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 superstructure 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 only 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 or otherwise prepared to enhance adhesion, is not opened and reclosed after match-casting, and is subject to the permanent compression imparted by 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 bridge superstructure cast with 55-MPa concrete in a severely aggressive environment may reach a longitudinal edge tensile stress of 1.85 MPa when the joints are designed with continuous 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 33.1 MPa in both cases, and the stress range allowed in a superstructure with continuous longitudinal reinforcement is therefore 5.6% broader. In reality, a prestressed-concrete box girder rarely reaches the limit compressive stress in the top slab due to the raised location of the cross-sectional centroid, and the actual saving in longitudinal post-tensioning may reach 10-15% at midspan.
Another major advantage of in-place casting is the processing of loose materials. Avoiding precasting facility, stockyard, heavy lifters and special transportation means for precast deck segments offers major cost savings. A precasting facility is often equipped with a batching plant, one or more batching plants are also necessary at the bridge site if the latter is far from the precasting facility, and saving one batching plant is a second major advantage. No segment storage is needed, 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 a span require repair and may delay the repositioning of the MSS, which would delay the entire production line. Damage of a precast segment during shipping and handling causes similar problems, and since the segments are shipped and handled individually, the cumulative risk 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 constant 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 the structural efficiency on the longest spans.
Advances in structural analysis programs and post-tensioning technology simplify the design and construction of cast-in-place continuous spans. Span-by-span casting offers a similar productivity as incremental launching and assures more flexibility in the geometric design of the bridge, and both methods are much faster than balanced cantilever construction. This stimulated the application of span-by-span technology to longer and longer spans to reduce the cost and duration of bridge construction. Movable equipment for the construction of continuous spans is evolving according to this trend and sometimes preceding it, allowing construction of longer spans with shorter cycle times.
In 41 pages, Movable Scaffolding Systems (MSS): Introduction introduces span-by-span casting with MSS through comparisons with alternate construction methods for medium-span prestressed-concrete bridges. It explores one-phase casting of box girders and ribbed slabs and two-phase casting with and without first-phase post-tensioning. It also discusses multi-phase casting of wide box girders by combining in-place precasting of pier tables, two-phase MSS casting of the central box core, and segmental casting of the side wings with forming carriages rolling over the central core.
For each span casting method, the eManual explores cage prefabrication and its impacts on the span cycle time, the filling sequences for the casting cell, the design of deck post-tensioning, and staged application of post-tensioning to control structure-equipment interaction.
- Introduction
- In-place casting on falsework
- Movable scaffolding systems
- One-phase casting
- Multi-phase casting
- Casting procedures
- Choice of the most appropriate type of MSS
- Bridge design recommendations for effective use of MSS
- References
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 in relation to the selected staged casting process.
Dr. Rosignoli’s course on Movable Scaffolding Systems (1 day, on-demand in the offices of bridge owners, designers and constructors) explores the use of MSS for span-by-span and balanced cantilever construction of prestressed-concrete bridges. You will learn under which circumstances is span-by-span casting a competitive alternative to incremental launching and precast segmental construction, will compare the use of telescopic MSS for balanced cantilever bridges with in-place casting with form travelers, and will explore bridge design and detailing for effective use of MSS.
With extensive illustrations and case studies, the course explores configurations, operations, loads, kinematics, performance, productivity, structure-equipment interactions and industry trends of overhead and underslung MSS for span-by-span casting. The course also examines telescopic MSS and form travelers for balanced cantilever casting, and the forming carriages for segmental casting of the concrete slab of steel girders.
The course explains the design of piers, abutments and superstructures for safe and efficient use of MSS and delivers a unique wealth of knowledge, learning and insights extracted from three decades of design, design review, construction and forensic engineering of bridges, bridge construction machines, and their interactions. The bestseller Bridge Construction Equipment (2013, ICE Publishing) and the eBooks of the Bridge Engineering eManuals Project integrate the course to provide exhaustive coverage of the topic.