P1: MRM/SPH PB262A-04
P2: MRM/UKS PB262/Seismic.cls
Comm. C4.]
QC: MRM/UKS January 3, 2003
T1: MRM 8:37
Char Count=
PART I – LOADS, LOAD COMBINATIONS, AND NOMINAL STRENGTH
77
provisions like those in these Provisions are not mandatory. However, special seismic provisions are mandatory in Seismic Design Categories D and E, which cover areas of high seismicity. In the 1997 Uniform Building Code (ICBO, 1997) and the 1999 SEAOC Seismic Provisions (SEAOC, 1999), the detailing required for buildings is based on the Seismic Zone in which the building is located. A new term “Applicable Building Code” has been introduced into this edition of the Seismic Provisions. While it is the intent that these Provisions be used in conjunction with the codes and standards previously listed, there can be no guarantee which building code edition a designer may use to design a steel building. To eliminate potential conflicts with the many building codes currently in use, these Provisions refer to the Applicable Building Code to establish loads and load combinations, system limitations and system factors (e.g. R, Cd and o ). However, where building codes differ from ASCE 7, it is the intent of these Provisions that the ASCE 7 criteria apply. C4.
LOADS, LOAD COMBINATIONS, AND NOMINAL STRENGTH These Provisions are intended for use with load combinations given in the Applicable Building Code. However, since they are written for consistency with the load combinations given in ASCE 7 (ASCE, 2002) and IBC 2000 (ICC, 2000), consistency with the Applicable Building Code should be confirmed. The earthquake load E is the combination of the horizontal seismic load effect and an approximation of the effect due to the vertical accelerations that accompany the horizontal earthquake effects. An amplification or overstrength factor o applied to the horizontal portion of the earthquake load E is prescribed in ASCE 7, the 2000 IBC, 2000 NEHRP Provisions and the 1997 Uniform Building Code; however, the relevant load combinations are not all expressed in exactly the same format, as shown in Table C-I-4.1. In prior editions of these Provisions it was felt that this difference could be clarified by including load combinations (Equations 4-1 and 4-2 in the 1997 Seismic Provisions (AISC, 1997b)), primarily to account for overstrength inherent in different systems or elements when determining the Required Strength of connections. Unfortunately, due to the difference in the various codes and source documents with which these Provisions are intended to be used, the specification of Load Combinations 4-1 and 4-2 proved confusing. It is not practical to specifically reference any load combinations from reference documents, and Load Combinations 4-1 and 4-2 were eliminated in favor of a new term “Amplified Seismic Load.” When used in these Provisions, this term is intended to refer to the appropriate load combinations in the Applicable Building Code that account for overstrength of members of the Seismic Load Resisting System. The load combinations containing the overstrength factor o should be used where these Provisions require use of the Amplified Seismic Load. The general relationship between the different structural steel systems is illustrated in Table C-I-4.2 based upon similar information in the ASCE 7 load standard. R is a seismic load reduction factor used to approximate the inherent ductility of the Seismic Load Resisting System. Cd is an amplification factor that is used with the Seismic Provisions for Structural Steel Buildings, May 21, 2002 American Institute of Steel Construction