South Road overpass investigation report released
02 March 2017
South Australian Government: The State Government has today released a report by consulting engineering firm Aurecon into what caused the damage to the structure of the pedestrian and cycling path over South Road at Glandore.
The report found that the incident can be attributed to a failure in the design of the pedestrian and cycling path.
The path 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 Engineers.
Copies of the report have been provided to the consortium of companies who designed and built the path. They will now have an opportunity to respond, after which the State Government will work through the report’s findings and deal with issues of liability.
Separately, Aurecon has also been tasked with designing a permanent solution for the pedestrian and cycling path, as well as to conduct a comprehensive review of the Government bridge maintenance and inspection regimes.
The report’s findings
The report found that the incident can be attributed to the failure of the design to adequately allow for out-of-balance effects. This resulted in girders tilting sufficiently to render the bearing restraint system ineffective and essentially free the bearings, allowing them to “walk” out of their design position.
The report states: “Once the bearings were free to move, 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.”
Aurecon has advised that other parts of the pedestrian and cycling path are exhibiting signs of this phenomenon, with noticeable bearing deformation, girder rotation and cracking of the keeper walls. Aurecon considers that there is no cause for concern over a similar incident occurring to the girders of the adjacent tram bridge structure, which are separated from and act independently of the shared path.
The report makes four recommendations to minimise any further damage to the cycling and pedestrian overpass and recommend what is required to return the path to full working order:
The anti-throw screens should be removed from the bridge, in order to reduce the lateral loading on the bearing system
A monitoring system of all spans should be set up to measure further ongoing movement, and alert the need for appropriate action
A design of remediation measures should be undertaken to all spans. As part of the remediation, all girders should be restored to their original design location by means of jacking. The design shall incorporate an appropriate lateral restraint system of sufficient capacity to reinstall the anti-throw screens and restore the functionality of the structure in accordance with the relevant Australian bridge design standard
Repair the damaged concrete headstocks should be undertaken, along with upgrade and replacement of failed outer keeper walls at all piers.
Now the cause of the fault has been identified, works to remove the anti-throw screens on the northern side of the overpass and the development of a design solution to return the cycling and pedestrian path to full working order will commence immediately.
The cycling and pedestrian path, and the anti-throw screens, were always included as part of the project detailed in the Department of Planning, Transport and Infrastructure’s 2008 Submission to the Public Works Committee.
On 18 January 2017, Department of Planning, Transport and Infrastructure became aware of a girder dislodgement of the cycling and pedestrian overpass.
As a result, South Road was temporarily closed to traffic while the structure was stabilised.
On 23 January, the State Government appointed national and international consulting engineering firm Aurecon to complete a thorough and comprehensive external review of the South Road overpass.
Aurecon, which had no involvement with the construction of this overpass, was commissioned to conduct the following work:
A detailed investigation into the cause of the incident.
Development of a proposal for the remediation of the shared path bridge.
Conduct an independent review of the DPTI bridge inspection processes. The deliverable for this scope item will be a report that assesses DPTI’s approach to periodic bridge inspections, in the context of best industry practice.
This report summarises the findings of the investigation into the cause of the incident, and includes recommendations on appropriate actions to repair and safeguard the structure.
Aurecon, which has a significant presence in South Australia, has more than 50 years of experience in all forms of bridge design, including large span bridges, rail and road bridges, and bridges for cyclists and pedestrians.
Quotes attributable to Transport and Infrastructure Minister Stephen Mullighan
I am pleased to have received such quick, thorough and independent advice about such a concerning issue with this cycling and pedestrian path.
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.
During day works, South Road will remain open in both directions at all times, and access to businesses and properties will be retained at all times. Any works which require lane closures or through traffic to be held up will occur at night.
Traffic may be held up for up to 15 minutes on up to nine occasions during night works, which will occur between 9pm to 6am later in the coming week.
The shared path remains closed and access points have been fenced up with increased security to be installed once the anti-throw screens are removed. In addition the overpass has extensive CCTV which can be used to monitor the path.
Source: South Australian Government - www.premier.sa.gov.au
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