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Rehabilitation of two self-launching gantries
TWO SELF-LAUNCHING OVERHEAD GANTRIES WERE REINFORCED TO OPERATE SAFELY WITH SEGMENTAL DECKS SPANNING UP TO 60M
Location: Dubai – UAE
Client: Salini Dubai Branch
Cost of total Works: 1,000,000 € (bridge superstructure)
Services: Consultancy service for the strengthening interventions of two self-launching overhead gantries
For the construction of 4 km of segmental bridges in Dubai, two self-launching gantries (one for each alignment) were reinforced to erect precast segments of up to 60 m long spans using the span-by-span method.
The consultancy service carried out for the steel equipment aims at checking by analysis and testing, all the steel components and then provide the design of the strengthening interventions, terminating with the final load test. Design activity has been supervised by Parsons (Engineer) and COWI (Independent design review of the launching gantry rehabilitation design).
Each main girder of the gantry is made up of 2 trusses spaced 7.5 m and 130 m long, usually leaning on 2 or 3 supports. Each truss has a triangular 6 m high section, composed of an upper chord, where a winch of 200 t runs, 2 lower chords which provide the temporary support to the deck segments and several pinned diagonals placed along the two connecting inclined planes. The truss consists of 11 segments, joined together on site by prestressing the end flanges of each chord by means of 4 to 10 large-diameter bars. The total weight of the main girder is around 600t.
Working on different fronts, going deeper into the project through numerical analysis (also with a non linear incremental analysis in order to find the ultimate capacity of the gantry) and experimental tests (samples tested in laboratory and full-scale load tests to measure girder deflections), a series of necessary reinforcements have been defined for the main girders, involving in particular the truss connections.
On the basis of the laboratory mechanical tests, the ultimate strength of the high-grade steel components (pins and bars) has been defined along with the elastic limit of pin connections (maximum permissible service load which prevents the accumulation of plastic deformation), enabling the achievement of a better understanding of materials and nodes resources and thus the optimization of the strengthening interventions.
Supports have been thoroughly revised and in particular a new scheme of restraint of the girder adopted, by producing new under-roller spreaders, provided with bigger wheels and jacks with higher capacity.
The full load test of the launching gantry has been performed on May 2012, checking the gantry behavior for 125% of the working load and achieving full confirmation of theoretical expectations.