Making The Connection
Bridges of all types are vital links in infrastructure development. Beam, arch, suspension, truss, cable stayed, cantilever, truss-arch, and lattice truss, from pedestrian walkways to awe inspiring spans of spectacular gorges, each bridge has unique requirements and challenges during fabrication. The same bridge design may require completely different construction methods simply due to site restrictions and accessibility. One site may have unrestricted areas for stockpiling of materials, equipment staging, and traffic control detours while another site may have limited access and require unrestricted traffic flow during construction. The former may be able to use in place fabrication techniques while the latter would probably require prefabricated methods. So while there is no single construction method for bridge erection, there are several broad categories of fabrication techniques. These are:
- Falsework or staging: temporary framing and scaffolding to support structural elements during construction.
- Span-by-span: structural elements are fabricated in situ between supporting structures to create each span in a sequential process.
- Full span erection: fully prefabricated spans are constructed off site, transported to the bridge construction project, and installed whole between supporting structures.
- Balanced cantilever: segments are installed or fabricated in situ on opposing sides of a supporting pier until the span is complete.
Additionally, there may be more than one technique employed on a single project to achieve the most efficient construction process.
To facilitate these techniques at least one of two types of heavy equipment are usually present:
- Crane assist: structural elements are placed and temporarily held in position by one or more movable lifting cranes located on the ground, barges, or on the bridge itself as construction progresses.
- Gantry or launch girder: typically a horizontal steel framework supporting a track and carriage which hoists and transports structural elements along a bridge span, then travels horizontally to the next span.
Stress Can Be A Good Thing
Another interesting implementation of modern bridge construction is accounting for pre and post completion loading. Since a bridge does not realize full structural strength until all the elements are connected, individual elements may be shifted out of position and/or pre-stressed prior to final connection. For example suspension and cable stayed bridges typically arc away from the true load line until the weight of the decking has been applied. For bridges using structural concrete a technique of post-tensioning is employed to ensure the structural element experiences primarily compressive loading. This is vital as concrete typically has poor tensile strength. Understanding these concepts is important not only during construction but also during bridge maintenance and inspection. A suspension bridge significantly deviating from its load line may indicate missing elements, or tensile microcracks in a concrete beam may indicate insufficient post-tensioning.
Easier Said Than Done
Of course all of these concepts are easy to describe, but challenging, costly, and time consuming to execute. With increasing demands for faster completion time, fewer construction injuries, lower costs, reduced inconvenience, to move in, move out and stay out, bridge construction engineers are constantly striving for the best possible construction solutions. Connecting the dots using the most efficient means available has resulted in extraordinary examples of bridge construction such as the Mackinaw Bridge, the Millau Viaduct, and the Beipanjiang River Railroad Bridge to name but a few. And with constant innovation in civil engineering there will be more, without a doubt.
About the Author
John Moehring is a practicing Engineering Technologist in civil, geological, biological, and electrical engineering fields. And one of these days he may actually get it right.
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