Engineers are to review 1,500 bridges in South Australia after a report identified the design flaws that led to a tram and cycleway overpass bridge over a busy Adelaide road being closed.
A displaced beam was noticed on the overpass section of the bridge on January 18 and the road was temporarily closed while it was stabilised. The bridge carries a tram line, with a pedestrian and cycle path incorporated along one side.
The overpass deck was designed to run along a t-shaped beam, which was designed to sit inside two vertical supports resting on cross beams which support the whole bridge structure. The t-shaped beam rests on a bearing which is supposed to accommodate movement. The overpass part of the structure also includes a vertical metal screen.
The South Australian government commissioned consultants Aurecon to investigate the incident. Aurecon’s report says that wind induced movement caused the overpass structure to move and shuffle the bearing out of position.
The report said: “We believe the mechanism for the movement is due to cyclic ongoing wind load effects on the anti-throw screen, coupled with a constant lateral force due to out-of-balance self-weight effects, promoting a caterpillar movement of the bearing. Lateral movement would then have occurred incrementally over a period of time, until the bearings had moved sufficiently for the girder to lose support.”
South Road Overpass
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The bridge was built under a design and construct contract with the Government in 2009 by McConnell Dowell, and was designed for McConnell Dowell by engineering firm Aecom, and certified by Wallbridge and Gilbert Consulting and Engineers.
Aurecon has also said that other parts of the pedestrian and cycling overpass are showing similar signs, with noticeable bearing deformation, beam rotation and cracking of the keeper walls. However there is no evidence that the tram line section is affected and its beams are independent from the overpass.
The report has four recommendations which are:
The anti-throw screens should be removed from the bridge, in order to reduce the lateral loading on the bearing system
All spans should be monitored for any further movement
Remedial design should be undertaken on all spans and all beams should be restored to their original design location by means of jacking.
The damaged vertical concrete supports for the t-shaped beam which carries the cycleway should be repaired
South Australia transport and infrastructure minister Stephen Mullighan said: “I have been reassured from the advice I have received that the problems with this bridge are not expected to be experienced elsewhere as the design on a single concrete “Super-T” structure and the supporting bearing arrangement is a design that has not been used elsewhere on our other bridges.
“Nonetheless, I have ordered a full review of all of our 1,500 road and rail bridges to ensure that the public can remain confident in these structures.
“DPTI will immediately commence implementing Aurecon’s recommendations, with works to remove the anti-throw screens to start today. A design solution to return the cycling and pedestrian to full working order will also commence immediately.
“Works to remove anti-throw screens will be managed to minimise disruption to motorists who use South Road and to businesses and residents located around the tram overpass.”
A statement from McConnell Dowell said: ”Aurecon have tabled thier report following thier investigation into the technical issue at the South Road Overpass. In conjunction with our design engineering consultants, Aecom and Wallbridge & Gilbert, we continue to review Aurecon’s report after receiving a copy this morning.
”As long-standing, well-regarded companies with a strong track record in designing and constructing major infrastructure projects in South Australia, we regard the movement experienced with the Shared Path Bridge as a matter of significant concern.
”We are committed to continuing to work with the Department of Planning, Transport and Infrastructure to find a safe, permanent solution.”