Articulated Self-Launching Gantries for Precast Segmental Bridges

 articulated self-launching gantry for span-by-span erection of precast segmental bridges

The first-generation single-girder overhead gantries include a main girder that suspends the segments for the span, a launch nose that controls overturning during launch, and a rear C-frame that provides the rear support of the gantry and rolls over the new span during launch. A short rear overhang of the main girder, behind the rear C-frame, is used to load precast segments delivered on the completed bridge with an underslung winch-trolley carried by the gantry and spanning the bottom chords of the main truss.

An inverted tower-crossbeam assembly anchored to the deck during launch may be used to provide the rear support of the gantry instead of the rear C-frame. The rear tower-crossbeam assembly requires a long launch tail in the main girder to control stability during repositioning of the rear support frame. Both types of rear supports are compatible with segment delivery on the ground and on the completed bridge.

A box girder, two braced I-girders, or two braced trusses are used for the main girder of the gantry in combination with a lighter front nose and a lighter rear tail. Bending-to-shear ratio, application points of localized loads, cost of hand and robotized welding, design of field splices, cost and availability of skilled labor for site assembly of the gantry, access to the working areas, and handling and shipping requirements govern the choice between plate-girders and trusses.

The geometry of a single-girder overhead gantry is similar to the geometry of a single-girder overhead movable scaffolding system (MSS), and some MSS may actually be reconfigured for span-by-span erection of precast segmental bridges. Launch nose and support and launch systems may be unaffected by the reconfiguration, especially when the segments are delivered on the ground and no rear C-frame is therefore needed.

Lateral braces connect truss chords and I-girder flanges for their entire length. Bracing includes crosses or K-frames, connections designed to minimize displacement-induced fatigue, field splices designed for fast assembly, and sufficient flexural stiffness to resist vibration stresses. Cross diaphragms connected to flanges and chords at the same locations as lateral bracing distribute torsion and provide transverse rigidity. Connections are often designed to develop member strength to take advantage of the modular nature of the assembly and facilitate the reuse of entire sections of the gantry in different configurations.

A front nose controls overturning during launch. No rear tail is necessary when a rear C-frame rolls over the new span during launch; these short gantries are easier to launch in curved bridges than the twin-girder overhead gantries. A rear nose is needed when an inverted tower-crossbeam assembly provides the rear support of the gantry, and the main girder is in this case more than twice as long as the typical span of the bridge.

The gantry carries a winch-trolley that at first loads all the segments for the span on two stacked levels, and then releases the segments from the hangers to move them into the assembly position. Overhead winch-trolleys spanning the top chords of the truss have been used for span-by-span erection of simply-supported spans to pick up segments delivered on the ground with wide crane bridges overhanging beyond the main girders.

Strand jacking platforms sliding along the top flanges have also been used in combination with segment delivery on the ground to save the cost of the winch-trolley. In the vast majority of cases, however, an underslung flat-frame winch-trolley spanning the bottom chords of the truss is used for handling of segments. Underslung winch-trolleys are compatible with segment delivery on the ground or on the completed bridge. Hangers and spreader beams are used to suspend the segments from a top working platform supported on the top chords of the truss. The hangers are located outside the truss to not interfere with the operations of the winch-trolley.

The gantry suspends an entire span of segments and the structural elements are heavily stressed. Gantries designed for long and wide spans have been equipped with prestressing cables that control the deflections of the gantry. PLC-piloted stressing jacks for real-time adjustment of the pull in the prestressing cables have been successfully used on long-span MSS to control the deflections of the casting cell during filling and application of span post-tensioning. Prestressed gantries for span-by-span erection of precast segments shorten the span cycle time as the virtual absence of deflections during loading and unloading allows gluing the segments as soon as they are loaded on the gantry. Span post-tensioning may also be applied in its entirety before span release as the PLC relieves the pull in the prestressing cables of the gantry during application of span post-tensioning to minimize structure-equipment interaction. Although PLC-piloted prestressing cables may also be used in the twin-girder overhead gantries and the underslung gantries, single-girder overhead gantries simplify the application of prestressing, diminish its cost, and provide superior stability under the axial compression generated in the truss by the prestressing cables.

