J, Ductile Moment-ResistingConnections, was added to provide reference to a protocolfor such testing and to reference other material developedby CISC, Moment Connections for Seismic Applications,on design procedures and size and other limitations for anumber of prequalified moment-resisting connections tosatisfy (a).

2. Type MD (Moderately Ductile) Moment-ResistingFrames, R = 3.5

Moderately ductile moment-resisting frames develop amoderate amount of inelastic deformation through plastichinging in the beams at a short distance from the face ofcolumns and thus have similar but more relaxed require-ments compared to ductile moment-resisting frames.Specifically, beams can be Class 1 or 2 and bracing is thatassociated with less plasticity; for columns, the factoredaxial load limit is raised to 0.50AFy ; and for the joint, theminimum interstory drift angle is 0.03 radians.

3. Type LD (Limited-Ductility) Moment-ResistingFrames, R = 2.0

Limited-ductility moment-resisting frames can develop alimited amount of inelastic deformation through plastichinging in the beams, columns, or joints. This system maybe used in buildings not exceeding 12 stories in height invelocity- and acceleration-related seismic related zones 3 orlower.

To develop a limited amount of inelastic deformation,sections must be Class 2 or better. It is anticipated that inmany cases joint details will conform to traditional forms ofconstruction used for moment resisting frames. If weldedjoints are used, the tensile resistance to a normal load onthe column flange is reduced to 60% of the value given in



Clause 21.3 to account for the highly non-uniform stressesin a beam flange when welded to an unstiffened columnflange. Also for welded joints, Clause 27 requires detailedremoval of weld backing and run-off plates, and the repairof potential notches. These procedures, together with weld-ed web connections and use of continuity plates wereshown in tests following the Northridge earthquake to sig-nificantly increase the inelastic rotation capacity. It isexpected that an interstory drift angle of 0.02 radians maybe anticipated as being acceptable.

CONCENTRICALLY BRACED FRAMES

1. Type MD (Moderately Ductile) ConcentricallyBraced Frames, R = 3.0

Moderately ductile concentrically braced frames areexpected to dissipate moderate amounts of energy throughyielding of bracing members. Energy dissipation is expect-ed to occurs under brace elongation, inelastic buckling ofthe braces and inelastic bending when the braces are subse-quently straightened. In low-rise V- or chevron braceframes, energy may also be dissipated through limitedbending of the beams at the brace intersection point.

These frames include(a) tension-compression bracing systems not exceeding

eight stories;(b) chevron braced systems not exceeding eight stories;(c) tension-only bracing systems not exceeding four sto-

ries; and(d) other bracing systems, provided that stable inelastic

response can be demonstrated.Multi-story concentrically braced frames have limited

capability of distributing vertically the inelastic demandafter buckling and yielding of the braces have developed ata given level. This phenomenon is more pronounced in tallframes in which the inelastic demand tends to concentratein the bottom floors, which are the first affected by theground motion, or in the upper levels due to higher modeeffects. Buildings up to the specified maximum buildingheight are expected to exhibit a stable inelastic responsewhen applying the provisions of Clause 27.

The stability of the inelastic response of these systems isachieved principally by means of the continuity of thecolumns which exhibit sufficient reserve strength and stiff-ness to prevent the formation of a soft-story response anddynamic instability under the design base earthquake.These results were obtained by means of inelastic time-his-tory analysis.

Most studies on concentric braced frames have beenconducted on regular multi-story buildings with uniformstory height varying between 3.5 to 4 m. Judgement must beexercised when the geometry of the frame deviates signifi-

cantly from such a uniform configuration (e.g. industrialbuildings or hangars in which the bracing system in any onelevel includes a stack of two or more bracing panels), asthese structures may also be prone to concentration of theinelastic demand in a few bracing members.

Knee bracing and K-bracing, including those systems inwhich parrs of braces meet a column on one side betweenfloors, are excluded from the Type MD braced frame cate-gory because plastic hinging that will develop within theclear length of the columns may lead to their instability.

In most cases, including tension-only systems, the post-buckling capacity of braces is necessary to contribute to sta-bility and therefore in all these systems, the slendernesslimits apply to braces in all Type MD concentrically bracedframes, including tension-only systems.

