Accounting for overstrength in seismic design of steel structures

Observations during many earthquakes have shown that building structures are able to sustain without damage earthquake forces considerably larger than those they were designed for. This is explained by the presence in such structures of significant reserve strength not accounted for in design. Relying on such overstrength, many seismic codes permit a reduction in design loads. The possible sources of reserve strength are outlined in this paper, and it is reasoned that a more rational basis for design would be to account for such sources in assessing the capacity rather than in reducing the design loads. As an exception, one possible source of reserve strength the redistribution of internal forces, may be used in scaling down the design forces. This is because such scaling allows the determination of design forces through an elastic analysis rather than through a limit analysis. To assess the extent of reserve strength attributable to redistribution, steel building structures having moment-resisting frames or concentrically braced frames and fi-om 2 to 30 storeys in height are analyzed for their response to lateral loading. A static nonlinear push-over analysis is used in which the gravity loads are held constant while the earthquake forces are gradually increased until a mechanism forms or the specified limit on interstorey drift is exceeded. It is noted that in moment-resisting frames the reserve strength reduces with an increase in the number of storeys as well as in the level of design earthquake forces. The P-Delta effect causes a further reduction. In structures having braced frames the main parameter controlling the reserve strength is the slenderness ratio of the bracing members. In these structures, reserve strength is almost independent of both the height of the structure and the effect of building sway.