|Department of Engineering|
|Structures Research Group|
|Engineering Department > Structures Group > Teaching > Prestressed Concrete (4D8)|
|This work was undertaken by Richard Scantlebury, under the supervision of Chris Burgoyne, as his 4th year undergraduate project. Richard now works for Benaim.|
|This web page provides background information and web links to support the paper "Why did Palau Bridge Collapse?", which appeared in The Structural Engineer, published by the Institution of Structural Engineers in London. The paper was awarded the Husband Priize of the Institution in 2007.|
The collapse of the Palau Bridge in 1996 received considerable attention at the time, but there has been very little reported in the literature about the investigation of the collapse mechanism, partly because of a legal agreement between the parties involved. This paper has been prepared from publicly available sources to ensure that the wider structural engineering community learns something from the failure. Since the collapse occurred soon after a repair to the bridge, it has been widely assumed that the repair was the cause of the failure, but it is shown that this is very unlikely. Instead it is concluded that lack of robustness in the original design meant that the structure had always been vulnerable to accidental damage, which eventually occurred as part of the resurfacing works.
The name Palau (or Belau) refers to a group of about 350 small islands (centred at about 134 deg 30'E and 7 deg 30'N) at the western end of the Caroline chain in the Western Pacific. The islands are about 900 km both from the Philippines and New Guinea. Palau passed from German control to that of Japan after WWI; the chief industry then was the exploitation of phosphate deposits. The islands were of strategic importance to Japan in WWII, especially after the fall of the naval anchorage at Truk. A fierce battle was fought for one of the islands (Peleliu) in September 1944 but most of the islands were left in Japanese hands (although blockaded), until the general surrender at the end of the war. The islands remained part of the US Trust Territory for the Pacific Islands but are now independent, although they retain close ties with the USA. The total land area is only 494 sq km and the population today about 20,000. The economy relies almost completely on tourism.
The bridge was designed to meet the need for a link between the two major islands of Palau; Koror and Babelthuap. The latter contained the country's international airport and was the source of most fresh water but approximately 70% of the population lived on Koror, where the capital is situated. The channel between the two islands is about 30 m deep with tidal flows of up to 3 m/s and steep banks, which is why a single 240 m span (then the longest concrete girder bridge in the world) was chosen. It was completed in 1977.
The failure occurred on 26th September 1996, at around 5:45 in the afternoon. The collapse was catastrophic, killing two people and injuring four more, and occurred under virtually no traffic load during benign weather conditions. Services passing through the bridge between the country's two most populated islands were severed; this caused the government to declare a state of national emergency and request international aid for the thousands of people left without fresh water or electricity.
In the nine years since the collapse, there has been speculation regarding possible causes of failure, and remedies that could have avoided it. Litigation and out-of-court settlements between the Palauan government and the engineers involved have meant that the cause was never officially confirmed, and analysis performed on site was never released. Only one paper has been presented based on the site investigations and the true cause remains unreported.
The present study was undertaken to ascertain whether there is something fundamentally wrong with the way prestressed concrete is understood, and in particular whether it should be taught differently in the light of what happened. The authors are not associated in any way with any of the companies involved and have had no access to any confidential information; everything presented here has been derived using information already in the public domain. The objective has been to undertake simple approximate analyses to determine the magnitude of various effects that might have happened.
Photos of the collapsed bridge are available on Mark Ketchum's page, and also here.
Two Virtual Reality Models of Palau Bridge are available. Both require
a VRML2 viewer to be installed as a plug-in to the web browser. A
Viewer is available and the images have been tested under Mozilla.
No guarantee is given that they will work in any other viewer but the files
conform to the VRML2 standard which is widely used.
|This model has various views of the bridge
at various stages of construction. The viewer can go straight to
the different views by using the predefined viewpoints. The one illustrated
to the right shows our best estimate of the bridge shape after creep and
immediately before the repair.
The user is free to move around the various images using the VR navigation controls. Click on the image to load the files.
|This model shows the bridge, as built, against a map of the locality. There is only one viewpoint but the user can move around the scene. Click on the image to load the files.|
The image of the warship is shown to give a sense of scale to the bridge. It is a model of HMS Petard, a WWI Royal Navy Destroyer, a VRML model of which is available for download at Trenches on the Web site. The ship is displayed to scale but would never have sailed under the bridge, being 40 years too old, and in fact its mast had to be lowered by about a metre to fit under!
The map in the second model is taken from the 1:25,000 scale map on the University of Texas web site.