Effect of Damping on Displacement Demands for Structures Subjected To Narrow Band Ground Motions

Abstract eng:
Soft soil sites have produced especially large lateral displacement demands that resulted in significant damage or collapse to medium and high rise buildings in past events such as the 1977 Vrancea, Romania earthquake, the 1985 Michoacan, Mexico earthquake, and more recently the 2010 Maule, Chile earthquake. However, in addition to large displacement demands these ground motions are characterized by narrow band spectra in which these demands exhibit a much larger sensitivity to damping ratios. This is especially true when the fundamental period of the structure is close to the site’s predominant period. Seismic provisions include displacement modification factors to account for structures having damping ratios smaller than 5% (e.g., high rise buildings) as well as structures having damping ratios larger than 5% (e.g., structures incorporating viscous dampers). However, most previous studies and seismic provisions are based on structures subjected to ground motions recorded on rock or firm soil and are typically period-independent and therefore may not be adequate for structures built on very soft soil deposits. This paper summarizes a statistical study of displacement modification factors (DMF) which have to be used to adjust 5% elastic spectral ordinates to those with the desired damping. Results of this study suggest that, using the seismic provision’s currently recommended DMF, displacement demands can be significantly underestimated for structures with low damping ratios and with fundamental periods close to the predominant period of the site, whereas the opposite holds true for structures with supplemental damping where displacement demands may be smaller. Finally, an equation to estimate the mean DMF as a function of the ratio between the fundamental period of the structure and the soil’s predominant period is proposed and a study on the variability of this factor is also analyzed and discussed.

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