Adjacent-field velocity pulse effect on strength reduction factors

Strong adjacent-field pulse ground motions pose severe potential damage to the structures. Based on the large number of adjacent-field velocity-pulse ground motions recorded in the Chi-Chi earthquake (Taiwan September 20, 1999), the effect of adjacent-field velocity-pulse on the strength reduction factors is statistically investigated and expressions for the modification of the strength reduction factors are established. It is concluded that in the long period range, the strength reduction factors for the adjacent-field velocity-pulse ground motions tend to be consistent with those for far-field ground motions. But, in the short or medium period range, the strength reduction factors for the adjacent-field velocity-pulse ground motions are significantly smaller than those from the far-field ground motions, making it unsafe, in adjacent areas, to use the current strength reduction factors, which were obtained for far-field ground motions. For a given period, the adjacent-field effect on the strength reduction factors increases with the structure ductility coefficient.