Heavy single-girder overhead gantries have been designed for simultaneous erection of adjacent bridges. These gantries are expensive and complex to assemble and operate and have found limited application. The gantry carries two underslung winch-trolleys that handle the segments for the two spans simultaneously and requires the use of T-piers that support the gantry between the adjacent superstructures, which involves the extra cost of post-tensioned pier-caps.

Conventional twin-girder overhead gantries can be easily shifted from one alignment to the other for simultaneous erection of adjacent bridges by just lengthening the support crossbeams. Even if the span cycle time is longer, these gantries are lighter and easier to reuse and do not require pier-caps.

In the last two decades, the twin-girder overhead gantries have progressively replaced the single-girder units, which have in the meantime evolved into the articulated telescopic gantries. Telescopic gantries are the current state-of-the-art for span-by-span erection of precast segmental LRT bridges with complex plan and vertical alignment.

In its basic configuration, a single-girder overhead gantry is not much fit for curved bridges because of torsion and stability issues and the risk of overturning during both span erection and self-launching. Telescopic single-girder gantries solve most of these issues and have been employed successfully to erect simply-supported LRT spans with tight plan radii.

A telescopic gantry includes a rear main girder and a front underbridge. The underbridge is supported on the leading pier-cap of the span to erect and on the next pier like in a launcher for precast spans. In the configuration for span assembly, the main girder of the gantry is supported on the leading pier segment of the completed bridge by means of a rear C-frame, and on the rear end of the underbridge by means of a hydraulic turntable. The main girder suspends all the segments for the span with hangers and spreader beams from a top working platform.

An underslung flat-frame winch-trolley spans the bottom flanges of the main girder to handle precast segments delivered on the ground or on the completed bridge through the rear C-frame. The rear overhang of the main girder above the completed bridge is used as a lifting bay for the winch-trolley to pick up segments delivered on the deck. Braced I-girders are used for main girder and underbridge instead of trusses to limit the total depth of the gantry as much as possible.

The turntable between main girder and underbridge in equipped with sliding systems at the top and the bottom, so that its position may be adjusted both along the main girder and along the underbridge. In order to cope with tight plan and vertical radii, the turntable is equipped with hydraulic controls for longitudinal translation (top and bottom) and rotation about the transverse axis (tilt of the underbridge) and the vertical axis (plan rotation).

During the first phase of launch, the turntable pulls the main girder over the underbridge. When the turntable reaches the leading pier of the new span, the underbridge is launched forward to clear the area under the main girder for erection of a new span of segments. In the second phase of launch, main girder and turntable provide the flexural capacity for the underbridge to cantilever out until the next pier is reached.

The main girder may slide on the turntable to operate the gantry on shorter spans. The underslung winch-trolley cannot be operated beyond the turntable, and these gantries are therefore unfit for erection of continuous spans with wet joints at the rear quarter or fifth of the span. Because of the presence of multiple support systems at the leading pier of the span to erect and the small dimensions of the pier-caps of LRT bridges, the telescopic gantries are mostly used for erection of simply-supported spans.

The underbridge has a rear C-frame and two sliding support frames. The main sliding frame provides the front support to the underbridge during repositioning of the main girder and supports the turntable during forward launching of the underbridge. The auxiliary sliding frame supports the underbridge on landing at the next pier to control negative bending in the underbridge and the turntable.

When the rear C-frame of the underbridge reaches the leading pier of the new span, it is inserted under the turntable to support the main girder for erection of the new span, and the main sliding frame is released and moved to the next pier to provide the front support to the underbridge. The rear C-frame of the underbridge has broad geometry adjustment capability (the main girder does not shift laterally at the turntable) and provides most of the torsional stiffness of the front section of the gantry. When the pier-caps are very narrow, a crossbeam is anchored to the pier-cap to widen the support base of the gantry.

In 21 pages in full A4/letter format, Span-by-Span Erection of Precast Segmental Bridges: Single-Girder Overhead Self-Launching Gantries explores the main components of a single-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 span post-tensioning, and the support and launch systems of the gantry.

The eManual discusses loads, self-launch kinematics, performance, productivity, span cycle times, and the labor demand of the various temporary static schemes of staged construction. The eManual also explores 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.

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