Many precast segmental box-girder bridges have been erected with underslung self-launching gantries. These twin-girder machines operate beneath the deck with one girder on either side of the piers. The gantry supports the precast segments for the span to erect under their side wings with adjustable carts that roll along the main girders. The main girders have front noses and rear tails to control overturning during launch. Noses and tails are lighter than those of the overhead gantries as they are not used to handle precast segments and pier crossbeams.
The underslung gantries are simple to design, assemble and operate. They do not require hangers and spreader beams to suspend the segments, allow fast loading of the segments on the gantry, and facilitate access to the segments with working platforms applied to the main girders for application of the epoxy adhesive. As a result, the underslung gantries are typically less expensive and labor-intensive than the overhead gantries, and they also offer shorter span cycle time.
The underslung gantries support the deck segments under the side wings, are designed to erect precast segmental box girders, and are hardly compatible with the U-segments of dual-track light-rail transit bridges. The geometry of these gantries prevents their use for simultaneous erection of adjacent bridges by side shifting the gantry from bridge to bridge, and these gantries are therefore addressed at linear erection from abutment to abutment. Reverse launching under the completed deck to side shift the gantry behind the abutment for linear erection of a second deck is expensive and time-consuming due to the need to reposition the supports of the gantry under the deck; in most cases, therefore, the two halves of the gantry are strand-jacked on the deck and rolled back to the abutment on the completed bridge.
The main girders of the underslung gantries are supported on pier brackets that are repositioned from pier to pier with ground or floating cranes during span erection not to disrupt the span cycle time. Three sets of pier brackets are used so that two sets support the gantry at the piers of the span to erect and the third set is removed from the previous pier and repositioned on the next pier to receive the gantry during the self-launch. Intermediate props from foundations may be used to increase the load capacity or the design span of the gantry when the piers are short.
Some new-generation gantries carry a portal crane bridging the main girders to handle the segments. The crane picks up the segments on the ground within the central clearance between the main girders, rotates them to longitudinal, lifts them up above the gantry, rotates them to orthogonal, moves them longitudinally along the gantry, and lower them down on support saddles for assembly.
The crane can also pick up segments delivered on the completed bridge. Support saddles for the segments overhanging from the inner webs of the main girders permit unrestricted operations of the portal crane. The last segments of the span may be lifted from a side overhang of the portal crane or between the main girders beyond the leading pier; this solution avoids anchoring the crane but increases the cost of the launch noses of the gantry due to the increased negative bending and shear.
A lifting frame anchored to the leading end of the completed bridge may also be used to load the segments on the gantry. Lifting frames with side overhangs lift the segments from the ground alongside the bridge, rotate the segments to orthogonal, move the segments inward to bridge centerline, and download the segments on the rear end of the gantry. Portal cranes pick up the segments on the deck, move them forward throughout the frame, rotate them to orthogonal, and load them on the rear end of the gantry. In both cases the segments are moved along the gantry to the assembly position with adjustable carts rolling along the main girders. The segment carts provide lateral and vertical geometry adjustment for setting of camber, crossfall and plan curvature.
Wheeled or crawler cranes may also be used on the completed bridge to load the gantry. Commercially available cranes are more complex to operate and maintain than anchored lifting frames, more expensive, and heavier as swinging requires counterweights, but they are easier to depreciate as multi-purpose machines. Crawler cranes require blocking to cope with deck gradient and crossfall and operate the crane horizontally, which increases the labor demand and complicates the operations. Wheeled cranes may require a new homologation to operate with partially extended outriggers in relation to the location of the deck webs. Cranes on deck lift the segments alongside the bridge and have two weak points:
- For the erection of simply-supported spans, crane operations (weight of crane and counterweight, hoist load, hoist torque and dynamic allowance) increase the longitudinal bending applied to the leading pier by the missing weight of the next span. Crane operations are also demanding in continuous decks with wet joints at the quarter or fifth of the span as the crane is placed on the front cantilever of the completed deck. Cranes on deck are unavoidable with span-by-span casting with MSS, but many alternatives do exist for handling of segments for precast segmental bridges.
- The torsion applied to the span supporting the crane may be critical. For normal operations, the counterweight of the crane is designed for the hoist load, the load radius and the boom configuration, so that an equal and opposite torque is applied to the span before and after picking up the segment. To assess span stability in case of accidental release of the segment, some design standards require application of a vertical dynamic allowance equal to -2 to the hoist load. The crane crawler or outriggers facing the load may lift from the deck when the torque generated by the negative hoist load is combined with the counterweight torque, the weight of the crane is suddenly transferred to the other crawler, and the load redistribution can overturn the span if the bridge bearings are closely spaced or the spans are short or curved. These design conditions are frequently found in the light-rail transit bridges.
