Energy considerations in ground motion attenuation and probabilistic seismic hazard studies
The severity of earthquake ground motion is generally characterized using peak ground acceleration and spectral acceleration, which are force-based or strength-based parameters. Modern seismic provisions adopt such strength-based design procedures, where seismic demand is represented in the form of an elastic response spectrum. Such design procedures implicitly account for the ductility capacity that a structure might possess by the use of reduction factors, but they do not include in a direct way consideration of the cyclic nature of the response that the structure undergoes and the resulting cumulative damage. In contrast with such strength-based parameters, energy-based parameters may be easily defined that include both the effects of oscillation amplitude as well as cycles experienced. Hence, these parameters may be expected to correlate better with structural damage. The correlation of structural damage measures with strength- and energy-based parameters is studied using inelastic dynamic analyses of reinforced concrete and steel buildings. The two earthquakes that occurred in Turkey in 1999 and the damage suffered by structures in those events offer a regional framework within which to study recorded ground motions to understand strength and energy demands over different distance ranges and for structures of different natural periods. This same framework, focused on a region in Northwestern Turkey, is used in probabilistic seismic hazard analysis (PSHA) studies where design ground motions associated with different return periods derived from both strength and energy considerations are compared for four sites as well as using maps for a larger area around Istanbul. Finally, new attenuation models for strength- and energy-based parameters are developed using an extensive database of earthquakes in Turkey. A noteworthy aspect of the new attenuation models is that they are developed using random effects models that account for distinctions between inter-event and intraevent variability in estimates of ground shaking. These attenuation models highlight differences in strength and energy demands felt at different sites. Results from hazard studies that use the new models are compared with those obtained for Western U.S. models and show some differences in design motions that are based on strength but smaller differences when energy is considered.