Introduction to the AISC Seismic Provisions

8/18/2019 Introduction to the AISC Seismic Provisions

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APRIL 2006 MODERN STEEL CONSTRUCTION

A A MAJOR CHANGE TO AISC’S 2005 SEISMICPROVISIONS FOR STRUCTURAL STEEL BUILD-

INGS , INCLUDING SUPPLEMENT NUMBER 1 IS IN ITS FORMAT. The 2005 Seismic Provisions are written as a supplement to the 2005 AISCSpecification for Structural Steel Buildings , combin-ing allowable strength design (ASD) and load and

resistance factor design (LRFD) into a single, uni-fied approach. The section that accommodated ASD in previ-

ous editions was absorbed into Parts I and II ofthe new provisions: “Structural Steel Buildings”and “Composite Structural Steel and ReinforcedConcrete Systems.” Other revisions have been in-corporated throughout the 2005 provisions, in ad-dition to new quality criteria and two new seismicsystems: buckling-restrained braced frames andspecial plate shear walls. (Throughout the provi-sions, exceptions to the 2005 specification and ad-ditional criteria are specified.)

Part I: Structural Steel Buildings The first four sections of Part I establish the

provisions’ relationship with the 2005 specifica-tion, as well as with the applicable building codeand other applicable national standards such as

ASCE and ASTM, among others.

Section Overviews

Section 1—Scope: Section 1 defines thescope of the 2005 provisions: they apply to build-ings classified by the applicable building code asseismic design category D or more severe. In seis-mic design categories A, B, and C, which are less

severe, the system must satisfy one of two condi-tions: a seismic response modification coefficient,

R of 3, must be used and elements must be de-signed to satisfy the 2005 specification only; or ahigher R value must be used and the system mustbe designed to meet all of the requirements of the2005 provisions. The second requirement is toensure that large R-values are not being used fora structure without meeting the ductile detailingrequirements of the 2005 provisions.

Sections 2-4—Applicable Codes and Stan-

dards: Section 2 lists referenced standards beyondthose given in the 2005 specification. Sections

3 and 4 direct the user to the applicable build-

ing code for determining the required strength, where the applicable building code is defined asthe “building code under which the structure isdesigned.”

Section 5—Construction Documents: Thenewly developed Section 5 outlines informationrequired in the construction documents prepared

by the engineers, fabricators, and erectors. De-sign drawings and specifications must identifyall elements of the seismic load resisting system(SLRS), demand critical welds, protected zones,connection configuration, welding requirements,etc. Shop drawings and erection drawings mustinclude similar information to verify that the fab-ricator and erector understand the design intent.

Section 6—Material Properties and Char-

acteristics: Section 6 considers acceptable mate-rial properties and characteristics for structuralsteel systems in seismic regions. One of this sec-tion’s most important points is that the expected

yield strength and the expected tensile strengthmust be considered in determining the requiredstrength. For each structural material, F y R y resultsin the expected yield strength of the material. Asecond term, Rt , has been introduced. When Rt ismultiplied by the nominal tensile strength, F u, theresult is the expected tensile strength of the ma-terial. The remainder of the provisions identifies

when the R y and Rt terms are to be used in deter-mining the required strength of the members.

Section 7—Design of Connections, Joints,

and Fasteners: The design of connections, joints, and fasteners in the SLRS is addressed in

Section 7. All connections should be detailed sothat a ductile limit state controls the strength ofthe components. It also defines the “protectedzone,” or the critical regions of elements in theSLRS where discontinuities must be avoided tominimize the chance of premature brittle fractureof the members.

All bolted connections must use pretensionedhigh-strength bolts, with the faying surface pre-pared for Class A or better slip-critical joints.However, bolted connections may be designedfor the strength in bearing. This requirement

is meant to avoid joint slip during small earth-quakes, while recognizing that bolts will eventu-

BY JAMES O. MALLEY, S.E.

James O. Malley is a senior principal with Degenkolb En- gineers of San Francisco. Hehas more than 22 years’ ex- perience in the seismic design,evaluation, and rehabilitationof building structures. He is amember of the AISC Specifi-cations Committee and servesas Chairman of the AISCSeismic Subcommittee, which

is responsible for developing AISC’s Seismic Provisions.

Introduction to the AISC Seismic Provisions

steelwiseApril 2006

The 2005 Seismic Provisions coordinate with the 2005 Specification for Structural

Steel Buildings , the 13th Edition Steel Construction Manual, and the upcomingSeismic Manual .

