Estimation of the Approximation Error for Hydrodynamic Mass Simplification for Bridge Piers with Large Cross-Sectional Aspect Ratios

For seismic analysis, evaluation, design, and retrofit of major water-crossing bridges, fluid-structure interaction needs to be properly modeled. Hydrodynamic mass and pressure have been used for such modeling. The formulation for estimating hydrodynamic mass developed by Goyal and Chopra (1989) is the standard practice by bridge engineers. While pier-walls with large cross-sectional aspect ratio are common as piers for water-crossing piers, the standard hydrodynamic mass approximation has limitation for crosssectional aspect ratio. In this paper, the limitation of such formulation is investigated through finite element analyses. The approximation errors for pier main design measures namely pier-wall requency, base shear, and drift are computed. Such errors are estimated by comparing results of finite element models of pier-walls with surrounding water with finite element models of pier-walls with added mass under harmonic base forces. It has been observed that added mass approximation could cause significant error in estimated pierwall design measures when pier-wall aspect ratio gets large. The main causes of such errors are frequencydependency of the added mass approximation and limitation of Goyal and Chopra’s formulation for large cross sectional aspect ratios. Further, investigation need to be directed toward behavior of pier-walls surrounded by water excited by seismic ground motions with rich frequency content and have nonlinear plastic hinging behavior.

Keywords:

Hydrodynamic added mass , Hydrodynamic pressure , Bridge pier-walls , Large crosssectional aspect ratio