The Provisions give member and element load requirements that supplement those in the applicable building code. In order to accommodate both load and resistance factor design (LRFD) and allowable strength design (ASD), the 2005 edition of the Provisions (AISC, 2005) was the first to provide two “available strengths,” one for LRFD and one for ASD. “Available strength” is the term used in the Specification to cover both design strength (LRFD) and allowable strength (ASD).
In some instances, the load effect defined in the Provisions must be combined with other loads. In such cases, the Provisions simply define the seismic load effect, which is combined with other loads using the appropriate load factor from the seismic load combinations in the applicable building code, and thus both LRFD and ASD are supported.
The Provisions are intended for use with load combinations given in the applicable building code. However, because they are written for consistency with the load combinations given in ASCE/SEI 7 and the International Building Code (ICC, 2021), consistency with the applicable building code should be confirmed if another building code is applicable.
The engineer is expected to use these Provisions in conjunction with the Specification. Typically, the Provisions do not define available strengths as these are given in the Specification. Additionally, the designer is directed to specific limit states or provisions in the Specification in certain cases.
An overstrength factor, , applied to the horizontal portion of the earthquake load, , is prescribed in ASCE/SEI 7, the International Building Code, the NEHRP Provisions (FEMA, 2015), and the NFPA Building Construction and Safety Code (NFPA 5000) (NFPA, 2021). However, these codes do not all express the load combinations that incorporate this factor in exactly the same format. In the future, if all codes adopt ASCE/SEI 7 by reference, it will be possible to directly reference the appropriate combinations within these Provisions.
These Provisions require the consideration of system overstrength for many elements. System overstrength effects on the required strength of such elements are addressed in two ways. For some elements, it is sufficient to approximate the effect using the overstrength factor for the system given in ASCE/SEI 7, Table 12.2-1. For other elements, this approximate method is not sufficient, and a more explicit calculation of required strength based on the expected or probable strength of adjoining elements is required. This latter approach had been used in past editions of these Provisions and is now addressed by ASCE/SEI 7, Section 12.4.3.2, and termed the “capacity-limited horizontal seismic load effect.” Per ASCE/SEI 7, Section 12.4.3.1, where consideration of overstrength is required but the capacity-limited seismic load is not, the approximate method based on the system’s overstrength factor is permitted. Loads determined using this approximate method need never be taken as larger than those calculated using the capacity-limited seismic load. In either method of addressing system overstrength, the horizontal seismic load effects are combined with vertical seismic and gravity load effects using the load combinations in ASCE/SEI 7 to obtain the required strength. The capacity-limited horizontal seismic load effect, , is intended to have a load factor of 1.0 for LRFD and 0.7 for ASD applied in the applicable ASCE/SEI 7 load combinations given in ASCE/SEI 7, Sections 2.3.6 and 2.4.5, for LRFD and ASD, respectively.
In some cases, the total load on an element (typically a connection) is limited by the yielding of an adjacent member. In such cases, these provisions directly specify the required strength of the element (both for ASD and for LRFD terms) and no combination is made with gravity loads.
Specific mention of designing composite systems incorporating reinforced concrete components by LRFD was removed from this section because the requirement was already addressed in Section A1.
The calculation of seismic loads for composite systems per the ASCE/SEI 7 provisions is the same as is described previously for steel structures. The seismic response modification coefficient, , and the deflection amplification factor, , for some structural systems have been changed in ASCE/SEI 7 to make them more consistent with similar systems in structural steel-only and reinforced concrete-only systems. This is based on the fact that, when carefully designed and detailed according to these Provisions, the overall inelastic response for composite systems should be similar to comparable steel and reinforced concrete systems. Therefore, where specific loading requirements are not specified in the applicable building code for composite systems, appropriate values for the seismic response modification coefficient can be inferred from specified values for steel and/or reinforced concrete systems. These are predicated upon meeting the design and detailing requirements for the composite systems specified in these Provisions. Unlike the requirements for steel systems, for composite systems that include reinforced concrete members, the design loads and the corresponding design strengths are limited to those defined based on LRFD. This is done to ensure consistency between provisions for steel, composite, and reinforced concrete members that are designed in accordance with the ACI Specification and the Building Code Requirements for Structural Concrete (ACI 318) (ACI, 2019).