When the segments are delivered on the ground, the area under the bridge is accessible, and the piers are not tall, the segments may be loaded with a ground crane. When the segments are delivered on barges, they may be lifted with a floating crane or a commercially available crane placed on a barge. Access to the deck must be maintained throughout bridge erection, which may be expensive when working on the water due to the cost of barges and tugboats. Weather and sea conditions and the distance from the docking facility also affect delivery and lifting of segments.
The crane is placed in front of the leading pier, and the segments are loaded on the gantry and rolled backward into position. A ground crane traveling alongside the gantry can load the segments directly into position, but the width of the segments after rotation requires a longer boom and therefore a more powerful crane.
Launch noses and tails are applied to the main girders of the gantry to control overturning during launch, and the length of the gantry is more than twice the typical span. The underslung gantries are unfit for bridges with tight plan curves because nose and tail of the inner girder conflict with the piers and the completed deck, and traversing the outer girder during launch becomes complex and requires more expensive pier brackets. Articulated girders equipped with hydraulic hinges have been used to overcome this limitation, although the articulated gantries are more expensive and complex to operate and to reposition.
Articulated gantries carrying a varying-span portal crane can erect varying-width spans and bifurcations. Articulations in the main girders prevent the use of capstans, chain/pinion systems, or rack/pinion systems to drive the crane along the gantry; the crane is driven by motorized wheels, which limits the maximum gradient achievable with the gantry due to the limitations of friction drive systems. Ground cranes are rarely used to load the articulated gantries because of the complexity of rolling the segment carts throughout the articulations.
The gantry projects beneath the deck, which may cause clearance issues when passing over existing highways or railways. The abutment walls are typically wider than the piers; geometry conflicts with the gantry during erection of the abutment spans may be solved by applying the launch tails to the gantry after erecting the first span with the gantry propped from foundations and by removing the launch noses before launching the gantry to the last span. Geometry conflicts may also be avoided by casting the abutments in two stages, where the first-phase pour emulates the geometry of the typical pier of the project to support the gantry with standard pier brackets and avoid props from foundations. Two-phase casting of the abutments facilitates gantry assembly on the access embankment behind the abutment and side shifting of the gantry to erect adjacent bridges. With both one- and two-phase casting of the abutments, the abutment must be taller than the gantry to avoid geometry conflicts with its foundations.
The underslung gantries cannot erect segments devoid of side wings. Varying-width decks and bifurcations may be erected by skewing the main girders of the gantry, which complicates the use of portal cranes on the gantry for loading of segments and the movement along the gantry of segments loaded with ground cranes or cranes on the deck. When straddle bents, L-piers, pier-tables of aerial stations and other custom substructures of light-rail transit bridges make the clearance beneath the deck wings insufficient for gantry operation, simply-supported spans may be erected at a raised elevation, temporarily released on jacks, and lowered onto the bearings after launching the gantry to the next span.
The main girders of the gantry are designed to support the segments at the top slab level. Triangular trusses with two braced bottom chords and one top chord have been used intensively in the past. Some new-generation gantries use box girders that allow robotized welding of webs to flanges with profile-tracking equipment and can be supported at any location, which simplifies the use of the gantry for spans of different length or in curved bridges where the pier brackets are aligned with the plan radius and their spacing is therefore different in the two girders. Box girders can have inclined webs with a narrower top flange to simplify the design of the carts that roll the segments into position. Triangular trusses are typically used for the launch noses and tails; the bottom chords have the same web spacing as the bottom flange of the main girder to roll on the same launch saddles during self-launching of the gantry.
Main girders and launch extensions are not braced to each other, and the two halves of the gantry are operated independently. When the gantry does not carry a portal crane, the spacing of the main girders can be modified to minimize the lateral load eccentricity on the piers during erection of curved spans. The main girders can also be operated at different elevations during span assembly to minimize the vertical geometry adjustment provided with the support systems for the segments.
