BEHAVIOUR OF BOX GIRDER BRIDGES

Abstract

A box girder bridge is a bridge in which the main beams comprise girders in the shape of a hollow box. The box girder normally comprises either pre stressed concrete, structural steel, or a composite of steel and reinforced concrete. The box is typically rectangular or trapezoidal in cross-section. Box girder bridges are commonly used for highway flyovers and for modern elevated structures of light rail transport. Although normally the box girder bridge is a form of beam bridge, box girders may also be used on cable-stayed bridges and other forms. The box girder bridge was a popular choice during the road building expansion of the 1960s and many new bridge projects were in progress simultaneously. If made of concrete, box girder bridges may be cast in place using false work supports, removed after completion, or in sections if a segmental bridge. Box girders may also be prefabricated in a fabrication yard, then transported and emplaced using cranes. For steel box girders, the girders are normally fabricated off site and lifted into place by crane, with sections connected by bolting or welding. If a composite concrete bridge deck is used, it is often cast in-place using temporary false work supported by the steel girder. Either form of bridge may also be installed using the technique of incremental launching. Under this method, gantry cranes are often used to place new segments onto the completed portions of the bridge until the bridge superstructure is completed. Compared to I girders, box girders have a number of key advantages and disadvantages. Box girders offer better resistance to torsion, which is particularly of benefit if the bridge deck is curved in plane. Additionally, larger girders can be constructed, because the presence of two webs allows wider and hence stronger flanges to be used. This in turn allows longer spans. On the other hand, box girders are more expensive to fabricate, and they are more difficult to maintain, because of the need for access to a confined space inside the box. Corrosion of the steel cables that provide the post-tensioning for box girder bridges has become a major concern. On December 13, 2009, the Indiana Department of Transportation (INDOT) closed the Cline Avenue (SR-912) bridge v over the Indiana Harbor and Ship Canal after a routine inspection revealed significant corrosion of the steel tensioning cables and rebar within the box girders due to water seeping through cracks in the bridge deck. After determining the level of corrosion compromised the bridge's structural integrity beyond repair, INDOT decided to permanently close and eventually demolish the span. The purpose of present study is the design of bridge structure. The most obvious choice of this span is T- Beam and Box Girder. They have their own characteristics and limitations as T-Beam has easy construction mythology, where as Box girder has sophisticated and costly formwork. In present study a two lane simply supported RCC T- Beam Girder and RCC Box Girder Bridge was analyse for dead load and IRC moving load. The dead load calculation has been done manually and for live load linear analysis is done on Staad Pro. The goal of study is to determine most favourable option from above two. The decisions based on obvious element of engineering that are safety, serviceability and economy. Following these aspect a design for both T-Beam and Box Girder has been performed. After calculation two basics material consumption steel and concrete the most economical has been selected. This study is on the basis of moment of resistance of section, shear capacity of section and cost effective solution from both T-Beam and Box Girder Bridge. The study gives the solution based on the prevailing rates of construction cost to be adopted by design Engineer.