Segmental precasting vs. in-place casting with MSS

underslung movable scaffolding system (MSS)

Span-by-span casting of 30-70m spans is a popular alternative to precast segmental construction in many countries. A typical 50m precast segmental span of a continuous deck includes 15 epoxy joints and 2 unreinforced wet joints, while a cast-in-place span requires only one vertical joint. Most of the 17 joints of a precast segmental span are located in deck regions subject to peak positive and negative bending, while the joint of a cast-in-place span is located at the counter-flexure point of the span and is subject to minimal bending. The joint is bush-hammered and prepared to enhance adhesion, is not opened and reclosed after match-casting, and is subject to permanent compression as a result of longitudinal post-tensioning. In addition to the durability and ductility advantages of a joint-less monolithic structure, the few joints are crossed by continuous longitudinal reinforcement, which allows bridge design for partial prestressing.

According to AASHTO standards, a superstructure cast with 55MPa concrete in a severely aggressive environment may reach a longitudinal edge tensile stress at the Serviceability Limit State (SLS) of 1.85N/mm2 in vertical joints with through reinforcement, while it must be designed for no edge decompression in the presence of epoxy joints. The limit longitudinal compressive stress at the opposite edge of the cross-section is 33.1N/mm2 in both cases, and for a given level of post-tensioning, the stress range allowed in a superstructure with continuous longitudinal reinforcement is therefore 5.6% broader. In reality, a box girder rarely reaches the limit compressive stress in the top slab at midspan, and the actual saving in longitudinal post-tensioning may reach 10-15%.

Another major advantage of span-by-span casting is processing loose materials. Avoiding precasting facility, segment stockyard, heavy lifters and special segment transportation means are major cost savings. A precasting facility is often equipped with a batching plant, a second batching plant is necessary at the erection site if the latter is distant from the precasting facility, and saving one batching plant is a second major advantage. No segment storage is needed in the precasting facility and on site, which also diminishes project right-of-way and disruption of the area under the bridge.

Span-by-span casting and span-by-span erection of precast segments involve one working point only, and operating a movable scaffolding system (MSS) or a self-launching gantry involves similar risk profiles. Casting defects in the new span would require repairs and might delay repositioning of the MSS, which would delay the entire production line. Damage of a precast segment during shipping and handling would cause similar problems, and since the segments are shipped and handled individually, the cumulative risk actually increases.

Span-by-span casting is compatible with simply-supported and continuous spans, and with different types of cross-section. Solid or voided slabs with or without pier haunches are used for 30-40m continuous spans, ribbed slabs with double-T section are rarely used on spans longer than 50m due to the poor torsional stiffness and strength of the cross-section, and box girders are used over the entire range of spans. Simply-supported spans are preferred in light-rail transit and high-speed railway bridges because of the favorable thermal interaction with the continuous welded rail and because they spread the longitudinal traction/braking loads to a greater number of piers. Continuous spans are preferred in highway bridges because of the higher structural efficiency and durability, the smaller number of bearings and expansion joints, and cost savings in longitudinal post-tensioning. Cast-in-place continuous spans typically have constant depth, but varying depth or pier haunches have been used to enhance structural efficiency on the longest spans.

Advances in structural analysis software programs and post-tensioning technology have simplified the design and construction of cast-in-place continuous spans. Span-by-span casting offers a similar productivity as incremental launching and more flexibility in the geometric design of the bridge, and both methods are faster than balanced cantilever construction. This time advantage has stimulated the application of span-by-span casting to longer and longer spans. Movable equipment for span-by-span casting is evolving with this trend and sometimes preceding it, allowing construction of longer spans with shorter cycle times.

In short bridges, the outer form for span-by-span casting is supported on advancing shoring. In longer bridges comprising multiple spans of constant length and large plan radius, the outer form is assembled on a self-launching frame.

The movable scaffolding systems (MSS) are available in overhead and underslung configuration. In an underslung MSS, the outer form is assembled on two self-launching girders supported on steel brackets applied to the piers of the span to cast. In some MSS, the two halves of the outer form shift over the girders to create the central launch clearance. In other machines, the two halves of the MSS shift outward along the pier brackets to simplify and accelerate form reconfiguration on launch completion.

In 41 pages, Movable Scaffolding Systems (MSS): Introduction illustrates span-by-span casting with MSS through comparisons with the other construction methods for medium-span prestressed-concrete bridges. It explores one-phase casting of box girders and ribbed slabs and two-phase casting of box girders with and without first-phase post-tensioning. It also discusses multi-phase casting of wide box girders combining in-place precasting of pier tables, two-phase casting with MSS of the central box core, and segmental casting of the side wings with forming carriages rolling on the central core.

For each span casting method, the eManual explores the prefabrication techniques for the rebar cage and their impacts on the span cycle time, the filling sequences for the casting cell, the design of post-tensioning, and staged application of post-tensioning to control structure-equipment interaction.

With extensive illustrations, the eManual introduces the different types of MSS and provides exhaustive guidance on the choice of the most appropriate type. It also explains how to design bridge piers, abutments and superstructures for effective use of MSS technology in relation to the staged casting process selected for the span. The eManual introduces span-by-span casting with MSS to bridge owners, designers and constructors unfamiliar with this construction method, and is an indispensable gateway to specialized monographs of the eManuals project dealing with the different types of MSS.


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