When designing structures to resist earthquake motions, each structure is categorized based upon its occupancy and use to establish the potential earthquake hazard that it represents. Determining the available strength differs significantly in each specification or building code. The primary purpose of these Provisions is to provide information necessary to determine the required and available strengths of steel structures. The following discussion provides a basic overview of how several seismic codes or specifications categorize structures and how they determine the required strength and stiffness. For the variables required to assign seismic design categories, limitations of height, vertical and horizontal irregularities, site characteristics, etc., the applicable building code should be consulted. In ASCE/SEI 7 (ASCE, 2022), structures are assigned to one of four risk categories; risk category IV, for example, includes essential facilities. Structures are then assigned to a seismic design category based upon the risk categories and the seismicity of the site adjusted by soil type. Seismic design categories B and C are generally applicable to structures with moderate seismic risk, and special seismic provisions like those in these Provisions are optional. However, special seismic provisions are mandatory in seismic design categories D, E, and F, which cover areas of high seismic risk, unless stated otherwise in ASCE/SEI 7.
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