Transmission Gully’s Bridge 20 is one of the project’s most significant structures.
According to NZTA Senior Manager Project Delivery Chris Hunt, Bridge 20 is a showcase of technical design and innovation.
“The design and construction of Bridge 20 is significant. The technical challenges associated with designing and building a long, high, curving bridge that is aesthetically pleasing yet able to withstand a 1 in 2500 year seismic event in an area of high seismicity require an innovative approach”.
Chris adds further challenges include:
Standing 60m above the stream below, Bridge 20 is the largest structure in the entire Transmission Gully motorway project. The finished bridge will be four lanes wide, 226 metres long and 24.5 metres wide. A total of 44 steel girders, each 1.8m wide and 3m deep make up the framework of the bridge deck which is supported by two abutments and two piers. Over 10,000 cubic metres of concrete will be used to build the bridge and all of its components – equivalent to more than four 2m deep Olympic-sized swimming pools.
The challenging geographical features of the site, in particular the steep sides of the gorge and the need to span a large distance, mean that launching is the most effective process for installing the bridge deck structure. Launching is where a pre-assembled bridge frame, in the form of steel girders, is “launched” across an open space to land on the bridge piers. This bridge launch is being undertaken in four phases and will eventually see the bridge reach the abutment on the other side of the gorge.
Phase 1 is the much-anticipated initial launch, after months of behind-the-scenes work to prepare the bridge for launching. Girders have been assembled in the launch yard, ready for the custom-made launch nose, pulling frame and precast panels to be attached. Prior to the brake being removed, the launch nose will be lifted, and the bridge will move from trestles onto rockers allowing the back end of the girders to be lowered to lift the nose, and the entire structure will slide effortlessly across onto the first pier - only to be repeated three more times.
The entire four-phase launching process is expected to take around ten months. Each phase will see tail assemblies removed and more girders added to the structure before the launching process continues, allowing the structure to safely reach the far side.
Watch an animation of the launching process.
Watch a timelapse of the construction of Bridge 20.
Watch a timelapse of the launch of Bridge 20 over Cannons Creek.
A good public viewing spot is from Belmont Regional Park a short (steep) walk from the regional park carpark located near the top of Takapu Road.