Erection of Precast Beam Bridges with Twin-Truss Launchers

Erection of precast beam bridges with twin-truss launchers

The most common erection method for precast beams is with ground cranes. Cranes usually give the simplest and most rapid erection procedures with the minimum of investment, and the deck may be erected in several spans at once in relation to the availability of piers, cranes and beam delivery means. Good access is necessary throughout the length of the bridge to position the cranes and deliver the beams. In the presence of tall piers, steep slopes, ground obstructions or river crossings, crane erection may be impossible. Ground transportation and crane erection of precast beams are often impossible in congested urban areas.

Tall straddle carriers and rail-mounted portal cranes have been used to lift the beams into position when a level access is available throughout the length of the bridge and the piers are short. The beams are transported under the crane, which lifts them up. Less versatile but also less expensive than commercial cranes, straddle carriers and portal cranes are used for erecting long urban viaducts along existing roads. Pairs of lifters may be necessary for long beams to halve the design load of the lifters and to limit negative bending in the beams due to long end cantilevers.

When ground cranes cannot be used efficiently, a beam launcher may solve most erection issues. The beam launchers were developed in the 60s; their configuration was soon standardized and has not changed much since then. These self-launching cranes may be grouped into two families in relation to their structural configuration: twin-truss gantries and single-girder gantries. In a twin-truss gantry, the precast beams are handled within the central clearance between the trusses; in a single-girder gantry, the precast beams are handled beneath the main girder of the gantry. The twin-truss gantries are wider and operate low on the deck; the single-girder gantries are narrower, which facilitates operations alongside existing bridges, but much taller and prone to lateral wind.

A twin-truss beam launcher comprises two triangular trusses supported on crossbeams anchored to the two pier-caps of the span to erect; additional support crossbeams are necessary for self-launching and longitudinal movements of the gantry along the bridge. The trusses carry two winch-trolleys bridging the trusses that suspend the precast beam at its ends within the central clearance of the gantry. The length of the trusses is 2.1-2.3 times the typical span of the bridge for self-launching reasons; this is rarely a problem as these gantries operate above the deck and do not interfere with substructures and the completed bridge.

The support crossbeams are equipped with transverse runways to shift the gantry laterally for placement of the edge beams and to traverse it according to multi-step launch procedures in curved bridges. The top chords of the trusses carry full-length runways for unrestricted operations of the winch-trolleys. Each winch-trolley carries two winches: the hoist winch lifts the beam at one of its ends, and the haulage winch acts on a capstan that drives the trolley in both directions along the gantry. The capstan is anchored at the ends of the gantry and is kept in tension by lever counterweights to avoid rope slippage at the haulage winch.

One of the winch-trolleys carries an electric generator that supplies power for gantry operation. In heavier machines, the generator may be carried by a third, specific trolley to lighten the winch-trolleys and diminish the traveling point loads that their wheels apply to the top chords of the trusses. Traveling point loads often govern the design of permanent connections and field splices in the truss.

The precast beams may be delivered on the ground or on the completed bridge. When the beams are delivered on the deck, the hoist winches may be replaced with long-stroke pull cylinders. The vertical movements are better controlled and the hoists are lighter; however, pull cylinders are unable to lift beams from the ground if necessary and diminish the use flexibility of the gantry.

Both types of hoist are assembled on crabs that roll or slide laterally along the crane bridge of the winch-trolley to increase the lateral eccentricity achievable with the gantry for placement of the edge beams. Capstans, hydraulic motors with racks and pinion, electric motors or double-acting cylinders are used to drive the crab along the crane bridge. The pull cylinders are gimbal-mounted on the crab to avoid bending in the plunger due to load oscillations. The hoist winches are directly fixed to the crab and typically equipped with redundant brake systems for increased safety of operations, especially during load lowering.

The edge beams of the span to erect are lowered beneath the bottom chords of the trusses and shifted outside to the end-of-stroke of the crane-bridge runways. Shifting the hoist crabs within the winch-trolleys shortens the side cantilevers of the crossbeams that support the gantry on the pier-caps and diminishes the lateral load eccentricity on the bridge piers, as the self-weight of trusses and winch-trolleys is kept closer to bridge centerline. Shifting the hoist crabs, however, overloads the outer truss of the gantry due to the transverse load eccentricity.

Proprietary embedded items, post-tensioning bars or strand loops are used to lift light precast beams; heavier units are typically lifted with through beams crossing the web and suspended from the lifting beam of the gantry as close to the webs as geometrically possible in relation with the top flange width of the beam. The lifting beams of the gantry are always connected to the hoists and are designed for modularity in order to lift I-beams, T-beams and box girders with the same equipment.

The twin-truss launchers are relatively inexpensive and easily adaptable in both the length and the spacing of the trusses. The trusses are very flexible but this is not cause of unexpected interactions with the precast beams as the latter are suspended at the ends and may rotate in the longitudinal plane without hyperstatic effects. These gantries are able to cope with shorter spans and complex deck geometries such as varying-width trapezoidal spans, bifurcations, skewed spans and bridges with piers with varying skew. The trusses are located above the deck and are therefore unaffected by ground constraints, atypical substructures and the plan curvature of the bridge. Twin-truss launchers can cope with 200-300m plan radii; moving the gantry from span to span on tight curves may require multiple traversing operations in relation to the effective width of the pier-cap crossbeams and the planned launch trajectory.

Traversing the gantry during launch rarely causes design-governing transverse bending in the piers. Placing the edge beams of the span is typically more demanding, especially when the deck is wide and the bridge is designed with a central pier alignment. The maximum longitudinal gradient is 3-4%; higher gradient may require more expensive drive systems for the winch-trolleys, as the gantry operates with the same gradient as the deck and the winch-trolley runways are therefore inclined. Deck crossfall can reach 3-4%; higher superelevation may require shimming of the supports of the pier-cap crossbeams in wide decks.

A few new-generation beam launchers use triangular box girders in lieu of modular trusses. A box girder with triangular cross-section is structurally stable and provides a top runway beam for the winch-trolleys and two bottom launch rails under the webs. The web plates can be castellated to save weight and diminish the wind drag area. Hexagonal windows in castellated webs generate principal stresses in the web panels similar to the axial stress distribution in a truss with crossed diagonals, and minimize hand welding. Rectangular windows generate stress distributions similar to a Vierendell truss and are preferred in box girders with rectangular cross-section. Both types of windowing allow robotized welding of webs to flanges with profile-tracking equipment. The extra costs include stiffening of window edges and a central full-length weld in castellated web plates.

Trusses and castellated box girders include long transportable welded modules with a minimum number of shear-pin connections for fast site assembly and decommissioning at the end of the project. A central stiffer portion of the gantry may be combined with lighter trussed extensions that prevent overturning during launch. The beam launchers do not require access to the inner cell of the main girders during operations, and the different types of web windowing do not affect productivity of the machine.

With extensive illustrations, Erection of Precast Beam Bridges with Twin-Truss Launchers explores configurations, operations, kinematics, loads, performance, productivity and structure-equipment interactions of this family of self-launching gantries for span-by-span erection of precast beam bridges.

In 20 pages in full A4/letter format, the eManual provides exhaustive coverage of the topic. It compares new-generation self-launching gantries made of castellated-plate girders with 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.

The eManual is an essential tool for bridge owners, designers and constructors interested in the design, design-build bidding and construction of precast beam bridges on difficult terrains.