Canadian Seismic Detailing

CANADIAN REQUIREMENTS FOR SEISMIC DESIGN OF DUCTILE STEEL STRUCTURES M. I. Gilmor graduated from the University of Toronto in 1966 and was subsequently employed by the Toronto consulting structural engineering firm of Robert Halsall and Associates. He began his career designing institutional, commercial and government buildings. In 1969 he obtained a masters degree in structural engineering from the University of Toronto and M. I. Gilmor later was a lecturer in the Department of Civil Engineering. In 1970, he joined the Canadian Institute of Steel Construction (CISC) and held various positions within the Institute. He is currently the Vice President of Operations. He is the editor of the Canadian Institute of Steel Construction Handbook of Steel Construction and co-author of the Canadian textbook, Limit States Design in Structural Steel. Mr. Gilmor is secretary of the Canadian Standards Association Technical Committees S16, Steel Structures for Buildings, and S473, Steel Offshore Structures. He is the past chairman of the Research Council on Structural Connection and has represented Canada on the ISO TC 167 SCI and SC2 committees for steel structures, materials, design and fabrication, and fabrication during the past ten years. Mr. Gilmor is a registered professional engineer in the Province of Ontario and, in 1992, was elected a Fellow of the Canadian Society for Civil Engineers for his excellence in engineering and for services rendered to his profession and to Canada. For his work on standards, in 2001 he received the Canadian Standard Association's Award of Merit.

2002 NASCC Proceedings

ABSTRACT The Canadian Standards Association formally approved CSA Standard S16-01 in October of 2001. This newest Canadian standard for the limit states design of steel structures continues to provide seismic requirements in its Clause 27. However, due to the significant amount of new information from research, such as the SAC project, the work in Canada on steel plate shear walls, bracing systems and connections, and that from other sources, Clause 27 underwent a complete overhaul. The basis of the new seismic requirements is capacity design principles. Clause 27 provides design and detailing requirements for eight lateralload-resisting systems over a broad range of ductile performance expectations. This paper provides an overview of these new seismic design requirements.

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CANADIAN REQUIREMENTS FOR SEISMIC DESIGN OF DUCTILE STEEL STRUCTURES M. I. GILMOR

INTRODUCTION On February 26, 1998 the Canadian Standards Association's Technical Committee for Structural Steel Buildings, S16, meet in Toronto to begin what was to become the biggest overhaul of the Canadian Standard for the design of steel structures since the introduction of the first 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 full Technical Committee meetings during the intervening time and many more days of Task Group meetings of over a dozen Task Groups charged with drafting the proposed technical changes to be discussed by the main Technical Committee. Seismic provisions for design of steel structures have been an integral part of the Canadian Standard for more than a decade as Clause 27. However, since the drafting and adoption of the 1994 edition, two significant seismic events, Northridge and Kobe, have had a lasting effect on how the design of all structures is to be approached. In Canada, about one half of the research grants from the Steel Structures Education Foundation have been directed at issues of the design and performance of steel structures due to earthquakes. These research projects have concentrated their focus on lateral load resisting systems such as ductile brace frames and steel plate shear walls as the SAC venture had ductile moment frames well covered. While most people generally do not associate Canada with earthquakes, perhaps a greater percentage of Canada's population lives in a seismic zone 1 or greater, with the areas of highest risk being Canada's west coast, than that of the population of the USA. Seismic zones in Canada's NBCC range from 0 to 6 for both velocity and for acceleration. Seismic load requirements are given in section 4.1.9 of Part 4 of the National Building Code of Canada 1995. Part 4 also provides the requirements for loads and load combinations, load factors, importance factors, and live loads due to snow, wind, rain and ice. In short, Part 4 of the NBCC M. I. Gilmor is a registered professional engineer in the Province of Ontario, Canada.