Compared with the tension-only system, the stockierbraces of the tension-compression system provide greaterpost-buckling capacity and stiffness. This, combined withthe stiffness provided by continuous columns has beenshown to provide stability in frames up to eight stories inheight.

Chevron bracing in which the braces (which may beeither both above the beam or both below it, meet within thecentral region of the beam) are now permitted in the TypeMD concentrically braced frame category provided that thebeams in the bracing bents remain essentially elastic afterbuckling of the bracing members has occurred. Braces inframes with such strong beams can develop their full yieldcapacity in tension and the structure exhibits a more stablehysteretic response than when weaker beams are used.

Several cycles of inelastic bending are anticipated athinge location in the bracing members and limits areimposed on the width-to thickness ratios of the braces toprevent premature fracture of these members. Physical test-ing has shown that HSS bracing members exhibit limitedfracture life and relatively more stringent limits are speci-fied for these sections. Relaxation of width-to-thicknesslimits is permitted when lower inelastic demand is expect-ed in the braces such as when slender bracing members areused (buckling becomes essentially elastic) or when thestructure is located in a region of low seismicity. The inelas-tic demand is also less critical in the vertical legs of doubleangle bracing members buckling about their plane of sym-metry and less stringent requirements are permitted by SI6-01 for this case.

Brace connections must be designed to resist brace axialloads that correspond to the probable buckling strength andtensile yielding strength of the braces. A realistic estimateof the actual compressive strength of a brace is obtained bymultiplying by 1.2 its nominal compressive resistance, thelatter being obtained with the probable yield stress of thesteel. Actual brace end restraint conditions and the presenceof intermediate supports must also be taken into account



when evaluating the buckling strength of the braces. In ten-sion, the maximum anticipated brace force corresponds tothe probable yield tensile strength.

In some cases, braces can be oversized to meet otherdesign criteria such as drift, width-to-thickness ratio, orslenderness limits. For such cases, the brace connectionloads need not exceed the forces induced by a story shearcalculated with R = 1.0.

The net section resistance of braces may be based on theprobable tensile strength of the material, since the load levelcorresponds to the probable yield stress. Recent research bySchmidt shows that a conservative factor equal to Ry can beapplied to Fu provided the value does not exceed 1.1. Also,since the principal geometrical parameter of the net sectionand gross section are identical, the resistance factor may betaken as 1.0.

Buckling of the braces will induce rotation demand atthe brace ends and the connections must be detailed toavoid any premature fracture at his location. Proper detail-ing must be provided to allow this rotation to developthrough controlled plastic hinging in the bracing members,away from the connections, or in the brace connections.

Columns, beams, and other connections in the lateral-load resisting system are designed to carry the gravity loadstogether with the brace forces that are expected to developunder the design earthquake. In a given story, it must beassumed that yielding in the tension braces develops simul-taneously with either the buckling or post-buckling strengthof the compression braces, depending upon which case pro-duces the most critical condition for the element beingdesigned. The value of the brace post-buckling compressivestrength corresponds to that observed in tests at a ductilityof 3.0.

Columns in multi-story structures are most often contin-uous over two or more stories and the flexural stiffness andstrength of these columns contributes in reducing the con-centration of inelastic demand in a given story along theheight of the building. This behavior is now explicitlyaccounted for in S16-01 and the columns must therefore bemade continuous to prevent soft-story formation.

2. Type LD (Limited-Ductility) Concentrically BracedFrames, R = 2.0

Braced frames of limited ductility are designed with an Rfactor of 2.0 as they are expected to undergo lower inelas-tic response than Type MD braced frames. Inelasticresponse is however still restricted to the bracing membersand to the beams of low-rise chevron braced frames. TheLD frames are designed to the requirements of the MDframes except that some relaxation is permitted in view ofthe lower anticipated ductility demand.

LD frames are

(a) tension-compression bracing systems not exceedingtwelve stories;

(b) chevron braced systems not exceeding twelve sto-ries;

(c) tension-only bracing systems not exceeding eightstories.

Single- and two-story braced frames with slender braces,including rods, bars, etc. having KL/r greater than 200 arenow permitted for tension-only systems. Other require-ments for ductile braced frames, including minimum braceconnection resistance, still apply, however. This representsa relaxation form the more severe requirements of the 1994edition of the Standard.

