Mitigation of Exterior Beam Rotation in Bridge Construction Through Experimental Investigation of Different Bracing Systems

Abstract

Bridge contractors use different bracing systems to prevent exterior girder rotation in bridge construction. The reason behind this rotation is the unbalanced eccentric loads induced from loading the deck overhang, which can cause excessive twisting in the exterior girders and excessive overhang deflections. This situation can lead to several problems during construction and over the life of the bridge. Contractors traditionally use block-and-tie systems, which are formed by a combination of tie bars and timber blocks, to mitigate exterior girder rotation. However, block-and-tie systems do not always perform as expected and their efficiency to prevent rotation not yet been evaluated. This paper investigates current block-and-tie systems and presents an analysis of improved rotation prevention systems that can be implemented in the field. Both the block-and-tie systems and the new proposed bracing alternatives were tested in a twin steel girder system [1.8 m × 4.6 m (6 ft. × 15 ft.)] that simulated a steel girder bridge bay. The prototype was eccentrically loaded at mid span through the application of a vertical load in a bracket which was mounted in one of the girders, simulating the loading from the bridge overhang. Forty-five bracing combinations were tested by varying the number of bracing elements and the spacing between them and obtaining girder rotation corresponding to the mid span of the loaded girder. The forty-five cases were divided into nine different groups to allow a clear and easy comparison between bracing alternatives. The results show very promising potential replacements of the current temporary bracing systems which, in many cases, offer minimal protection against exterior girder rotation.