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The Czech government is investing heavily in upgrading and modernising the country’s top-tier transportation infrastructure. This year alone, building works with a total investment volume of over 1.6 billion euros are planned or underway on the Czech motorway and main-road network. Completion of the D8 motorway between the Czech capital Prague and Dresden in eastern Germany has a high priority here. As part of this large-scale project, the contracting consortium of SMP and Metrostav is erecting the 585 m long Prosmyky Bridge across the River Elbe. Being built using the balanced cantilever method, the bridge is intended to relieve congestion on the existing, old Elbe bridge and is scheduled to be opened for traffic in 2009. This ambitious construction project marks the world debut for the newly developed Doka cantilever forming traveller. Among the many advantages of this rentable cantilever forming traveller are its modularity, which gives it maximum adaptability; its workplace-friendly design thanks to the use of optimised longitudinal trusses; its innovative drive concept using plastic slide bearings, and its unrivalled standard of safety. The perfect design match between the formwork and the cantilever forming traveller makes for troublefree, cost-effective forming operations. It also cuts down the formwork/traveller interface to one single contact person for the superstructure planner and client to deal with, and ensures certainty regarding the planning and the costs, as well as optimising the construction workflow.
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The contracting consortium of SMP and Metrostav is erecting the motorway bridge over the River Elbe in the Czech Republic using the newly developed Doka cantilever forming traveller.
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The Prosmyky Bridge spans the Elbe with a length of 585 m. The box-girder superstructure rests on six piers, which are also being constructed using Doka Formwork Technology. The three spans of the superstructure are being cast by means of two Doka cantilever forming travellers in a total of 26 casting steps, each 15 m wide and max. 5 m long. The widest span is 151 m, giving the Prosmyky Bridge the longest span of any bridge in the Czech Republic. Working outwards from a hammerhead, the segments are being poured and alternately post-tensioned on both sides of the piers. The cantilever forming traveller is fitted with Doka large-area formwork Top 50 elements. With sidewalls tapering from 7.5 to 3.5 m, anchor blocks to be integrated in the middle of the carriageway slab, a clothoid of A=325 m and the resulting change in the cross-section gradient from zero to 3.5%, this project makes some tough demands of the formwork solution. The Top 50 large-area formwork elements were delivered to the site by the Česká Doka Ready-to-Use Service on a JIT basis, pre-assembled with complete dimensional accuracy and ready for immediate use. The cover-slab anchor blocks, positioned in the middle of the carriageway slab, present a particular challenge here. The site crew are dealing with this by using specially made anchor-block boxouts, also fabricated by Česká Doka’s Ready-to-Use Service. These anchor-block boxouts have a cover flap which makes it possible to pour casting sections either with or without cover-slab anchor blocks, as the case may be, with no need for any time-consuming modifications. This custom solution keeps work moving swiftly ahead, without costly downtimes. The high degree of pre-assembly of both the Doka cantilever forming traveller and the timber-beam formwork greatly reduced the time needed for the site installation work, hugely simplifying the overall site logistics.
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The result of Doka’s long-standing experience in the field of cantilevered construction, the newly developed Doka cantilever forming traveller sets a new benchmark in terms of workplace ergonomics, safety and cost effectiveness. The innovative cantilever forming traveller is dimensioned for casting five-metre long bridge segments weighing up to 250 tonnes. This is sufficient to cover more than 90 percent of all balanced cantilever bridge projects anywhere in the world. The Doka cantilever forming traveller’s modular, flexible structure allows it to be used for forming both straight, inclined and multi-cell box girder cross-sections without any time-wasting, cost-intensive modifications. It can also be used to form T-beams. The Doka Cantilevering Expertise Centre made an in-depth analysis of the potential for improvement on existing cantilever forming travellers, and translated these insights into reality by developing the new Doka cantilever forming traveller to meet the needs of on-site practice.
The most striking improvement over the cantilever forming travellers previously available on the market relates to the geometry of the longitudinal trusses. These take the form of parallelograms and are deliberately designed to be much greater in height.This means that the workspaces around the cantilever-arm and slab formwork are much roomier, and also much easier to get to. In addition, the risk of injury to the crew is much lower.
Another advantage is that the 60 cm high slide-rails are easy to step over, and are no obstacle to crew-members. Because the bracing is located on the outside, the new Doka cantilever forming traveller also offers a maximum of handling space for lifting the reinforcements and placing the concrete.
Another crucial improvement is the tie-rod girder. This special twin-webbed plate girder ensures a maximum of flexibility: the tie-rods can be freely positioned in a tightly spaced grid anywhere along the entire length of the tie-rod girder, enabling them to be perfectly tailored to the bending moment. The tie-rods are operated from a generously sized, safeguarded working platform. Thanks to the modular design of the traveller, the tie-rod girder can even be arranged asymmetrically, without difficulty, as needed when e.g. neighbouring structures mean that space is tight on the site. Like all the other universal girders of the new cantilever forming traveller, the tie-rod girder is available in lengths of 6.6 m and 8.8 m. In addition, the girder comes with a steel section enabling the post-tensioning jack to be repositioned by means of a travelling crab. This does away with the need for field-built improvisations such as welding extra steel sections onto the cantilever forming traveller. The cantilever-arm bracket, on which the formwork is put down while the cantilever forming traveller is being repositioned, is also modularly designed. This provides the necessary flexibility for adapting the cantilever arm formwork to different superstructure geometries. Furthermore, the versatile cantilever-arm bracket makes it possible to locate the tie-rods in the right position for the sheathing ducts needed for the post-tensioning tendons. To facilitate casting of the upward-tapering sidewalls, Doka further optimised its new cantilever forming traveller with a telescopic timber-beam formwork. There is thus no need for any improvised on-site modifications to the formwork, such as cutting back or shortening system components. The surplus formwork is dismounted in a few simple steps and the universal girders of the sidewall formwork are raised by turning spindles, as required by the upward taper.
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The newly developed cantilever forming traveller makes Doka the only manufacturer anywhere in the world that can offer an all-in-one solution for cantilevering. The perfectly matched combination of Doka’s cantilever forming traveller and its Top 50 large-area formwork does away with the often fault-prone interface between traveller and formwork. Construction firms benefit from the certainty this provides regarding planning and costs, and can handle their cantilevering projects more quickly, safely and economically than ever before. The Doka cantilever forming traveller is a rentable all-in-one solution that ensures the very maximum in cost efficiency and cost transparency.
Doka also blazed an innovative trail in the development of the drive for the cantilever forming traveller. Instead of rolling on expensive, service-intensive armour-plated rollers, as competing systems do, the Doka cantilever forming traveller is slid from one casting section to the next in a safe, controlled way on plastic slide bearings. Made from a special plastic compound, these slide bearings are designed to have a friction behaviour with sufficient static friction to prevent unwanted travelling, yet while still allowing the cantilever forming traveller to be hauled to the next casting section by hydraulic cylinder.
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As well as optimising the geometry and flexibility of the cantilever forming traveller by way of its modular structure, another main focus of the development effort, spread over several years, was on enhancing the workplace safety. For example, unlike on competing products, safe access to the tie-rod girder is ensured by a built-in ladder system with integrated ladder cages and fully enclosed working platforms from the Doka standard range. The two other work-deck levels, for the cantilever and slab formwork and the bottom formwork, are also equipped with vertical access systems and working platforms. This extensive package of safety features is rounded off by self-locking manhole lids.
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