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MODERN STEEL CONSTRUCTION APRIL 2006

ally develop bearing during a design-levelseismic event. Standard holes are requiredat bolted joints; short-slotted holes are ac-ceptable when the axis is perpendicular tothe direction of load. Oversized holes alsomay be used if they are in only one plyof the slip-critical joint. Bolts and weldsare not allowed to share load at the same

joint. Welded connections must be made with filler metals that have a minimumCVN toughness of 20 ft-lbs at 0 ºF. Thisis a relaxation from the previously adopt-ed temperature of -20 ºF. Demand critical

welds still require filler metal with a mini-mum CVN toughness of 20 ft-lbs at -20 ºF.

An additional requirement of 40 ft-lbs at70 ºF CVN toughness is placed on de-mand critical complete-joint-penetrationgroove welds in various systems (for ex-ample: welds of beam flanges to columns,

column splice joints, and welds of beam webs to column flanges). Specific detailingrequirements for continuity plates are alsoprovided in this section.

Section 8—Local and Global Insta-

bilities: Requirements for local and globalinstabilities, as well as other general mem-ber requirements, are considered in Sec-tion 8. The limiting width-to-thicknessratios of flanges and webs for members inthe SLRS are provided. These ratios aremore restrictive than the compact sec-tion ratios given in the 2005 specification

because of the expected inelastic demandduring seismic behavior. The remainingportion of this section emphasizes col-umn design. Splices for columns that arenot part of the SLRS now have specialdesign requirements since research indi-cates these columns may have significantflexure and shear demand during a severeseismic event.

Sections 9 through 17 provide designrequirements for each of the codifiedstructural steel building systems:

Section 9—Special Moment Frames

(SMFs): SMFs are considered highly duc-tile and therefore have the highest R fac-tor of the steel buildings systems discussedhere. The proposed use of a particularmoment-resisting joint must have a dem-onstrated capability of accommodating aninterstory drift of 0.04 radians. This is ac-complished by one of the following:➜ Using a connection prequalified for use

in a SMF in accordance with Prequali- fied Connections for Special and Inter-mediate Moment Frames for Seismic Ap-

plications , (ANSI/AISC 358-05), a new

standard developed by the AISC Con-nection Prequalification Review Panel(CPRP). In the first edition, approvedin 2005, ANSI/AISC 358 includedprequalification of the reduced beamsection and end plate connections. Ef-forts continue to prequalify all widelyused connections.

➜

Using a connection prequalified foruse in a SMF in accordance with Ap-pendix P criteria, where minimum re-quirements for any moment-resisting joint are established. A CPRP will beestablished to review all test resultsand other data to ensure that the con-nection satisfies all minimum require-ments.

➜ Providing qualifying test results inaccordance with Appendix S. This ap-pendix requires the test assembly tobe consistent with joints proposed in

the prototype building, defines essen-tial test parameters, and identifies thetest program implementation and theadequacy of the joint to sustain therequired seismic demand. Test resultscan be taken from tests reported in theliterature or from tests performed spe-cifically for the project under consider-ation.

The panel zone must be consistent withthe prequalified test configuration and theexpected strength must be approximately

“balanced” with the yield strength of the

beams. In addition, all column splicestrengths in bending and shear must bedesigned to develop the full flexural ca-pacity of the smaller column spliced.

Section 10—Intermediate Moment

Frames (IMFs): Like SMFs, IMFs musthave moment-resisting connections quali-fied in accordance with ANSI/AISC 358

Appendix P or Appendix S. The qualify-ing interstory drift limit is reduced to 0.02radians for these connections to reflectthe more limited ductility demand ex-pected from these systems. Current build-ing codes limit the use of IMFs in highseismic design categories. Other than theprequalified connection and the more re-strictive lateral bracing requirements, the2005 specification governs the design re-quirements of these frames.

Section 11—Ordinary Moment

Frames (OMFs): OMFs are accepted inlight metal buildings and small buildingapplications in the more severe seismicdesign categories. OMFs may be designed without the prequalified performance

testing requirement. In an effort to induce

inelastic behavior into the adjoining ele-ments, the connection strength must ex-ceed 1.1 times the expected strength ofthe connected members. Specific require-ments such as continuity plates, removing

weld backing and run-off tabs, and weldaccess holes help ensure minimum ductileperformance of OMF connections.

