The Overstrength Factor
The overstength factor, Ωo, is automatically obtained according to the structural systems selected from ASCE 7-16 Table 12.2-1.
Ωo = Overstrength factor
R = Response modification coefficient
The standard specifies an overstrength factor, Ω0, to amplify the prescribed seismic forces for use in design of such elements and for such actions. This approach is a simplification to determining the maximum forces that could be developed in a system and the distribution of these forces within the structure.
According to the ASCE 7 design approach, the structure can be designed for a base shear equal to Ve/R. Coefficient R is the response modification coefficient. The base shear Ve/R is the minimum expected yield strength in the building. Actual first yield in the structural framing and actual expected strength of the building are expected to be higher than the design base shear. Several factors contribute to the overstrength, including material overstrength and strain-hardening, conservatism in design load combinations and strength reduction factors, three-dimensional interactions among structural (and nonstructural) parts of the building that are not considered in analytical models used in design, and other factors such as drift limits that may control building proportions. The expected base-shear strength is designated as Ωo (Ve/R) in which Ω0 is the system overstrength factor.