2002 NASCC Proceedings

1995 provides Canadian engineers what ASCE 7 provides for American engineers. It is to the requirements of the earthquake requirements of the 1995 NBCC that Clause 27 of CSA- SI6-01 responds. Load combinations follow a companion action approach as earthquakes are rare events but of high intensity. Thus the load combinations are given as: (a) and (b) either (i) for storage and assembly occupancies; or (ii) for all other occupancies, where D is the Dead load; E is the Earthquake load; and is the Importance factor. Note that the load factors are 1.0 or less when combined with the earthquake loads. TYPES OF LATERAL-LOAD-RESISTING SYSTEMS The 1995 NBCC assigns force modification factors, R, to various lateral-load resisting systems in relation to their capacity to dissipate energy. This energy dissipation is assumed to occur as the structural elements undergo inelastic deformations. The greater the ability of the structure to dissipate energy, the higher is the assigned value of R The force modification factor, R, is then used to reduce the magnitude of the seismic base shear and to increase deflections under seismic loading. While the requirements of section 4.1.9 of the 1995 NBCC are normally required, the designer may, alternatively, determine the maximum anticipated seismic loads from non-linear time-history analyses using appropriate structural models and ground motions. Clause 27 provides requirements for members, elements and connection details of the lateral-load resisting system that will exhibit ductility consistent with the R values assumed in the analysis and applies to all steel structures in Canada for which energy dissipation capability is required. In previous editions of S16.1, this has included all structures for which In this newest edition, structures for which R = 1.5 some minimum requirements are introduced in Clause 27.10 to achieve the traditionally assumed energy dissipation properties of these framing systems to require that brittle failure is avoided in higher seismic zones.

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Eight classes of lateral load resisting systems, all with R > 1.5, are described in Clause 27: • ductile moment resisting frames (Type D, with R = 5.0) • moderately ductile moment resisting frames (Type MD, with R = 3.5) • moment resisting frames with limited ductility (Type LD with R = 2.0) • moderately ductile concentrically braced frames (Type MD, with R - 3.0) • limited ductility concentrically braced frames (Type LD, 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 segments, seismic isolation, or other energy-dissipating devices are permitted under Clause 27.11. The lateral-load-resisting systems are thus designed according to capacity design principles to resist the maximum anticipated seismic loads. In capacity design (a) specific elements or mechanisms are designed to dissipate 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 vertical load paths are designed to resist these seismic loads; (e) diaphragms and collector elements are capable of transmitting the loads developed at each level to the vertical 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 capable of supporting gravity loads when subjected to seismically induced deformations.

STRUCTURAL STEEL AND WELD REQUIREMENTS

Structural steels used must limit the ratio of yield to tensile strengths to less than or equal to 0.85 and the limit the specified minimum yield to 350 MPa. Under specific conditions, however, the yield strength of columns can be up to 480 MPa. Thus, steels anticipated for use are CSA G40.21 350W and ASTM A992. To determine the probable yield stress for determining the developable capacity of a member, the factor R y has been introduced in this edition. As the applicable steels are limited, a value for Ry of 1.1 has been selected. However, a minimum value of R y Fy of 385 MPa is

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prescribed in recognition that steel making practices have achieved much higher than specified yields for some steels

such as A36. Toughness is a requirement for velocity- or accelerationrelated seismic zones 4 or higher, for rolled shapes with flanges 40 mm or thicker, or plates and built-up shapes over 51 mm in thickness, used in energy-dissipating elements or welded parts. These shall have minimum average Charpy V-Notch impact test values of 27 J at 20°C. Welds of primary members and connections in velocityor acceleration-related seismic zones 2 or higher are to be made using filler metals that have minimum average Charpy V-Notch impact test values of 27 J at -30°C. However, this requirement may be waived in velocity- and acceleration-related seismic zones 3 or lower when the welds are loaded primarily in shear. BOLTED CONNECTIONS

Except where needed to conform to the details of a tested assembly, in order for bolted connections to ensure that friction plays a role in load transfer, and that too rapid slip into bearing 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 normal to the slot; and (f) to have end distances in the line of seismic force not less than two bolt diameters when the bearing force due to seismic load exceeds 75% of the bearing resistance.

Second-order effects have been required in performing analyses for moments and forces since the introduction of limit states design. However, for seismic forces and moments, the calculation involves using the deflections under 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 provision ensures that the prescribed lateral resistance can be developed from the anticipated inelastic seismic deformations.

