Experiments, Analysis and Design Models for Slab on Prestressed Concrete Girder Bridge Structures

Abstract

A new class of spread slab beam bridge superstructure has recently been developed and implemented in Texas. In order to investigate its structural performance in general, and load distribution behavior in particular, comprehensive static and dynamic tests are performed on an in-service spread slab beam bridge, the US 69 Bridge located in Denison, Texas. Different computational techniques including the historic grillage analysis and the more rigorous finite element method (FEM) are utilized to model the moment and shear actions for this new bridge system. Satisfactory agreements are obtained with comparisons between experimental and computational results.

Current service load design practice reveals that the asymmetric AASHTO HS20 truck load and complicated LRFD LDF formulas bring unnecessary inconvenience to the design process. Alternative symmetric live load models and new design models with a familiar “S/D” format are developed in this dissertation for the purpose of providing bridge engineers a more straight-forward option to determine the moment and shear demands at the service load design or for the rapid checking of computer output. The applicability of the proposed design models for the prestressed concrete girder bridges commonly used in Texas and elsewhere is evaluated by comparing accuracy with more exacting FEM analysis results. Comparative results show that the proposed design formulas are mostly conservative for these bridge types.

Following service load design, the adequacy under factored ultimate strength conditions requires checking. Due to their expediency and ease of use, plastic limit analysis methods are used to evaluate the reserve strength capacity of slab-on-beam bridge systems. It is shown that by taking a holistic view of several different potential failure modes the “balance” of the design can be judged in terms of a hierarchy of failure mechanisms. Therefore, it is possible to make minor adjustments to the design in order to obtain a preferred outcome.

The proposed design methods are adopted in the service load design of a multi-span spread slab beam bridge to explore the potential for extending the span length of this low profile bridge system. The design results indicate that span lengths up to 21.3 m are viable with only four spread slab beams. To achieve this span it is necessary to make at least three spans continuous and use load balancing principles along with some supplementary post-tensioning.