Section 2.3.6/Load Combinations Including Overstrength Factor
ASCE 7-22 Section 2.3.6 defines load combinations using the seismic overstrength factor (omega-0) for elements that must remain elastic during the design earthquake.
Section 2.3.6 provides load combinations that use the seismic overstrength factor (omega-0) in place of the standard seismic load effect E. The amplified seismic load Emh equals omega-0 times QE, where QE is the horizontal seismic force from the equivalent lateral force procedure per Section 12.8. The overstrength factor omega-0 ranges from 2.0 to 3.0 depending on the seismic force-resisting system and is tabulated in Table 12.2-1. The overstrength combinations are: (6) 1.2D + Ev + Emh + L + 0.2S and (7) 0.9D - Ev + Emh. These combinations are required for the design of specific elements identified in Section 12.4.3, including collectors and their connections, elements supporting discontinuous walls or frames, and connections where a stronger element frames into a weaker element. The overstrength combinations are also required by material design standards for specific connection types (e.g., column base plates in special moment frames).
Why this section exists
The standard seismic load combinations in Section 2.3 use a reduced seismic force that accounts for ductility and energy dissipation in the yielding elements of the structure. However, certain elements must remain elastic to maintain load path integrity during the earthquake. If a collector yields, for example, it can no longer transfer diaphragm forces to the lateral system, creating a progressive failure. The overstrength factor approximates the maximum force that the yielding elements can actually deliver (due to material overstrength, strain hardening, and higher mode effects), ensuring the non-yielding elements are designed for the real forces they will experience, not the reduced design forces.
What plan reviewers look for
Plan reviewers check the structural calculations for elements identified in Section 12.4.3 that require overstrength combinations. The most common are collector elements (drag struts) at diaphragm boundaries, columns supporting discontinuous braced frames or shear walls, and transfer structures. They verify the correct omega-0 value from Table 12.2-1 for the seismic system used. They check that the overstrength combinations are applied to both member design and connection design for the identified elements. For ASD combinations, the overstrength equivalents in Section 2.4.5 must be used.