C-B2B2 Loads and load combinations

The loads, load combinations, and nominal loads for use with this Specification are given in the applicable building code. In the absence of an applicable specific local, regional, or national building code, the loads (for example, $D, L, L_{r}, S, R, W$, and $E$ ), load factors, load combinations, and nominal loads (numeric values for $D$, $L$, and other loads) are as specified in ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE, 2022). The 2022 edition of ASCE/SEI 7 has adopted many of the seismic design provisions of the NEHRP Recommended Seismic Provisions for New Buildings and Other Structures (FEMA, 2020), as have the AISC Seismic Provisions for Structural Steel Buildings (AISC, 2022c). The reader is referred to the commentaries of these documents for an expanded discussion on loads, load factors, and seismic design.

This Specification is based on strength limit states that apply to structural steel design in general. The Specification permits design for strength using either load and resistance factor design (LRFD) or allowable strength design (ASD). It should be noted that the terms “strength” and “stress” reflect whether the appropriate section property has been applied in the calculation of the available strength. In most instances, the Specification uses “strength” rather than “stress.” In all cases, it is a simple matter to recast the provisions into a stress format. The terminology used to describe load combinations in ASCE/SEI 7 is somewhat different from that used by this Specification. ASCE/SEI 7, Section 2.3, defines Combining Factored Loads Using Strength Design; these combinations are applicable to design using the LRFD approach. ASCE/SEI 7, Section 2.4, defines Combining Nominal Loads Using Allowable Stress Design; these combinations are applicable to design using the ASD approach.

LRFD Load Combinations. If the LRFD approach is selected, the load combination requirements are defined in ASCE/SEI 7, Section 2.3.

The load combinations in ASCE/SEI 7, Section 2.3, are based on modern probabilistic load modeling and a comprehensive survey of reliabilities inherent in traditional design practice (Galambos et al., 1982; Ellingwood et al., 1980; Ellingwood et al., 1982). These load combinations utilize a “principal action-companion action format,” which is based on the notion that the maximum combined load effect occurs when one of the time-varying loads takes on its maximum lifetime value (principal action) while the other variable loads are at “arbitrary point-in-time” values (companion actions), the latter being loads that would be measured in a load survey at any arbitrary time. The dead load, which is considered to be permanent, is the same for all combinations in which the load effects are additive. Research has shown that this approach to load combination analysis is consistent with the manner in which loads actually combine on structural elements and systems in situations in which strength limit states may be approached. The load factors reflect uncertainty in individual load magnitudes and in the analysis that transforms load to load effect. The nominal loads in ASCE/SEI 7 are substantially in excess of the arbitrary point-in-time values. The basis for the LRFD load combinations can be found in the Commentary to ASCE/ SEI 7, Section 2.3.

The return period associated with earthquake loads was revised in both the 2003 and 2009 editions of the NEHRP Recommended Seismic Provisions for New Buildings and Other Structures (FEMA, 2003, 2009). In the 2009 edition, adopted as the basis for ASCE/SEI 7-10 (ASCE, 2010), the earthquake loads at most locations were intended to produce a collapse probability of 1% in a 50 year period, a performance objective that is achieved by requiring that the probability of incipient collapse, given the occurrence of the maximum considered earthquake (MCE), is less than 10%. This continues to be the basis of the current version of ASCE/SEI 7 (ASCE, 2022). At some sites in regions of high seismic activity, where high intensity events occur frequently, deterministic limits on the ground motion result in somewhat higher collapse probabilities. The ASCE/SEI 7, Chapter 1 Commentary, provides information on the intended maximum probability of structural failure under earthquake and other loads.

Load combinations of ASCE/SEI 7, Section 2.3, which apply specifically to cases in which the structural actions due to lateral forces and gravity loads counteract one another and the dead load stabilizes the structure, incorporate a load factor on dead load of 0.9.

ASD Load Combinations. If the ASD approach is selected, the load combination requirements are defined in ASCE/SEI 7, Section 2.4.

The load combinations in ASCE/SEI 7, Section 2.4, are similar to those traditionally used in allowable stress design. In ASD, safety is provided by the safety factor, $\Omega$, and the nominal loads in the basic combinations involving gravity loads, earth pressure, or fluid pressure are not factored. The reduction in the combined time-varying load effect in combinations incorporating wind or earthquake load is achieved by the load combination factor 0.75 . This load combination factor dates back to the 1972 edition of ANSI Standard A58.1 (ANSI, 1972), the predecessor of ASCE/SEI 7. It should be noted that in ASCE/SEI 7, the 0.75 factor applies only to combinations of variable loads; it is irrational to reduce the dead load because it is always present and does not fluctuate with time. It should also be noted that certain ASD load combinations may actually result in a higher required strength than similar load combinations for LRFD. Load combinations that apply specifically to cases in which the structural actions due to lateral forces and gravity loads counteract one another, where the dead load stabilizes the structure, incorporate a load factor on dead load of 0.6 . This eliminates a deficiency in the traditional treatment of counteracting loads in ASD and emphasizes the importance of checking stability. The earthquake load effect is multiplied by 0.7 in applicable combinations involving that load to align ASD for earthquake effects with the definition of $E$ in the sections of ASCE/SEI 7 defining seismic load effects and combinations.

The load combinations in ASCE/SEI 7, Sections 2.3 and 2.4, apply to design for strength limit states. They do not account for gross error or negligence. Loads and load combinations for nonbuilding structures and other structures may be defined in ASCE/SEI 7 or other applicable industry standards and practices.