Initial page for our documentation of the saving of the historic Hampden Bridge in Wagga Wagga. (Technical details in the form of a paper are being produced and will be posted later).

We use the term saving as when we were commissioned by the Wagga Wagga City Council for inspection and advice on what to do with the bridge, all the experts who had investigated the Hampden Bridge indicated that the bridge was unsafe and needed to be demolished.

The bridge had been superseded by a modern concrete bridge a number of years ago and the Hampden Bridge was left in its original state, as when it was made redundant. In 2007 the bridge suffered a major partial failure due to one of the main timber chords failing due to extensive attack by white pocket rot and the photographs show the failed area 0.5 meters below its original profile.

The top deck view of the bridge indicating the partially failed span in 2007.

Side view shows the dramatic collapse

Reference picture

Background

The Hampden Bridge was designed and constructed by P Allan in 1897 and at the time it was the longest timber truss bridge in the World. There are longer in existence to date though one is a foot bridge the other a composite steel/timber truss. Allan gave a paper about its construction to The Institute of Civil Engineers UK. We have a copy of P Allan’s original presentation to Institute of Engineers UK in our papers page. Of note is the fact that the bridge was designed for a 16 ton axle load by P Allan in 1897!

See www.integritytesting.com.au/papers
The failed chord indicating the approximately 0.5 meters of white pocket rot which caused the partial failure of the bridge and the initial cleaned section.

Integrity Testing Pty. Ltd were commissioned to give advice to the client on what they should do, either demolition or preserve the bridge. Initially Integrity tested the bridge using their DBTS™ system to measure the remaining capacity in the bridge. The results showed there was sufficient residual capacity to facilitate a fix. The method of the fix was to replace the stumps in the shoes with steel section rather than replace the whole failed chord. There was also the problem that large hard wood timber was scarce.

Capacities

The table of capacities gives gross vehicle mass for three types of trucks and their principle wheel configurations. The method employs an equivalent beam method to calculate the present capacity of the bridge. Effectively the truss acts as a large beam with the top and bottom chords acting as flanges to this equivalent beam. The DBTS™ method is extremely accurate with a five order of magnitude accuracy better than the conventional static load testing.

Span	EI N.m^2	Total Load KN	Moment Capacity KN. m	B-double GVM Triple or Quad Axles	T-44 GVM Tandem Axles	Rigid Truck GVM Single axle	Pedestrian Traffic As a Crowd
1	1.05 x 10^10	300	1080	N/A	N/A	N/A	18% *
2	1.68 x 10^10	1180	4970	66t	44t	18t	95%
3	1.50 x 10^10	920	3890	52t	44t	18t	75%

Budgets for this work was limited, but luckily Wagga Council were able to procure a launching beam, Integrity recommended preservation and by means of modifying the beam launching frame (Used to launch the beams for the new Hampden Bridge) with which the bridge was restored to its original line and level.

View of re stumped section

Eventually the steel boxed sections will be covered in a timber veneer.

The bridge after completion of the lifting showing the lifting frame and the new level line of the bridge.

The bridges line and level restored. See along the line of the hand rail.

The soffit of the bridge indicating how much the deck was raised and that the deck was not relying on the support frame.

The aim of the works was to make the bridge safe, through ingenuity, value engineering with sound Engineering practices the Bridge is now considered safe and the client is investigating ways to fully preserve the bridge as a tourist attraction.

Our gratitude and thanks go to the staff of Wagga Wagga City Council and in particular Lindsay Tanner and Bryan Short who had the faith in us to save the bridge. It was jointly their management of the project and Integrity Testing’s engineering input in particular its Director John Higgs that saved an historic bridge, with the costs being considerably less than half of the cost for demolition.

Copy of original Drawing

More info can be found at www.integritytesting.com.au and follow the link to contact us.