COMMENTARY Seismic Provisions AISC PART. 3

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PB262/Seismic.cls

December 17, 2002

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PART III. ALLOWABLE STRESS DESIGN (ASD) ALTERNATIVE

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SCOPE Part III has been included in these Seismic Provisions for designers that choose to use ASD in the seismic design of steel structures. A limited Supplement to the ASD Specification (AISC, 1989) has been issued (AISC, 2001). As noted in Part I, the seismic requirements are collateral provisions related to the LRFD Specification. Part I is based upon the limit-state seismic load model used in the NEHRP Provisions (FEMA, 2000g). Since the seismic requirements in Part I are based upon the expected nonlinear performance of a structure, the use of ASD in its traditional form is somewhat complicated because a knowledge of Design Strengths, not allowable stresses, is required to assure that connectors have sufficient strength to allow nonlinear behavior of the connected member(s). The provisions in Part III allow for the selection of members in an ASD format to provide the performance intended in Part I. Part III is intended as an overlay to Part I and, when using ASD, the designer will use Part I for the seismic design of a structure except where a section is replaced by or modified by a section shown in Part III. Provisions have not been included for the use of ASD with the composite structural steel and reinforced concrete systems, members, and connections in Part II because ACI 318 is in limit-states format.

C4.2. Nominal Strength The procedures in this section provide a methodology for the conversion of allowable stresses into Nominal Strengths, in most cases by removing the factor of safety from the ASD equations. These Nominal Strengths are converted to Design Strengths when multiplied by the Resistance Factors given in Part III, Section 4.3. In general, the Resistance Factors given are consistent with those in the LRFD Specification. The remainder of the provisions in Part III translate the provisions of Part I into ASD terminology to correlate with the appropriate sections of ASD.

Seismic Provisions for Structural Steel Buildings, May 21, 2002 American Institute of Steel Construction