Torsional Analysis for Exterior Girders

Abstract

Concrete deck placement imposes eccentric loading on exterior steel bridge girders. This report describes a design tool that aids bridge engineers in evaluating the response of the exterior girder due to this eccentric loading. Computer analyses are conducted in order to gain a detailed understanding of the factors influencing the response of the girder. It is shown that the “flexure analogy” is correct and can be used in the design tool. The “flexure analogy” is the assumption that torsional loads on the girder are mainly carried by the flanges in minor axis bending. Top and bottom flanges need to be analyzed independently since the boundary conditions for them vary significantly. Furthermore, analyses indicate that a substantial improvement in accuracy can be achieved if the boundary conditions on the local system used to analyze the behavior of the girder are changed. The influence of dynamic loads, such as the movement of the finisher and the impact of concrete during the placement process, is investigated and found to be negligible. Based on these findings, a design tool in the form of a Visual Basic © application, TAEG (Torsional Analysis of Exterior Girders), for Windows 95/NT © has been created. It uses the stiffness method to calculate the stresses and deflections of the flanges due to torsional loads. Results for bracket forces and diaphragms are also calculated. TAEG can be used to evaluate the effect of temporary support in the form of tie rods and blocking. Three examples are provided to justify the results and are compared with existing methods or field data. TAEG uses a 3-span fixed end continuous beam analysis model for finding torsional stresses while the AISC Design Guide method uses a less accurate single span fixed end model. Therefore, in comparison to the AISC Design Guide method stress results calculated with ATEG are approximately 20% higher for the positive moment region and approximately 20% lower for the negative moment region. Generally, stresses at the negative moment region govern.