Section 12—Special Truss MomentFrames (STMFs): STMF provisions de-fine a special segment of the truss that isintended to be the primary location ofinelastic behavior in the system. All otherframe elements are designed with suffi-cient over-strength to develop yielding inthe special segment. Both vierendeel andcross-braced special segment panels areallowed. The requirements also providelateral bracing requirements similar tothose required for SMF systems to pro-hibit out-of-plane instability.

Section 13—Special ConcentricallyBraced Frames (SCBFs): The conceptfor SCBF systems is that diagonal bracesbuckle and dissipate energy resulting fromthe design earthquake. The 2005 provi-sions have been modified to improve theductility of the system. For example, braceorientation in each line of framing musthave approximately the same number ofbraces in compression and tension. Con-nections in SCBFs must develop the fulltensile capacity of the brace or the maxi-mum force that can be delivered to the

brace by the rest of the system. Full flexur-al strength must also be considered unlessthe connection includes a yield-line gus-set plate that allows ductile post-buckledbehavior of the brace. Special limitationsare provided for V and inverted-V bracingto reflect the potentially undesirable char-acteristics of these bracing configurations.Column splices in SCBFs are required todevelop a shear capacity of approximately50% of the member capacity to reflect thesubstantial demands on these elementsduring the earthquake.

Section 14—Ordinary Concentri-

cally Braced Frames (OCBF): LikeOMFs, OCBFs have highly restricted ap-plications in high seismic design catego-ries due to their limited expected ductility.Connections in OCBFs may be designedto consider the amplified seismic load.

The previous requirement of member de-sign in OCBFs for the amplified seismicload was removed to address the reduced

R factor given in ASCE 7-05, MinimumDesign Loads for Buildings and Other Struc-

tures . Like OMFs, OCBF applications are


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strength in the web of the steel plate shear wall.

Similar to plate girder behavior, tensionfield action develops as the relatively thin web buckles during lateral loading. Limi-tations on configuration, width-thicknessratios, and other design parameters areprovided to be consistent with the suc-

cessful test results.Section 18—Quality Assurance: Section 18, the final section of Part I, ad-dresses a comprehensive quality assuranceplan that is required to demonstrate thatthe structural design intent is accom-plished during construction. Newly devel-oped Appendix Q discusses requirementsrelated to quality assurances and qualitycontrol to be provided by the contractor.Inspection requirements, both visual andnon-destructive evaluation (NDE) inspec-tions, for welds are presented in tabular

form. A similar table for bolted connec-tions is also provided.

Part II: Composite Structural Steel

and Reinforced Concrete BuildingsPart II of the Seismic Provisions consid-

ers the design of composite systems ofstructural steel and reinforced concrete.

This part contains individual sections

governing design requirements for beamscomposite with concrete slabs, compositecolumns, and the design of connectionsbetween concrete and steel elements. (Across-reference with ACI 318 is an impor-tant new feature.)

Composite connections have been de-signed using the basic principles of me-

chanics, existing standards for steel andconcrete construction, test data, and engi-neering judgment. The connection sectionis intended to standardize and improvedesign practices by establishing basic be-havioral assumptions for developing designmodels that satisfy equilibrium of internalforces in the seismic design connection.

The remaining sections of Part II ad-dress the design of various compositestructural system types. These sectionsparallel those found in Part I and gener-ally have R factors and system applica-

tion limitations similar to the comparablestructural steel systems. In addition to thecomposite SMF, IMF, and OMF system re-quirements, there is a composite partiallyrestrained moment frame (C-PRMF) sys-tem having connection details similar tothose shown in Figure 2.

Similar to Part I, there are two concen-

trically braced composite systems and oneeccentrically braced composite system ad-dressed in Part II. Part II also identifiesthree composite systems that use wall el-ements as the primary component in theSLRS. Two types of composite walls, onespecial and one ordinary, parallel the rein-forced concrete wall specifications of ACI

318; however, structural steel elements areused in the boundary elements (as shownin Figure 3). Finally, a composite steelplate shear wall system is also codified.

More information may be found in thepaper “The 2005 AISC Seismic Provisions for Structural Steel Buildings,” published inthe 2005 North American Steel Construc-tion Conference Proceedings, availablefor AISC members to download free at www.aisc.org .

The 2005 Seismic Provisions for Struc-tural Steel Buildings, Including Supplement

Number 1 is available to download freefrom AISC’s web site at www.aisc.org .It is also available in print at www.aisc.org/bookstore . This document has beenadopted by reference in the 2006 Interna-tional Building Code, and, as a result, willsoon govern the seismic design of all steelbuildings in the United States.

MODERN STEEL CONSTRUCTION APRIL 2006