Erecting box bridges
Box girder bridges are typically erected using segmental construction, cantilever methods, and specialized lifting equipment such as launching gantries or cranes to position precast or cast-in-place segments.
Overview of Box Girder Bridges
A box girder bridge uses hollow, box-shaped beams as the main supporting elements. These beams can be made from prestressed concrete, structural steel, or a composite of both, and may have a rectangular or trapezoidal cross-section. The hollow interior reduces material usage while providing superior torsional stiffness, which is essential for resisting twisting forces under uneven loads . Box girders can be single-cell or multi-cell, depending on the bridge width and load requirements .
Segmental Construction
Modern box bridges are often built using segmental construction, where the bridge is assembled from smaller precast or cast-in-place segments . Precast segments are manufactured off-site or in a temporary yard, then transported to the site by truck or barge. They are lifted into position using gantry cranes, floating cranes, or launching girders, and connected with post-tensioning tendons to form a continuous structure . Cast-in-place segments are poured directly into formwork supported by temporary falsework until the concrete cures .
Erection Methods
- Balanced Cantilever Erection: Segments are added symmetrically on either side of a pier, maintaining balance as the cantilever extends. This method is suitable for long spans and minimizes the need for ground-based supports .
- Span-by-Span Erection: Entire spans are assembled on a launching gantry and then placed onto piers. This method is efficient for continuous spans up to 50 meters and can accommodate curved alignments .
- Incremental Launching: The bridge superstructure is constructed behind an abutment and then hydraulically pushed forward across the supports. This reduces disruption below the bridge and is ideal for sites with limited access .
Post-Tensioning and Stability
After segments are positioned, post-tensioning tendons are threaded through the segments to provide compressive forces that counteract tensile stresses during service. Cantilever tendons, top and bottom slab continuity tendons, and draped tendons through pier diaphragms are commonly used to ensure structural integrity . Inclined struts are often incorporated to stiffen the trapezoidal cross-section transversely, reducing weight and improving fabrication efficiency .
Equipment and Considerations
- Launching Gantries: Fully automated or mobile gantries lift and place segments with precision, often capable of self-launching for multiple spans .
- Cranes: Gantry or floating cranes are used for heavy or irregular segments, especially in river crossings or urban environments .
- Site Access and Span Length: The choice of erection method depends on span length, site constraints, and minimizing disruption below the bridge .
Applications
Box girder bridges are widely used for highway flyovers, elevated urban roads, and light rail viaducts, where their slender profile, long-span capability, and aesthetic clean lines are advantageous . Segmental erection methods allow for efficient construction, reuse of equipment, and high-quality control of precast elements .
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