The design had to overcome overcome challenges of the terrain and the programme schedule.
The single-carriageway post-tensioned Doro viaduct is just 9.5m wide but forms an important part of the realignment of the E39 Harangen-Høgkjølen route in the Trondheim region. It is being built for the Norwegian Public Roads Administration and the formwork and shoring for the project has been supplied to concrete contractor Sobstad by local formwork specialist supplier, Hansmark.
RMD Kwikform’s engineering team was able to combine existing stocks of Rapidshor shoring and Superslim soldiers with the heavy duty Megashor shoring and R700 trusses, for a complete system for the viaduct.
It was essential to ensure that Sobstad could complete the single pour during August and so the RMD Kwikform team had to make sure the right amount of equipment was shipped to site in Norway for start of erection works in March.
In order to achieve this, a 3D CAD model of the viaduct was created using information received from the site. This enabled very detailed design drawings to be produced, allowing accurate erection of the equipment, with automatic equipment take-offs.
A total of three spans, measuring 29m, 35m and 29m, required shoring support. The main engineering challenge was to span an existing river running through the Doro valley. Fortunately, the width of the river only required a clear-span falsework solution for one of the three viaduct spans.
To support this first 29m span, RMD Kwikform designed a clear-span solution using five pairs of 25m-long R700 trusses, each weighing 11 tonnes. One end of the trusses was supported by a six-legged Megashor tower, which was mounted on the pier base, and the other end sat directly on a foundation near the abutment. Loadings from the trusses were transferred to the Megashor tower by an 11.5m-long header beam supporter by three 3m distribution beams.
The five R700 trusses were then craned into position, with steel Superslim soldiers then mounted on top of the trusses. A partial access platform was installed on top of the trusses to allow for the installation of a Rapidshor steel shoring birdcage arrangement, some 3.5m tall. By combining Rapidshor U heads at the base and top of the legs with bracing supports, the birdcage-style shoring structure was then able to support another layer of Superslim soldier beams and timber secondary beams for the bridge deck soffit.
The additional second and third spans were supported by Rapidshor birdcage arrangements from the ground, varying in height from 8m up to 14m. The complete system enabled the viaduct, with its 4.2% deck incline, to be cast in just one continuous pour.
Odd Inge Fremo, from Hansmark said: “The whole site was very exposed and so the solution we came up with needed to not only support the loadings pressures from the single concrete pour, it also needed to be robust enough to cope with high winds. With the truss arrangement we wanted to minimise the amount of equipment needed on site, whilst being able to pre-assemble units next to the structure. With the Megashor tower we were able to lift sections into place, sequencing the build and similarly, because the R700 trusses were kept under the capacity of our site crane, we were able to simply manoeuvre them into place with minimal effort."
The system also proved straightforward to extract. With the use of guide plates on the main header beam, the site team was able to simply move the trusses to one side, swinging them out safely using the crane. “This saved both time and money,” he said.
In addition to supplying the equipment for the single deck pour, RMD Kwikform also engineered Rapidshor access towers used to construct bridge piers that would eventually support the viaduct.
Commenting on the project Adam Fixter, RMD Kwikform business development manager said: “Prior to the Doro viaduct project we had supported Hansmark AS with Rapidshor solutions for the construction of the smaller Hallstein and Graven bridges on the same highway improvement programme. So when it came to helping them tender for the much larger Doro viaduct, we were well placed to combine existing equipment stock from previous projects with the additional heavy-duty shoring equipment required. This reduced the overall cost of the project, making the RMD Kwikform and Hansmark AS solution both competitive and achievable in the tight timescale.”