Self-Launching Erection Machines for Precast Concrete Bridges

Self-launching erection machines for precast concrete bridges

The technological aspects of construction influence the modern bridge industry from the very first steps of design. Entire families of prestressed concrete bridges, such as launched bridges, span-by-span bridges, and balanced-cantilever bridges, take their names straight from the construction method.

Construction of precast concrete bridges with spans ranging from 30m to more than 180m is mostly based on the use of self-launching machines. The launching units are complex and delicate structures. They resist high loads on long spans under the same constraints that the obstruction to overpass exerts on the final structure. They are adaptable for reuse on different projects. They must be as light as possible, which involves designing for high stress levels in different load and support conditions, and they are assembled and dismantled many times and reused by different crews.

Little has been written on these machines in spite of their cost, complexity, and sophistication. Self-Launching Erection Machines for Precast Concrete Bridges (2010, PCI Journal) illustrates the main features of these machines and some lessons learned during many years of the author’s practice in the bridge industry and as an independent design-checker of launching units. The paper deals with most types of special erection equipment for precast concrete bridges.

  • Introduction
  • Types and features of launching units
  • Launching gantries for precast concrete girders
  • Launching gantries for span-by-span erection of precast concrete segmental box girders
  • Launching gantries for balanced-cantilever construction
  • Wheeled carriers for full-span precasting
  • Design loads
  • Modeling and analysis
  • Instability of main trusses
  • Support member instability
  • Launch and lock systems
  • Load testing
  • Conclusions
  • References

The paper is one of the first publications ever written on the design of self-launching bridge construction machines. It includes thorough information and guidance on design loads, modeling and analysis, instability of main trusses and support members, launch and lock systems, and load testing at the end of site commissioning. Many e-books of the Bridge Engineering eManuals project expand the discussion from this paper.

  • Erection of Precast Beam Bridges with Twin-Truss Launchers (20 pages) explores configurations, operations, kinematics, loads, performance, productivity and structure-equipment interactions of this family of self-launching gantries. The eManual compares gantries made of castellated-plate girders and modular trusses with shear-pin connections, examines the span cycle time for beam delivery on the ground and on the deck with progressive casting of the concrete slab and advancing of the pickup bay on the completed deck, and explores the loads and stiffness interactions to consider for the design of bridge piers and bent caps.
  • Span-by-Span Erection of Precast Segmental Bridges: Single-Girder Overhead Self-Launching Gantries (21 pages) explores the evolution of the first-generation single-truss overhead gantries into modern non-articulated machines comprising front integral L-legs and rear C-frame, and the state-of-the-art telescopic gantries featuring main girder and underbridge connected by a central turntable. For both families of self-launching gantries, the eManual provides exhaustive coverage of loads, self-launch kinematics, performance, productivity, and when these machines should be selected in lieu of twin-girder overhead gantries. The eManual also discusses deck construction, the stiffness interactions to consider for the design of bridge piers and superstructures, and staged application of post-tensioning to avoid decompression of epoxy joints and the risk of brittle span failure.
  • Span-by-Span Erection of Precast Segmental Bridges: Twin-Girder Overhead Self-Launching Gantries (31 pages) explores the main components of a twin-girder overhead gantry, the pros and cons of modular trusses and box girders, the use of hangers and spreader beams to hold the segments in-place during gluing, and the support and launch systems of the gantry. It also discusses loads, self-launch kinematics, performance, productivity, the span cycle time, the labor demand of the different temporary static schemes of staged construction, and the pros and cons of simultaneous erection of adjacent decks by side shifting the gantry from bridge to bridge on wide pier crossbeams. The eManual explores the stability of tall bridge piers under the loads applied by brackets supporting the front pendular leg of the gantry and the stiffness interactions to consider for the design of bridge piers and superstructures.
  • Span-by-Span Erection of Precast Segmental Bridges: Underslung Self-Launching Gantries (21 pages) provides exhaustive coverage of span-by-span erection with underslung self-launching gantries. It explores loads, kinematics, support and launch systems, and performance and productivity of these machines. It also discusses span assembly, the stiffness interactions to consider for the design of bridge piers and superstructures, and staged application of post-tensioning to avoid decompression of epoxy joints and the risk of brittle span failure.
  • Balanced Cantilever Construction of Precast Segmental Bridges (81 pages) is a milestone of the Bridge Engineering eManuals project. The eManual explores the load and stiffness interactions to consider for the design of piers and superstructures, the stability of tall bridge piers under the loads applied by self-launching gantries, the midspan closure operations, the different decking solutions for cable-stayed precast segmental bridges, and the expansion joints of balanced cantilever bridges. It also explores loads, kinematics, performance, productivity and structure-equipment interactions of the self-launching gantries for balanced cantilever construction, and the different families of lifting frames.
  • Full-Span Precasting of High-Speed Railway Bridges (17 pages) explores the organization of large-scale precasting facilities designed for just-in-time delivery of precast spans. It explains how to optimize the productivity of casting cells and rebar jigs, the pros and cons of span post-tensioning vs. pre-tensioning and hybrid prestressing systems, and the organization of the stockyard in relation to the span curing time at delivery. The eManual explores the use of portal carriers with underbridge and span launchers fed by tire trolleys for transportation and placement of precast spans. It compares loads, kinematics, performance, productivity, structure-equipment interactions and span curing time at delivery of the two solutions and explains how to choose the most appropriate span delivery method in relation to the length and weight of the precast units, the length and number of bridges to erect, the presence of tunnels and crossover embankments along the delivery routes, and the area available for the span stockyard.
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The Bridge Club and the Bridge Industry host a rich collection of exclusive articles and free publications. The articles below are a selection for further reading on the topic.

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