Response spectral damping modification factors for structures built on soft soils

The seismic hazard intensity at a site is usually defined in terms of 5% pseudo-acceleration spectral ordinates. However, as structures can have damping ratios different from 5%, it is necessary to apply damping modification factors to adjust spectral ordinates. This study examines damping modification factors corresponding to damping ratios ranging from 1% to 30% for structures built on soft soils. A statistical study is presented based on 90,000 ratios computed using a database of 100 ground motions recorded in the soft soils of Mexico City. It is shown that damping modification factors are very sensitive to changes in the period of vibration of the system, with the largest sensitivity occurring for periods close to the predominant period of the site. A simplified equation to estimate mean damping modification factors is proposed as a function of the level of damping and of the period of vibration normalized by the predominant site period.