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The major works being carried out just outside Vienna to upgrade the “Knoten Prater” motorway junction has presented scaffolding specialists XERVON Austria with three complex tasks: the company has had to plan and erect the scaffolding needed to build two bridges over the Danube Canal; a further shoring structure is also needed to build an access ramp road.
This is one of the big infrastructure projects being carried out in Austria: due to last several years, the “Knoten Prater” motorway junction near Vienna is in the process of being upgraded. Linking the A4 motorway (“East Motorway”) and the A23 motorway (“South East Tangent”) on the outskirts of Vienna, this is one of the most important motorway junctions east of the city. The roads and bridges here, therefore, have to cope with large amounts of traffic.
The building contractor for this project is ASFINAG, an Austrian state-owned company responsible for roads and motorways in the country. ASFINAG has commissioned the “Umbau Knoten Prater” syndicate – consisting of the companies Porr Bau GmbH and Habau GmbH – to execute the project. This, in turn, has commissioned the scaffolding specialists XERVON Austria to develop a solution for the complex shoring structures needed to complete the work.
The main focus of this project is on the ‘Erdberger Brücke’ (Erdberg Bridge) which was built in the 70s and crosses the Danube Canal. Over the decades, the heavy volume of traffic has had such a detrimental effect on its structure that it must now be demolished and completely rebuilt. Two new additional bridges are currently being constructed either side of the bridge, so-called flyovers, to ensure the traffic does not come to a standstill and is not affected, for the most part, by the work being carried out on the main bridge. When the new bridge has been completed in 2017, these additional bridges will remain in place and continue to be used to prevent traffic jams during rush hour periods.
The complex work to erect the shoring scaffolding over the Danube Canal for these two new flyovers began in May 2014. In addition, a further stretch of road has to be built that also needs shoring scaffolding: a ramp road to connect the A23 motorway exit to the southern flyover.
”What is most important here is that our shoring structures have the necessary heights and widths to allow traffic to flow unimpeded.“
Anton Stricker, head of the shoring scaffolding division at XERVON Austria
Engineer Anton Stricker, head of the shoring scaffolding division at XERVON Austria, explained what the company had to take into account when designing the shoring scaffolding for these two additional bridges: “Both bridges will consist of eight sections (hollow section framework, 12 metres wide) with a total length of 223 metres and 237 metres respectively. They will cross over both the east and westbound lanes of the A4 motorway and have a main span over the Danube Canal of around 45 metres. What is most important here is that our shoring structures have the necessary heights and widths to allow traffic to flow unimpeded.” This includes both the lanes on the A4 motorway and the various exit and access roads to the A23 motorway as well as over the Danube Canal itself which is a busy shipping route.
The traffic should, for the most part, be unimpeded by the demolition and rebuilding work
In order to fulfil all these requirements, the scaffolding experts developed a complex structure based on steel girders (rolled steel girders), scaffold beams and scaffold supports (to transfer the load vertically) – all in all comprising a total of approx. 1,000 tonnes of material. The horizontal shoring scaffold section, on which the bridge superstructure and carriageway will be built, is made up of rolled steel girders (HEB 300 to HEB 800) up to 80cm thick and of lengths varying between 5 and 20 metres. The 34 metre section in the middle of the shoring scaffold, that crosses the Danube Canal, has been erected using heavy duty scaffold beams as it has such a long span. 16 scaffold beams have been joined together to create a 12m wide structure over the water. This, in turn, is connected to further structures – erected using cross girders, lowering devices and heavy duty vertical scaffold beams – which transfer the load into the ground or into the bridge foundations that have already been constructed.
There were two main challenges that both teams had to face whilst erecting the shoring scaffolding: firstly, the night shift work, when the motorway was closed off so they could set up the rolled steel girders above the carriageway, and secondly placing the 34m long scaffold beams over the Danube Canal. To keep the crane work to a minimum, all 16 scaffold beams were preassembled at the building site on the smallest of spaces and lifted into place – in some cases individually and in others as complete boxes (each weighing ca. 18 tonnes). This work was completed in a single day.
Apart from the final dismantling tasks, the shoring scaffolding work has already been completed for the northern flyover bridge. The installation teams are currently working on the southern structure. By the end of the shoring scaffolding work in April 2015, XERVON will have erected and dismantled a surface area totalling 6,300 square metres.
When in-situ concrete bridges are built, heavy duty shoring scaffolding is needed as a temporary support structure to carry the weight of the new bridge until it is able to bear its own load – i.e. after the concrete has reached full structural strength. In such projects, the shoring scaffolding creates a base for the formwork which holds the liquid concrete and gives it its shape. In the majority of cases, the main component used for shoring structures for spans up to 20 metres (e.g. over major roads and motorways) are rolled steel girders (HEB), which are heavy duty steel beams that can be up to one meter in height. So-called truss girders, however, are deployed for larger spans, for example for erecting bridges over rivers. These are special steel frameworks that bear the load of the structure being built and transfer the load, via heavy duty props, to the auxiliary foundations or the foundations of the new structure itself. Thanks to this type of system, shoring structures can be used for spans between 18 and 48 metres – without the need for any other type of support. Such complex shoring systems can only be designed and erected by specialist companies such as XERVON Austria which specialises in finding solutions for such major construction projects.