The scope of AISC Specification for Structural Steel Buildings (AISC, 2022d), hereafter referred to as the Specification, and the Provisions includes buildings and other structures designed, fabricated, and erected in a manner similar to buildings, with building-like vertical and lateral force-resisting elements. For simplicity, the Commentary refers to steel buildings and structures interchangeably.
It should be noted that these provisions were developed specifically for buildings. The Provisions, therefore, may not be applicable, in whole or in part, to some non-building structures that do not have the building-like characteristics described in the preceding paragraph. Extrapolation of their use to such nonbuilding structures should be done with due consideration of the inherent differences between the response characteristics of buildings and these nonbuilding structures.
Structural steel systems in seismic regions are generally expected to dissipate seismic input energy through controlled inelastic deformations of the structure. The Provisions supplement the Specification for such applications. The seismic design loads specified in the building codes have been developed considering the energy dissipation generated during inelastic response.
The Provisions are intended to be mandatory for structures that have been specifically referenced when defining a seismic response modification coefficient, , in ASCE/ SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE, 2022), hereafter referred to as ASCE/SEI 7. For steel structures, typically this occurs in seismic design categories D, E, and F, where is greater than 3 .
However, there are instances where of less than 3 is assigned to a system and the Provisions are still required. These limited cases occur in ASCE/SEI 7, Table 12.2-1, for cantilevered column systems and Table 15.4-1 for nonbuilding structures similar to buildings. For these systems with less than 3, the use of the Provisions is required. In general, for structures in seismic design categories B and C, the designer is given a choice to either solely use the Specification and the given for structural steel buildings not specifically detailed for seismic resistance (typically, a value of 3) or the designer may choose to assign a higher to a system detailed for seismic resistance and follow the requirements of the Provisions. Additionally, for composite steel-concrete structures, there are cases where the Provisions are required in seismic design categories B and C, as specified in ASCE/SEI 7, Table 12.2-1. This typically occurs for composite systems designated as "ordinary" where the counterpart reinforced concrete systems have a designated and specific design requirements for seismic design categories B and C.
The Provisions include requirements for splices and bases of columns not part of the seismic force-resisting system (SFRS) in Sections D2.5a, D2.5c, and D2.6.
The provisions for the seismic design of composite structural steel and reinforced concrete buildings were originally based upon the 1994 NEHRP Provisions (FEMA, 1994) and subsequent modifications were made in later editions of those provisions and in ASCE/SEI 7. Because composite systems are assemblies of steel and concrete components, the portions of these Provisions pertaining to steel, the Specification, and ACI Building Code Requirements for Structural Concrete (ACI 318 and ACI 318M) (ACI, 2019), hereafter referred to as ACI 318, form an important basis for provisions related to composite construction.
There is limited experience in the United States with composite building systems subjected to extreme seismic loads, and many of the recommendations herein are necessarily of a conservative and/or qualitative nature. Extensive design and performance experience with this type of building in Japan clearly indicates that composite systems, due to their inherent rigidity and toughness, can equal or exceed the performance of reinforced concrete only or structural steel only buildings (Deierlein and Noguchi, 2004; Yamanouchi et al., 1998). Composite systems have been extensively used in tall buildings throughout the world.
Careful attention to all aspects of the design is necessary in the design of composite systems, particularly with respect to the general building layout and detailing of members and connections. Composite connection details are illustrated in Commentary Chapters G and H to convey the basic character of the force transfer in composite systems. However, these details should not necessarily be treated as design standards. The cited references provide more specific information on the design of composite connections. For a general discussion of these issues and some specific design examples, refer to Viest et al. (1997).
The design and construction of composite elements and systems continues to evolve in practice. Except where explicitly stated, these Provisions are not intended to limit the application of new systems for which testing and analysis demonstrates that the structure has adequate strength, ductility, and toughness. It is generally anticipated that the overall behavior of the composite systems herein will be similar to that for counterpart structural steel systems or reinforced concrete systems and that inelastic deformations will occur in conventional ways, such as flexural yielding of beams in fully restrained moment frames or axial yielding and/or buckling of braces in braced frames. However, differential stiffness between steel and concrete elements is more significant in the calculation of internal forces and deformations of composite systems than for structural steel-only or reinforced concrete-only systems. For example, deformations in composite elements can vary considerably due to the effects of cracking.
When systems have both ductile and nonductile elements, the relative stiffness of each should be properly modeled; the ductile elements can deform inelastically while the nonductile elements remain nominally elastic. When using elastic analysis, member stiffness should be reduced to account for the degree of cracking at the onset of significant yielding in the structure. Additionally, it is necessary to account for material overstrength that may alter relative strength and stiffness.