Fling in near-fault ground motions and its effect on structural collapse capacity

Lynne S. Burks, & Jack W. Baker

Published 2014, SCEC Contribution #2090

We evaluate the collapse capacity of a nonlinear single degree of freedom (SDOF) system using ground motion records with varying fling properties, including records with static offsets preserved via baseline correction, records with static offsets removed via filtering, and records with artificial static offsets added. Fling is caused by a permanent static offset of the ground and appears as a ramp function in the displacement time history. Due to baseline errors in many acceleration recordings, these static offsets are typically removed via filtering before ground motion records are added to an engineering database, such as the Next Generation Attenuation (NGA) database. Therefore, fling is neglected by default in many engineering applications even though it may affect the dynamic nonlinear response of structures, extreme nonlinear behavior such as collapse, and structures crossing a fault. Some analysts account for fling by adding artificial static offsets to filtered records, but this method has not been rigorously tested and there has been little study on the effects of fling on nonlinear structural behavior and collapse capacity.
We found that the collapse capacity of a degrading nonlinear SDOF is similar for two versions of the same ground motion: one with the static offset preserved via baseline correction and one with the static offset removed via filtering. In most cases, the baseline corrected record and the filtered record result in the same collapse capacity, indicating that filtering preserves the dynamic effect of fling even though the static offset is removed. We also found that adding artificial static offsets to filtered records typically results in a conservative estimate of the collapse capacity. In particular, increased amplitude, or static offset, and decreased period, or duration of fling, cause decreased collapse capacity.

Citation
Burks, L. S., & Baker, J. W. (2014). Fling in near-fault ground motions and its effect on structural collapse capacity. Oral Presentation at Tenth U.S. National Conference on Earthquake Engineering.