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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 are designed and detailed to develop significant inelastic deformation through plastic hinging in beams a short distance from the face of columns. This is possible either by locally strengthening of the beams near the columns (by haunches, cover plates or other methods), by locally weakening of the beams at selected plastic hinge locations some distance from the columns or, by using special detailing that ensures ductile response. In order that beams develop plastic moments, beams sections must be Class 1 and laterally supported such that:

Columns may be either Class 1 or 2 and also braced when plastic hinging is expected at the column base. In zones 4 or greater, the factored axial load must not exceed 0.30AFy for all seismic load combinations. Column panel zones are restricted as to the amount of plasticity that can be permitted due to the detrimental effect on welds of beam flanges to column flanges when there is too much deformation. For joints in which the beams are welded to the columns, these curvatures may precipitate cracking of the beam weld at that location. Panel zone yielding without considerable concurrent beam yielding is generally not desirable for these connections, and the current provisions limit this behavior except when using a connection detail for which panel zone yielding has been found appropriate by testing. Yielding in the panel zone is perceived by some as beneficial 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 not reached prior to development of the plastic moment enhanced by strain hardening in the adjacent beams. Extensive research (SAC) was initiated following the Northridge earthquake to identify the reasons that led to the numerous observed beam-to-column connection fractures, and to formulate new connection design requirements. The result of this large research endeavor is a database of connection types that have been experimentally proven able to provide satisfactory seismic performance, with specific information regarding configurations, details, quality control, and other requirements (FEMA 350). The S16-01 requirement is that of demonstrated performance. The beam-to-column joint shall maintain a

2002 NASCC Proceedings

strength at the column face of at least the nominal plastic moment resistance of the beam, Mpb, through a minimum interstory drift angle of 0.04 radians under cyclic loading. Satisfaction of these criteria shall be demonstrated by physical testing. When reduced beam sections are used, or when local buckling limits the flexural strength of the beam, the beam need only achieve 0.8 Mpb at the column face when an interstory drift angle of 0.04 radians is developed 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 those already proven satisfactory by tests, or (b) conduct tests to demonstrate that under a number of cycles of loading the required total drift specified in this clause can be reached. A new Appendix, J, Ductile Moment-Resisting Connections, was added to provide reference to a protocol for such testing and to reference other material developed by CISC, Moment Connections for Seismic Applications, on design procedures and size and other limitations for a number of prequalified moment-resisting connections to satisfy (a).

2. Type MD (Moderately Ductile) Moment-Resisting Frames, R = 3.5 Moderately ductile moment-resisting frames develop a

moderate amount of inelastic deformation through plastic hinging in the beams at a short distance from the face of columns and thus have similar but more relaxed requirements compared to ductile moment-resisting frames. Specifically, beams can be Class 1 or 2 and bracing is that associated with less plasticity; for columns, the factored axial load limit is raised to 0.50AFy ; and for the joint, the minimum interstory drift angle is 0.03 radians. 3. Type LD (Limited-Ductility) Moment-Resisting Frames, R = 2.0

Limited-ductility moment-resisting frames can develop a limited amount of inelastic deformation through plastic hinging in the beams, columns, or joints. This system may be used in buildings not exceeding 12 stories in height in velocity- and acceleration-related seismic related zones 3 or lower. To develop a limited amount of inelastic deformation, sections must be Class 2 or better. It is anticipated that in many cases joint details will conform to traditional forms of construction used for moment resisting frames. If welded joints are used, the tensile resistance to a normal load on the column flange is reduced to 60% of the value given in

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Clause 21.3 to account for the highly non-uniform stresses in a beam flange when welded to an unstiffened column flange. Also for welded joints, Clause 27 requires detailed removal of weld backing and run-off plates, and the repair of potential notches. These procedures, together with weld-

ed web connections and use of continuity plates were shown in tests following the Northridge earthquake to significantly increase the inelastic rotation capacity. It is expected that an interstory drift angle of 0.02 radians may be anticipated as being acceptable. CONCENTRICALLY BRACED FRAMES

1. Type MD (Moderately Ductile) Concentrically Braced Frames, R = 3.0 Moderately ductile concentrically braced frames are expected to dissipate moderate amounts of energy through yielding of bracing members. Energy dissipation is expected to occurs under brace elongation, inelastic buckling of the braces and inelastic bending when the braces are subsequently straightened. In low-rise V- or chevron brace frames, energy may also be dissipated through limited bending 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 stories; 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 demand after buckling and yielding of the braces have developed at a given level. This phenomenon is more pronounced in tall frames in which the inelastic demand tends to concentrate in the bottom floors, which are the first affected by the ground motion, or in the upper levels due to higher mode effects. Buildings up to the specified maximum building height are expected to exhibit a stable inelastic response when applying the provisions of Clause 27. The stability of the inelastic response of these systems is achieved principally by means of the continuity of the columns which exhibit sufficient reserve strength and stiffness to prevent the formation of a soft-story response and dynamic instability under the design base earthquake. These results were obtained by means of inelastic time-history analysis. Most studies on concentric braced frames have been conducted on regular multi-story buildings with uniform story height varying between 3.5 to 4 m. Judgement must be exercised when the geometry of the frame deviates signifi-

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cantly from such a uniform configuration (e.g. industrial buildings or hangars in which the bracing system in any one level includes a stack of two or more bracing panels), as these structures may also be prone to concentration of the inelastic demand in a few bracing members.

