The vast majority of precast segmental bridges are erected span-by-span with overhead and underslung self-launching gantries. Twin-girder overhead gantries provide unrivalled flexibility of operations in large-scale highway bridges, light-rail transit bridges with box girders and dual-track U-spans, urban bridges in median, marine operations, and other conditions of reduced clearance under the bridge.
A twin-girder overhead self-launching gantry comprises two parallel trusses or box girders supported on crossbeams. The main girders carry runways for a winch-trolley or portal crane that bridges the girders and runs the entire length of the gantry to pick up precast segments from the ground or the leading end of the completed bridge. Trusses are lighter and facilitate site assembly, access, and mobility of workers. A triangular truss with two braced bottom chords and one top chord is also perfectly fit to carry a runway on top. Box girders are stable and robust and allow robotized welding of webs to flanges. With both types of girders, field splices designed for fast assembly develop member strength to allow alternative assembly configurations that take advantage of the modular nature of design.
Short 1.7-span gantries were often used in the past. Placing electric generator and gas tank over the rear end of the gantry controlled overturning during launch and if necessary, a counterweight could be lifted with the winch-trolley parked close to the generator to further increase the stabilizing couple. These operations were easy when the segments were delivered on the completed bridge as the pier segment of the next span could be used as a counterweight. Picking up the next pier segment before starting of self-launching also accelerated repositioning of the gantry, as placing the pier segment was one of the operations in the self-launch critical path.
Modern self-launching gantries are more than twice as long as the typical span they erect. The most expensive components of an overhead self-launching gantry are the winch-trolley and the support crossbeams, and the extra cost of longer trusses does not increase the total cost prohibitively. Longer trusses increase the longitudinal stability of the machine, accelerate self-launching and shorten the span cycle time. The winch-trolley can be operated freely during launch and can therefore be used to reposition the support crossbeams to avoid the use of auxiliary ground cranes.
The segments can be delivered on the completed bridge or on the ground, beneath the gantry. When the segments are delivered on the deck, the crane carried by the gantry picks them up at the rear end of the gantry, moves them out within the central clearance between the main girders and above the rear support crossbeam, lowers them beneath the girders, and rotates them by 90° to the assembly alignment. When the segments are delivered on the ground, the crane reaches them down, rotates them to orthogonal to bridge centerline, and lifts them up to the deck level. In both cases, hanger bars or ropes are used in combination with spreader beams to suspend the segments from the gantry and release the crane for another cycle.
Portal cranes are being used more and more frequently in lieu of flat-frame winch-trolleys in modern self-launching gantries. A portal crane may be more expensive than a winch-trolley, but it offers extra vertical clearance and may be easily reused in self-launching gantries for balanced cantilever erection of precast segmental spans to handle taller varying-depth segments at the root of the cantilever. A portal crane may also be used as a service crane in the precasting facility before being applied to the gantry for segment erection, and is also able to lift the hanger bars of the segments to avoid conflicts with the support crossbeams during self-launch.
The geometry of a twin-girder overhead gantry is conceptually similar to a twin-truss launcher for precast beams, but main girders and support crossbeams are designed to sustain the weight of an entire span of segments, and most components are therefore heavier and stiffer. The beam launchers carry two winch-trolleys to lift the precast beams at the ends, while a self-launching gantry for span-by-span erection carries only one crane.
The main girders are often braced to each other at the ends to enhance the lateral and torsional stability of the gantry. End bracing trusses have been used for decades on these machines. Sometimes it may be necessary to temporarily remove the end trusses to facilitate traversing during launch on tight plan curves. This operation lengthens the span cycle time, requires anchoring of the winch-trolley to avoid derailment during the staggered launch of the main girders, and requires a ground crane for handling of the braces.
Some new-generation gantries use box-girder braces at the ends to further increase the torsional stiffness of the assembly. Telescopic box-girder braces driven by hydraulic cylinders allow rear-time adjustment of the width of the gantry. This simplifies simultaneous erection of adjacent bridges with different width as the alignment of the hanger bars for the precast segments can be adjusted rapidly during side shifting of the gantry from one deck alignment to the other.
The overhead gantries are not much affected by ground constraints, straddle bents, C-piers and variations in span length and deck width and geometry. The overhead gantries, however, are more complex to design, assemble and operate than the underslung gantries, they are more expensive and slower in erecting the segments, and they complicate the access to the segments for application of epoxy and gluing bars.
Lighter extensions are applied to the main girders to control overturning during launch. The length of new-generation gantries is 2.1-2.3 times the typical span of the bridge but this is rarely a problem because overhead machines provide minimal interference with the bridge. Launch noses and tails are heavier than those for underslung gantries because the tails are used to pick up the segments when delivered on the deck, the noses are designed for placement of the pier-head segments, and both extensions are also used to reposition the support crossbeams with the winch-trolley during self-launching. Launch noses and tails of most overhead gantries have therefore the same depth as the main girders to operate the overhead crane throughout the length of the gantry.
Twin-girder overhead gantries are also available in telescopic configuration. The geometry of these machines is similar to a typical launcher for precast spans, and also the weight and cost are not extremely different as both machines are designed to sustain the weight of an entire bridge span. A central underbridge supported on the leading pier of the span to erect and on the next pier supports the front crossbeam of the rear main frame. The main frame comprises the front crossbeam, two parallel girders carrying winch-trolley and hangers for the precast segments, and a rear C-frame that rolls over the completed bridge during launch. The segments may be delivered on the ground or on the completed bridge through the rear C-frame.
The front crossbeam of the main frame rolls along the underbridge during launch. When the front crossbeam reaches the next pier, the underbridge is launched to the next span to clear the area under the main frame for erection of a new span of segments. These telescopic gantries are fit to bridges with tight plan and vertical radii as the connection between main frame and underbridge allows rotation about the vertical and transverse axis while providing full torsional continuity. The total length of main frame and underbridge is 2.1-2.3 times the typical span of the bridge. Despite the length of these machines, the articulation between main frame and underbridge provides unrivalled geometry adjustment capability and flexibility of operations on complex bridge alignments.
The telescopic gantries are tall and sensitive to lateral wind. They must be anchored to the bridge with great care as tall support frames are less stable than the support crossbeams of conventional twin-girder overhead gantries, and the central articulated underbridge diminishes the torsional stiffness of the machine.
In 31 pages in full A4/letter format, Span-by-Span Erection of Precast Segmental Bridges: Twin-Girder Overhead Self-Launching Gantries 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 application of epoxy, and the support and launch systems of the gantry. The eManual also discusses loads, self-launch kinematics, performance, productivity, span cycle times, the labor demand of 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, the stiffness interactions to consider for the design of bridge piers and superstructures, and staged application of post-tensioning to avoid opening of the epoxy joints and the risk of brittle span failure.
The eManual provides exhaustive coverage of the topic for bridge owners, designers and construction professionals interested in span-by-span construction of precast segmental bridges. Other monographs of the eManuals Project discuss segment fabrication, the span assembly operations common to all families of self-launching gantries, and the other types of self-launching gantries used for span-by-span 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) 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.