Structural Stability Provisions in IBC and ASCE 7

To view the figures and tables associated with this article, please refer to the flipbook above.

The methodology for analyzing and designing building structures using specified loads and load combinations in the International Building Code (IBC) and ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures has undergone significant evolution over the years. Structural engineering practice has progressively transitioned from traditional allowable stress design methods to reliability-based strength design methods. The updates in IBC and ASCE 7 aim to allow a consistent set of load combinations throughout the entire design engineering lifecycle. For assessments related to overturning and sliding stability, irrespective of the building risk and seismic design categories, seeing a factor of safety of 1.5 has been a norm for structural engineers throughout the U.S., particularly for those accustomed to traditional approaches. Design coordination with geotechnical engineers in the building industry who typically follow allowable or working stress design practices and require a minimum factor of safety of 1.5 for soil slope stability failures under sustained loads may also reinforce this opinion. Despite the well-intentioned nature of the IBC and ASCE 7 code revisions, they are often misinterpreted/misunderstood when evaluating the stability of structures against overturning and sliding.

Sections 7.2.9 and 7.2.10 of ASCE 41-23 Seismic Evaluation and Retrofit of Existing Buildings require that the minimum factor of safety of 1.0 shall be used for the overturning and sliding checks for seismic evaluation of existing buildings; ASCE 41-23 commentary clarifies that these requirements are “consistent with prevailing practice specified in current codes for new buildings”. Furthermore, Section 5.3.5 of ASCE Report Seismic Evaluation and Design of Petrochemical and Other Industrial Facilities mentions that the minimum factor of safety for overturning and sliding stability checks for seismic loads shall be 1.0 for petrochemical and other industrial facilities and recommends using the alternative allowable stress design (ASD) load combinations in IBC. However, quantitative structural stability requirements in IBC (2024) and ASCE 7-22—the codes of record for analysis and design of new building structures—are only provided in IBC Section 1807.2, which stipulates that the retaining walls shall be designed to produce a stability factor of safety (called safety factor in IBC) of 1.5 for nominal [or service] loads.

Does this mean that new building structures are exempted from the global structural overturning and sliding stability checks, and have IBC and ASCE 7 always been silent on the required factor of safety for stability calculations for new building structures? The answer is obviously no for both questions. IBC (2024) Section 1605.1.1 states that “where overall structural stability … is being verified, use of load combinations specified in Section 2.3 or 2.4 of ASCE 7, and in Section 1605.2 shall be permitted”. ASCE 7-22 Section 1.3 requires that the building structural systems “… shall be designed and constructed with adequate strength and stiffness to provide structural stability …” and “… designed to resist forces caused by earthquakes, wind, and tornadoes, with consideration of overturning, sliding, and uplift …” However, these standards do not provide any explicit requirement for the required factor of safety for these checks for new building structures. A quick review of the evolution of building codes shows that ASCE 7 used to contain explicit stability check requirements just a few decades ago, but those requirements were removed with the migration from the allowable stress design approach towards the reliability-based strength design methods.

ASCE 7-95 Section 2.4.4, applicable to load combinations for allowable (or working) stress design for service level loads, required that “Buildings and other structures shall be designed so that the overturning moment due to lateral forces…does not exceed two-thirds of the dead load stabilizing moment… The base shear due to lateral forces (wind or flood) shall not exceed two-thirds of the total resisting force due to friction and adhesion…” This ASCE 7-95 provision meant that a minimum factor of safety of 1.5 was required for service level loads. Similar requirements were not explicitly included or deemed necessary either in ASCE 7-95 Section 2.3 pertaining to load combinations for strength design or in the UBC-1997.

The code updates in ASCE 7-98 revised Section 2.4.4 to remove the above requirement but added two new allowable stress design load combinations involving 0.6D+W and 0.6D+0.7E, where D, W, and E represent dead loads, wind loads, and seismic loads, respectively, to supplement the existing D+W and D+0.7E load combinations; it is noted that basic wind loads were at service level and basic earthquake loads were at strength level in ASCE 7-98. The intent of these changes was that the use of 0.6D implicitly provides a factor of safety exceeding 1.5 for service level lateral loads when using the allowable stress design method. The commentary to ASCE 7-98 noted that these new load combinations are to “eliminate inconsistency in the treatment of counteracting loads in allowable stress design and strength design and emphasize the importance of checking stability.” In principle, these provisions have remained the same until now, with the only change occurring in ASCE 7-10 when the wind loads were defined to be at strength level, thus, necessitating the redefinition of both strength design and allowable stress design load combinations involving wind loads. Additionally, ASCE 7-22 now includes tornado loads (WT) in addition to wind loads. IBC standards since their inception have also followed the framework of UBC standards with no specific quantitative requirements for stability checks of building structures except for the retaining walls, as noted earlier.

