The use of precast concrete segments has the advantage that the superstructure can be erected at a faster rate compared to cast-in-place construction. The precast concrete segments are made while the substructure is being built and then stored until needed for erection. The precast segments are built using either the short-line or long-line method.
In the short-line method, each segment is cast next to the previous segment in a special adjustable casting machine. This ensures that the interface between the two segments matches exactly when erected. Each successive segment is then cast next to the previous one.
In the long-line method, formwork matching the shape of the soffit is erected on the ground. A traveling form for the webs and deck is moved along the soffit form for the casting of each segment.
Precast segments are usually erected using the span-by-span method, balanced cantilever method, or progressive placement method.
In the span-by-span method, an entire span is assembled, post-tensioned, and erected so that it is self-supporting before the next span is erected. The method is appropriate for span lengths up to about 150 ft. Beyond 150 to 180 ft, the method is less cost effective.
In one variation of this method, all the segments are supported by an erection truss before the segments are post-tensioned together. The erection truss may be located either above or below the segments. Once the segments are post-tensioned together and the span is resting on its bearings, the erection truss is moved to the next span. When space permits, the segments may be assembled at ground level, post-tensioned together, and the entire span lifted into place.
In the balanced cantilever method, the superstructure is erected by cantilevering out from opposite sides of the pier. The segments are added either at the same time or alternately to each cantilever to maintain a relatively balanced system. Often segments are offset by one-half segment length to reduce the out-of-balance moment. After the cantilevers from each adjacent pier reach midspan, a cast-in-place closure segment is placed followed by additional post-tensioning.
The balanced cantilever method is most economical for span lengths greater than about 160 ft. For span lengths greater than about 500 ft, the weight of the segments near the piers reduces the feasibility of using precast segments in balanced cantilever construction.
In the balanced cantilever method, segments are lifted into place using ground- or water-based cranes, deck mounted lifters at the end of each cantilever, or an overhead gantry. The selected method depends on the number of spans, contractor’s preference, and available access. An overhead gantry will typically be supported at three piers.
The progressive placement method involves starting at one end of the bridge and erecting segments in sequential order. This method of construction is particularly suitable for environmentally sensitive areas or where construction access is limited. It is often called “top-down” construction because the substructure and superstructure can all be erected from the superstructure. The method usually requires the placement of temporary piers at about the middle of each span and is suitable for span lengths of 100 to 300 ft.
Cast-in-place segmentally constructed bridges are generally built using the balanced cantilever method. A form traveler is used at the end of each cantilever to support the formwork and new concrete segment prior to post-tensioning. The form travelers at opposite ends of the cantilevers may be advanced simultaneously or alternately.
Cast-in-place segmental construction is used when the precast segments are too heavy to be shipped or access is too restrictive. It has been used for span lengths up to 760 ft in the United States.
The superstructure at the abutment ends cannot be easily constructed as part of the cantilevering process because of the large out-of-balance moment, so these are often constructed on falsework resting on the ground.