A numerical model for design of pile-supported bridge approach slabs
Description
An approach slab that follows a bridge structure is designed to provide a smooth transition between the bridge and the roadway. The current design practice of pile-supported approach slabs is purely empirical and consists of using piles of variable lengths along the slab length. Field observations showed that some pile-supported approach slabs have performed relatively well, while others have settled significantly to create bumps, or even gaps, at the bridge/roadway interface To quantitatively evaluate the performance of pile-supported approach slabs, a numerical model was developed that predicts the long-term deflection profile of the approach slab. By accounting for downdrag forces imparted on the piles due to excessive settlement of the embankment foundation soil, the model establishes a force-deflection curve based on pile/soil skin resistance interaction for each pile. It predicts the approach slab deflection profile from independent pile settlements along the longitudinal direction while ignoring the approach slab stiffness. A spreadsheet program entitled TU-DRAG was developed based on the numerical model to automate the pile stiffness calculations and to predict the slab deflection profile The general embankment and approach slab information, field and laboratory test results obtained at three selected sites of pile-supported approach slabs in southeastern Louisiana were used to validate the model. The predicted deflection profiles by the finite element method (FEM) and considering the slab stiffness were compared with the field measurements and the results of TU-DRAG. In addition, design modifications obtained using TU-DRAG to optimize pile lengths and the resulting deflection profiles were compared with the FEM predictions. Studies to quantify the effects of parameters related to slab stiffness, pile/soil spring stiffness, boundary conditions and truck load magnitude on the slab deflection profile were performed using the FEM and TU-DRAG Results of the foregoing analyses showed that the proposed numerical model is somewhat conservative but could be used for design and analyses of pile-supported approach slabs. The results also showed that TU-DRAG is a valid tool for preliminary designs and that the associated built-in conservatism may be reduced using a modified empirical method that approximates the contribution of the slab stiffness