Knee bracing and K-bracing, including those systems in which parrs of braces meet a column on one side between floors, are excluded from the Type MD braced frame category because plastic hinging that will develop within the clear length of the columns may lead to their instability. In most cases, including tension-only systems, the postbuckling capacity of braces is necessary to contribute to stability and therefore in all these systems, the slenderness limits apply to braces in all Type MD concentrically braced frames, including tension-only systems. Compared with the tension-only system, the stockier braces of the tension-compression system provide greater post-buckling capacity and stiffness. This, combined with the stiffness provided by continuous columns has been shown to provide stability in frames up to eight stories in height. Chevron bracing in which the braces (which may be either both above the beam or both below it, meet within the

central region of the beam) are now permitted in the Type MD concentrically braced frame category provided that the beams in the bracing bents remain essentially elastic after buckling of the bracing members has occurred. Braces in frames with such strong beams can develop their full yield capacity in tension and the structure exhibits a more stable hysteretic response than when weaker beams are used. Several cycles of inelastic bending are anticipated at hinge location in the bracing members and limits are imposed on the width-to thickness ratios of the braces to prevent premature fracture of these members. Physical testing has shown that HSS bracing members exhibit limited fracture life and relatively more stringent limits are specified for these sections. Relaxation of width-to-thickness limits is permitted when lower inelastic demand is expected in the braces such as when slender bracing members are used (buckling becomes essentially elastic) or when the structure is located in a region of low seismicity. The inelastic demand is also less critical in the vertical legs of double angle bracing members buckling about their plane of symmetry and less stringent requirements are permitted by SI601 for this case. Brace connections must be designed to resist brace axial loads that correspond to the probable buckling strength and tensile yielding strength of the braces. A realistic estimate of the actual compressive strength of a brace is obtained by multiplying by 1.2 its nominal compressive resistance, the latter being obtained with the probable yield stress of the steel. Actual brace end restraint conditions and the presence of intermediate supports must also be taken into account

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when evaluating the buckling strength of the braces. In tension, the maximum anticipated brace force corresponds to the probable yield tensile strength. In some cases, braces can be oversized to meet other design criteria such as drift, width-to-thickness ratio, or slenderness limits. For such cases, the brace connection loads need not exceed the forces induced by a story shear calculated with R = 1.0. The net section resistance of braces may be based on the probable tensile strength of the material, since the load level corresponds to the probable yield stress. Recent research by Schmidt shows that a conservative factor equal to Ry can be applied to Fu provided the value does not exceed 1.1. Also,

(a) tension-compression bracing systems not exceeding twelve stories; (b) chevron braced systems not exceeding twelve stories; (c) tension-only bracing systems not exceeding eight stories. Single- and two-story braced frames with slender braces, including rods, bars, etc. having KL/r greater than 200 are now permitted for tension-only systems. Other requirements for ductile braced frames, including minimum brace connection resistance, still apply, however. This represents a relaxation form the more severe requirements of the 1994 edition of the Standard.

since the principal geometrical parameter of the net section and gross section are identical, the resistance factor may be taken as 1.0. Buckling of the braces will induce rotation demand at the brace ends and the connections must be detailed to avoid any premature fracture at his location. Proper detailing must be provided to allow this rotation to develop through controlled plastic hinging in the bracing members, away from the connections, or in the brace connections. Columns, beams, and other connections in the lateralload resisting system are designed to carry the gravity loads together with the brace forces that are expected to develop under the design earthquake. In a given story, it must be assumed that yielding in the tension braces develops simultaneously with either the buckling or post-buckling strength of the compression braces, depending upon which case produces the most critical condition for the element being designed. The value of the brace post-buckling compressive strength corresponds to that observed in tests at a ductility of 3.0. Columns in multi-story structures are most often continuous over two or more stories and the flexural stiffness and strength of these columns contributes in reducing the concentration of inelastic demand in a given story along the height of the building. This behavior is now explicitly accounted for in S16-01 and the columns must therefore be made continuous to prevent soft-story formation.