DUCTILE ECCENTRICALLYBRACED FRAMES, R = 4.0

As the requirements for this system are based on work ofPopov, et al, and parallel what is already well known andused elsewhere, no further discussion is given here.

[STEEL]PLATE [SHEAR] WALLS

In S16-01, steel plate shear walls have been incorporateddirectly into the Standard as Clause 20, Plate Walls. Clause20 provide the designer with the requirements necessary toperform a preliminary design using a simple truss analogyand a final design using more refined procedures. Clause 20ensures that the boundary elements of the plate are suffi-ciently stiff to allow for the formation of the appropriatetension field action under lateral loads. The requirementsfor plate walls under seismic forces are given in Clause 27.8as either ductile plate walls, in which moment connectionsare made between the beams and columns surrounding theweb panels, or limited ductility plate walls, in which thebeams are attached to columns using pin-ended connec-tions.

1. Type D (Ductile) Plate Walls, R = 5.

Ductile plate walls are vertical plate girders comprisingweb plates framed by rigidly connected columns andbeams. Ductile plate walls can develop significant inelasticdeformation by the yielding of the web plates and develop-ment of plastic hinges in the framing members. Much of theresearch on this lateral-load-resisting system type has beenconducted over the past 20 years and offers potential wherestiffness, redundancy and ductility are required.

For ductile plate walls, energy is dissipated by yieldingof the web plates and through the development of plastichinges in the members surrounding the web plates.

The basis of the capacity design is that energy is dissi-pated by the yielding of the web plates of the wall prior tocolumns attaining their factored resistances. For Type D



plate walls, it is anticipated that most of the inelastic ener-gy will be absorbed near the base of the wall.

The boundary members must be braced as for Type Dmoment resisting frames, and the design requirements forbeam-to-column joints and connections are similar to thoseused for proportioning Type LD moment resisting frames.Plastic deformations are permitted in the beams, and beam-to-column joints, and special detailing of the columns isrequired in the vicinity of the base plates.

The results of tests indicated that the columns of platewalls can carry axial forces in excess of the forces allowedfor Type D moment resisting frames. The tension fields thatdevelop in the web members of plate walls tend to brace thecolumns against buckling.

Tests also indicate that the relative story drifts for build-ings with plate walls will be less than the drifts for buildingswith moment frames. The amount of energy that is dissi-pated in the moment frame members of Type D plate wallsduring earthquakes is much less than the amount of energydissipated by moment frames designed in accordance withClause 27. Consequently, the design requirements for themoment frames in plate walls are less stringent than therequirements for ductile moment frames acting alone toresist earthquake loads.

The columns of plate walls must be detailed so that theflanges of the columns do not buckle prematurely in thevicinity of the base plates.

2. Type LD (Limited-Ductility) Plate Walls, R = 2.0

For ductile plate walls, energy is dissipated during earth-quakes primarily by yielding of the steel web plates. No

special attempt is made to develop moment connectionsbetween the beams and columns of walls to dissipate energy.

CONVENTIONAL CONSTRUCTION, R = 1.5

For the first time the Standard includes provisions for struc-tures with R = 1.5, a category that until now has had no spe-cial requirement to provide for ductility. The new provi-sions are considered necessary because it is recognized thatmany R = 1.5 structures will be built in regions with non-negligible seismic risk, and that most steel structure failuresin seismic events are associated with brittle connectiondetails. The provisions relate only to connections, and aimat preventing brittle failure either by providing ductile con-nection details, or increasing the connection design loads.The provisions apply in velocity- or acceleration-relatedseismic zones of 3 and above.

Some details that may be considered as achieving ductilefailure modes when appropriately proportioned includeextended-end-plate moment connections, flange platemoment connections, gusset plates proportioned for ductil-ity, welded connections comprising fillet welds loaded pri-marily in shear, and bolted connections in which the gov-erning failure mode corresponds to bolt bearing failure.

ACKNOWLEDGMENTS

The author would like to acknowledge that much of thispaper has been taken from the proposed CISC Commentaryto S16-01 and the material supplied by the members of theTask Group on Clause 27, headed by R. G Redwood withthe contributions of M. Bruneau and R. Tremblay.



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Canadian Seismic Detailing