Box girder bridge
Advantages and disadvantages
Compared to I-beam 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 plan. 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.
Construction
If made of concrete, box girder bridges may be cast in place using falsework 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 falsework 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.
Development of steel box girders
The key events in the development of the steel box girder bridge were three serious disasters, when new bridges collapsed in 1970 (West Gate Bridge and Cleddau Bridge) and 1971 (Koblenz Bridge). Fifty-one people were killed in these failures, leading to the formation of the Merrison Committee and considerable investment in new research into steel box girder behaviour.
Single box girder bridge (concrete), Australia. A similar bridge on this river was fabricated ashore and pushed across its pylons
Single box girder bridge (steel), flyover above eastern approach of the San Francisco Oakland Bay Bridge. The traffic could only be rerouted for a few hours during the night, so this entire segment was prefabricated nearby, brought in on specialized trucks, and placed on its pylons using large cranes. Note numerous bolts used to fasten the segments together, required as this is designed to survive a nearby 8.5 magnitude earthquake without damage and it is also founded upon poor quality soils that tend to induce intense ground motion
Corrosion Concerns
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 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.
Erskine Bridge, near Glasgow.
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Categories: Box girder bridges | BridgesHidden categories: Articles lacking sources from December 2009 | All articles lacking sources
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