The main girders of a few underslung gantries have been permanently connected with crossbeams at both ends to avoid the use of launch noses and tails. The front crossbeam slides on a self-launching underbridge, and the rear crossbeam rolls over the new span. These telescopic gantries are shorter than the typical underslung gantries and better adaptable to curved bridges, as the connection between front crossbeam and underbridge is designed to allow rotation in the vertical and horizontal plane, and the absence of launch tails in the main girders avoids conflicts with the completed deck. These gantries avoid the cost and crane and labor demand of pier brackets but require a V-shaped design of the pier-caps to create a central launch clearance for the underbridge.
Underslung gantries comprising a central truss supported on the pier-caps and two edge trusses supported on pier brackets have been designed for simultaneous erection of adjacent box girders. So specialized machines have found limited application because of their difficult reuse, and adjacent bridges are erected more efficiently by side-shifting an overhead gantry. The vast majority of new-generation underslung gantries include two independent main girders equipped with launch noses and tails, supported on pier brackets, and launched individually. Hydraulic articulations may be used to connect launch noses and tails to the main girders to provide some extent of geometry adjustment in curved bridges. Bridges with tight plan curves are erected more efficiently with telescopic overhead gantries comprising main girder and underbridge.
Span-by-Span Erection of Precast Segmental Bridges: Underslung Self-Launching Gantries provides exhaustive coverage of the topic. 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.
The eManual provides exhaustive coverage of the topic for bridge owners, designers and constructors interested in span-by-span construction of precast segmental bridges with underslung self-launching gantries. Other eManuals of BridgeTech discuss segment fabrication, the span assembly operations common to all families of erection gantries and lifting frames, and the other types of self-launching gantries used for span-by-span and balanced cantilever construction.
The collection Span-by-Span Construction of Precast Segmental Bridges (134 pages in full A4/letter format) provides exhaustive coverage of the entire span-by-span construction process, ranging from segment fabrication by short- and long-line casting to the different types of self-launching gantries used for span erection, and is offered with a 10% discount on the cover price of the five eManuals. Combined with Balanced Cantilever Construction of Precast Segmental Bridges (81 pages), the monographs provide 215 pages of exhaustive coverage of all the construction methods and all the types of special construction equipment for precast segmental bridges. If you are interested in the design, construction and inspection of precast segmental bridges, the combination will provide you with a unique wealth of knowledge, learning and insights.
Last but not least, the eManual Construction Cost of Precast Segmental Bridges (134 pages in full A4/letter format) and the companion estimation spreadsheet explore the construction cost of precast segmental decks. The segment fabrication costs include the setup costs of precasting facilities and the production costs of the short- and long-line method. Segment transportation includes trucking, trains and barges, with or without intermediate staging areas. Segment erection includes span-by-span and balanced cantilever construction and the setup and production costs of the different types of special equipment. The estimation spreadsheet includes 1004 cost items (yes, you have read well: one thousand and four) and three columns for each cost item: construction costs, opportunities (potential of cost savings), and risks (potential of extra costs).
When combined, the monographs of BridgeTech provide 349 pages of exhaustive coverage of all the construction methods and all the types of special construction equipment for precast segmental bridges. If you thought that ASBI Construction Practices Handbook for Concrete Segmental and Cable-Supported Bridges was the international reference for the design and construction of precast segmental bridges, you will be greatly surprised.
The eManuals complement Precast Segmental Bridges, the 1-day course that Dr. Rosignoli teaches on-demand in the offices of bridge owners, designers and constructors. The bridge courses of Dr. Rosignoli originated within the ASCE Continuing Education Program. For more than 40 years, the American Society of Civil Engineers has ensured high-quality professional development and the latest innovations for bridge engineers. The ASCE Continuing Education Program is accredited by IACET to meet the premier benchmark for adult learning and undergoes review by a committee of professionals to select instructors that are authorities in their field, have decades of practical experience, and provide an outstanding level of expertise, in-depth training, and dedication to engineering.
The courses that Dr. Rosignoli teaches for the ASCE Continuing Education Program and on-demand in the offices of bridge owners, designers and constructors are true learning experiences to train bridge teams in modern bridge design and construction technology while meeting continuing education objectives. The courses foster personal research, innovation and professional development; promoting technical culture is indeed an excellent way to motivate, train and retain staff.
Learning is very effective with small groups of bridge professionals; 10-30 attendees is often the best compromise between interaction and the time constraints of hundreds of slides, although a 2010 seminar for the IABSE gathered 162 attendees in Singapore with excellent results. Richly illustrated with hundreds of photographs, the courses are constantly top-rated for material and presentation.