Damping Modification Factors for Pile–Induced Filtering Effect

Period–dependent Damping Modification Factors are usually applied to the 5%–damped pseudo–acceleration response spectrum at free field conditions to derive the seismic demand for structures with damping ratio other than 5%. Likewise, the response spectrum at the foundation level of pile–supported structures may differ from the free field spectrum due to the kinematically–induced filtering potential of piles, which is also period–dependent. We explore the combined effect of structural damping and piles’ filtering action on the free field response spectrum by means of a rigorous Finite Element model of a single, fixed–head pile embedded in a continuously inhomogeneous soil. Ratios of mean spectral acceleration between the pile head and the free field are derived for various soil–pile configurations and structural damping ratios, leading to the definition of a kinematic–induced spectral reduction factor. A practice–oriented formula is proposed which provides the acceleration response spectrum at the pile head for any structural damping ratio and a given 5%–damped spectrum at free field conditions. A case study of a piled foundation in an actual soil profile is presented as an application example of the proposed approach.