2. Type LD (Limited-Ductility) Concentrically Braced Frames, R = 2.0 Braced frames of limited ductility are designed with an R factor of 2.0 as they are expected to undergo lower inelastic response than Type MD braced frames. Inelastic response is however still restricted to the bracing members and to the beams of low-rise chevron braced frames. The LD frames are designed to the requirements of the MD frames except that some relaxation is permitted in view of the lower anticipated ductility demand. LD frames are

2002 NASCC Proceedings

DUCTILE ECCENTRICALLY BRACED FRAMES, R = 4.0 As the requirements for this system are based on work of

Popov, et al, and parallel what is already well known and used elsewhere, no further discussion is given here.

[STEEL]PLATE [SHEAR] WALLS In S16-01, steel plate shear walls have been incorporated directly into the Standard as Clause 20, Plate Walls. Clause 20 provide the designer with the requirements necessary to perform a preliminary design using a simple truss analogy and a final design using more refined procedures. Clause 20 ensures that the boundary elements of the plate are sufficiently stiff to allow for the formation of the appropriate tension field action under lateral loads. The requirements for plate walls under seismic forces are given in Clause 27.8 as either ductile plate walls, in which moment connections are made between the beams and columns surrounding the web panels, or limited ductility plate walls, in which the beams are attached to columns using pin-ended connections.

1. Type D (Ductile) Plate Walls, R = 5. Ductile plate walls are vertical plate girders comprising web plates framed by rigidly connected columns and beams. Ductile plate walls can develop significant inelastic deformation by the yielding of the web plates and development of plastic hinges in the framing members. Much of the research on this lateral-load-resisting system type has been conducted over the past 20 years and offers potential where stiffness, redundancy and ductility are required. For ductile plate walls, energy is dissipated by yielding of the web plates and through the development of plastic hinges in the members surrounding the web plates. The basis of the capacity design is that energy is dissipated by the yielding of the web plates of the wall prior to columns attaining their factored resistances. For Type D

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plate walls, it is anticipated that most of the inelastic energy will be absorbed near the base of the wall. The boundary members must be braced as for Type D moment resisting frames, and the design requirements for beam-to-column joints and connections are similar to those used for proportioning Type LD moment resisting frames. Plastic deformations are permitted in the beams, and beamto-column joints, and special detailing of the columns is

required in the vicinity of the base plates. The results of tests indicated that the columns of plate walls can carry axial forces in excess of the forces allowed for Type D moment resisting frames. The tension fields that develop in the web members of plate walls tend to brace the columns against buckling. Tests also indicate that the relative story drifts for buildings with plate walls will be less than the drifts for buildings with moment frames. The amount of energy that is dissipated in the moment frame members of Type D plate walls during earthquakes is much less than the amount of energy dissipated by moment frames designed in accordance with Clause 27. Consequently, the design requirements for the moment frames in plate walls are less stringent than the requirements for ductile moment frames acting alone to resist earthquake loads. The columns of plate walls must be detailed so that the flanges of the columns do not buckle prematurely in the vicinity of the base plates. 2. Type LD (Limited-Ductility) Plate Walls, R = 2.0 For ductile plate walls, energy is dissipated during earthquakes primarily by yielding of the steel web plates. No

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

CONVENTIONAL CONSTRUCTION, R = 1.5 For the first time the Standard includes provisions for structures with R = 1.5, a category that until now has had no special requirement to provide for ductility. The new provisions are considered necessary because it is recognized that many R = 1.5 structures will be built in regions with nonnegligible seismic risk, and that most steel structure failures in seismic events are associated with brittle connection details. The provisions relate only to connections, and aim at preventing brittle failure either by providing ductile connection details, or increasing the connection design loads. The provisions apply in velocity- or acceleration-related seismic zones of 3 and above. Some details that may be considered as achieving ductile failure modes when appropriately proportioned include extended-end-plate moment connections, flange plate moment connections, gusset plates proportioned for ductil-

ity, welded connections comprising fillet welds loaded primarily in shear, and bolted connections in which the governing failure mode corresponds to bolt bearing failure. ACKNOWLEDGMENTS The author would like to acknowledge that much of this paper has been taken from the proposed CISC Commentary to S16-01 and the material supplied by the members of the Task Group on Clause 27, headed by R. G Redwood with

the contributions of M. Bruneau and R. Tremblay.

2002 NASCC Proceedings

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.