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Truss aids Bath suspension bridge repair

The main span of Victoria Bridge in Bath was completely demolished and reassembled while the structure remained open.

In 1836, beer brewer James Dredge faced the problem of how to get his beer across the river Avon near Bath without going through the city centre. His solution was to design his own bridge.

Suspension bridges built around that time were built with a series of wrought iron bars connected by pinned joints for the “cables”.

However the heavy weight and amount of iron required made the bridges expensive to build. Dredge came up with a new idea.

Victoria Bridge

Span solution: Feeding a truss through the bridge piers enabled contractors to keep the bridge open as reconstruction work progressed

He designed a bridge which looked a lot like a conventional suspension bridge, but instead was more like two cantilevers supported by cables mounted on piers at each end - akin to a modern day cable stayed bridge.

Dredge’s design inclined the hangers instead of placing them vertically, so the load in the hangers could be resolved into compression in the deck and tension in the suspension chain.

These forces gradually decreased towards the middle of the bridge allowing Dredge to reduce the number of bars in the chain - in turn reducing the weight and consequently the cost. He called this design the “tapered bridge” principle.

Bath’s Victoria Bridge was the first of 50 of this type to be built. It spans 45m with 6.4m high arched masonry piers at either end supporting a “cable” of wrought iron chains, with hangers carrying the 5.8m wide deck below.

Victoria Bridge

Deck removal: Runway beam is used to transport dismantled deck section

However there was a design flaw. Dredge arranged the inclined hangers along each chain in pairs, with a single connecting point at the top, but where they connected to the deck, the pairs were separated and staggered.

This caused a problem, and as a load passed over the bridge, one of the hangers would go into tension, with the other forced into compression.

The elements were not designed to take compression, so all of the load would then be transferred into the tension hanger, overloading it.

During a survey of the bridge by Bath and North East Somerset Council in 2011, cracks were found in the wrought iron hangers and connections.

Victoria Bridge

Temporary truss: Allows replacement of staggered hangers and deck

A regular programme of monitoring revealed that the number and size of cracks began to significantly increase during the autumn and winter in 2011.

This led to the imposition of a loading restriction on the bridge and the implementation of emergency temporary works - props to reduce the span, and hanger straps to strengthen it. But the cracks continued to worsen and the bridge was closed to the public for safety reasons.

The solution to re-opening the bridge while carrying out the vital restoration work lay with Mabey Hire. Working with Galliford Try, Mabey Hire installed a 54m long panel truss across the river between the 3.5m wide arches within the masonry piers

“The task was to support the bridge deck in a way that allows the contractor to demolish it but also keep the river course open,” says Mabey Hire technical director Dave Holland.

“This was a semi-emergency job. It had to be developed, approved and installed in the blink of an eye”

Dave Holland, Mabey Hire

“But we had to span something almost 60m which was stiff enough, robust enough, with that sort of slenderness to go through the arches and also overcome the torsional effects.” In just six weeks the team designed and installed a bespoke truss which modified its standard universal panel trusses with new arrangements of bracing to keep it as light and strong as possible.

“A big part of it was the pressure of time,” says Holland.

“This was a semi emergency job. It wasn’t something which could be planned for a long
time, it had to be developed, approved and installed in the blink of an eye.”

“The task was to support the bridge deck in a way that allows the contractor to demolish it but also keep the river course open ”

Dave Holland, Mabey Hire

But installing it across the river was no mean feat.

The truss was launched from one side using hydraulic winches to provide the finite control necessary. It was cantilevered across the river to span onto a grillage of screw piles on the opposite bank. But the team was unable to use a tail or a backspan to balance the truss while it was cantilevered into place because of the steep incline in the road behind the bridge. Instead, counterweights were used to balance the load as it was jacked into place.

Victoria Bridge

Long span: The truss eliminated the need for supports in the river during the refurbishment programme

Guiding the truss

“We had to keep the nose very light, but a clever bit really was that we had to support the bridge in a way that wouldn’t stress the bridge,” says Holland.

Once the truss was in place, a footpath was installed within it and the bridge reopened to the public. Then there was the challenge of removing the existing bridge and rebuilding it.

“Temporary works is superb,” says Holland.

“You’ve got to have the foresight to think about what you will be supporting, what you’re going to do, what the final condition will be - you’ve got to think about the whole process.

“We didn’t know exactly how they would demolish and rebuild the bridge, but we gave them the options with something with enough redundancy for them to build from.”

At this point, Balfour Beatty stepped in. It designed and installed the temporary works to support the demolition phase and start removed the existing bridge around the truss.

Reduced cable tension

First, it hung the existing structure from beams it had installed across the top of the truss. This reduced tension in the “cables” allowing them to be cut sequentially without the chains springing back.

Once the hangers were removed, the deck could be demolished in sections. Working above the river meant the team had to devise a method to cut and transport the sections safely to the bank.

“Once we’d taken out all the hangers and the chains out, all that was left was the deck,” says Balfour Beatty senior temporary works engineer Tom Newton.

“It was over the river so that’s wasn’t particularly easy, so we installed a runway beam all the way along the underside of the truss.”

Deck sections were then cut out and moved along the runway beam using a small crane which then lowered them down onto the tow path.

Cable replacement

With the existing bridge demolished, the team then started the process of replacing the bridges cables, hangers and deck. Although the structure is grade II* listed, the team was allowed to modify its design, replacing the staggered hangers with pairs of parallel ones and new materials which are stronger and lighter.

“They’ve maintained Dredges’ concept but not brought along the structural problems,” says Newton.

The final piece of work was to remove the truss from the now complete bridge. Balfour Beatty did this, working closely with Mabey Hire and permanent works designers Aecom.

Removing the truss meant breaking it into sections, putting them on the restored deck and then removing them.

Dismantling the truss

“The worst case loading [for the permanent structure] was the dismantling of the truss, as the load of the truss was more than that of the pedestrian loading,” explains Newton.

“So we worked really closely with the permanent works designers to understand what the loads would be.”

All in all the truss solution allowed the bridge to remain open for 90% of the construction time.

A Twitter feed was set up by Balfour Beatty project manager on site Gareth Harris to give up to date accessible information to the local community about when the bridge would be closed.

“The truss was the whole back bone of the whole project,” says Newton.

“The truss was there to hold up all of the bridge, not just for people to walk through - it held up the whole bridge. It was integral.”

The project cost £3.4M and was completed in December 2014. Its innovative use of the truss and temporary works won the project a place on the British Construction Industry Awards 2015 Temporary Works Award short list.

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