IBC (2024) and ASCE 7-22 prescribe two sets of load combinations, one for the strength design and the other for allowable stress design. Different material design standards, such as American Institute of Steel Construction (AISC), American Concrete Institute (ACI), National Design Specification (NDS), and The Masonry Society (TMS), permit the use of different design approaches (Fig. 1). Specifically, AISC, NDS, and TMS for structural steel, wood, and masonry design, respectively, allow design engineers to use either the strength design (SD) method (called load and resistance factor design [LRFD] in AISC and NDS) or the allowable stress design (ASD) method, noting that AISC uses the “allowable strength” design method that is not formulated on traditional allowable stress based design practice though uses the allowable stress design load combinations in IBC and ASCE 7. However, ACI 318 now exclusively adopts the strength design method for concrete design and includes the strength design load combinations consistent with those in IBC and ASCE 7. In this article, the term “strength design” encompasses LRFD, and the terms “allowable stress design” and “allowable strength design” are collectively referred to as ASD. The current building codes aim to provide a comprehensive framework using either design approach, allowing structural engineers to conduct complete design evaluations, including structural stability, using either the SD or ASD method. The load combinations in IBC (2024) and ASCE 7-22 reflect this framework to lead to similar though not identical design outcomes using the two design approaches.

Based on the author’s opinion, typical design engineering firms for building structures have generally adopted the use of SD methods though some designers still prefer using the ASD methods. However, the soil checks to verify the adequacy of the subgrade supporting the buildings are typically performed for service conditions using demands based on allowable stress design load combinations irrespective of the method used to perform the design of the structural system. ASCE 7 Section 12.13.5 now includes the necessary design requirements that allows using the strength design method for soil checks and is permitted by IBC Section 1605.1.1, but this approach is still catching on in the building industry. Therefore, for analysis of the building structures, both strength design and allowable stress design load combinations are generally used and become necessary for concrete structural systems if the geotechnical capacity is based on allowable stress design.

The designers typically perform the analyses for all applicable load combinations in IBC and ASCE 7 and use the enveloping results from the strength design load combination set and allowable stress design load combination set for various design checks. Absent any direct discussion on this topic in IBC and ASCE 7, many designers misinterpret that the traditional code intent of minimum factor of safety of 1.5 for stability for service loads applies for all allowable stress design load combinations. However, an important though unstated design objective of the building codes is that the strength design and allowable stress design load combinations involving 0.9D and 0.6D, respectively, implicitly provide a minimum factor of safety to ensure structural stability when subjected to lateral loads. Alternatively, performance-based design procedures may be used to meet the target reliabilities defined in ASCE 7 Section 1.3.1.3.

For the building structure depicted in Figure 1, the structural analysis and design (including stability checks) for the wind (or tornado in ASCE 7-22) and earthquake load effects with the factored loads shown in Figure 2a for SD method and Figure 2b for ASD method implicitly meet the traditional minimum factor of safety requirements for stability. For seismic stability evaluations, the vertical load effect, not shown in Figure 2 for simplicity, shall be appropriately considered. As mentioned earlier, IBC Section 1807.2 requires the traditional minimum factor of safety of 1.5 for overturning and sliding stability of earth retaining walls for service level loads, which correspond to 1.0D+1.0H. The same objective would be automatically achieved by checking the stability of the retaining wall for the allowable stress design load combination 0.6D+1.0H or strength design load combination 0.9D+1.6H, though noting that the design of the different structural elements and soil bearing checks may still require analyses using both sets of load combinations.

The load combinations in IBC and ASCE 7 for the structural analysis and design of building structures have evolved over the years and are intended to produce a consistent design whether the strength design (SD) or allowable stress/strength design (ASD) methods are utilized. The traditional practice of maintaining a minimum factor of safety of 1.5 against overturning and sliding failures is only meant for situations where the evaluations are based on service level (nominal) loads, such as design of retaining walls. The building codes do not specifically address factors of safety against overturning and sliding failures since the appropriate application of either strength design or allowable stress design load combinations inherently result in a stable structural design configuration; additionally, ASCE 7 provides the target reliability for stability evaluation for use of performance-based design approaches. Therefore, there is no need to apply an additional factor of safety beyond those implicit in building code requirements and, if used, will result in overconservative and inefficient designs. ■

About the Author

Jaspal Singh Saini, PE, is a Principal Structural Engineer at Bechtel Corporation. He is an active member of various ACI, ASCE, and ASME committees with focus on analysis and design of nuclear structures and turbine-generator foundations.