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Bridges special: Melbourne's West Gate Bridge

Melbourne’s West Gate bridge carries the busy M1 freeway over the River Yarra, connecting the Australian city’s central business district to its south eastern suburbs.

Commuter crossing

But since the M1 opened in 1978, daily traffic along this stretch has grown to around 160, 000 vehicles a day, prompting the A$1.39bn (£800M) Monash-CityLink-West Gate (M1) upgrade, which started last year. The work will widen the existing motorway in southern Melbourne from eight to 10 lanes to alleviate congestion. Critical to the success of the project is the strengthening of the West Gate Bridge and, with over half of the internal strengthening already complete, engineers are well on the way to the December completion date.

The 2.5km long crossing is divided into three sections - two concrete approach viaducts at either end of the five-span steel cable stayed central steel box girder section.

An alliance comprising highways authority Vic Roads, contractor John Holland, steel bridge consultant Flint & Neill and concrete bridge consultant SKM is carrying out the work.

No additional bridge width is needed for the widened road - there is an existing hard shoulder that cantilevers from each side of the central box girders, and which will be used for the extra lanes of traffic. But the existing structure must be strengthened so the bridge can take the extra load. This work must be done without affecting the operation of the bridge as the motorway must remain open throughout.
The key has been careful assessment of how close to capacity the existing bridge is to minimise the strengthening required, explains Flint & Neill Associate Chris Murphy.

“The best approach is to squeeze out every last available reserve of strength in the structure and do away with the need to strengthen in the first place,” says Murphy.

“The best approach is to squeeze out every last available reserve of strength in the structure”

Chris Murphy

To do this, Flint & Neill has used several different methods, adding strengthening as a last resort. The first of these techniques has involved extensive onsite measurement of the structure’s behaviour. Current bridge design rules contain assumptions about the straightness of members under compression and therefore their load bearing capacities.

Members with a bigger curvature have a lower strength as they are more prone to buckling. By measuring the straightness of compression members insitu Flint & Neill has been able to plug those values back into fundamental first principles calculations to work out the actual strength of the compression members.

“Actual structures have an extra reserve of strength when the actual curvature in them isn’t as large as the out-of-straightness curve assumed in the code,” says Murphy.

Measuring the straightness of these stiffeners was an operation in itself, as the inside of the box girder over the length of the bridge has many different configurations of stiffeners, all which needed checking to some extent.

“An army of people at the start of the job were crawling all over the stiffeners armed with straight edges just measuring the actual out of flatness that we could then put into all of our design calculations,” says Murphy. “This was carried out over three months - there were hundreds and hundreds to check. After we had checked a reasonable sample, we were then able to apply statistics to be able to predict actual out of flatness of all the others.”

Chequered past

Current bridge codes also have a degree of conservatism in them as they have been developed for ease of use and to apply to a wide spectrum of different bridges. In response to this and to reduce the need for strengthening as much as possible, Flint & Neill went back to what could be called the original steel box girder code, the Merrison rules. These were produced following a series of steel box girder collapses - including one on the West Gate bridge itself - in the 1970s (see box).

“The bridge has a chequered history,” says Murphy. “Construction began in 1968, but in October 1970, part of the steel box girder collapsed during construction, killing 35 people. It was Australia’s worst ever industrial accident to date.

This collapse occurred a few months after the Milford Haven collapse in Wales that triggered the Merrison enquiry. Box girder rules changed as a consequence, influencing subsequent British Standards.”

“The Merrison rules are predecessors to BS5400 (the British Standard for design of bridges). I would say BS5400 is a simplified form of those codes. In order to simplify the rules some conservatism also inevitably crept in.”

After extensive calculations and finite element modelling, a certain amount of strengthening was still needed in the 32-year-old bridge.
Props are being added to support the cantilever and angles to strengthen existing stiffeners and existing splice plates, which hold the sections of the bridge together. But fitting in the stiffening elements was no easy task.

Bolts and splices

When the West Gate bridge collapsed during construction, one span was already up and another was in the process of being fabricated. As a result, a lot of the original steelwork was re-used, but was strengthened and stiffened to comply with the new rules emerging from the Merrison enquiry, resulting in a congested box girder interior in which steelwork is bolted together rather than welded.

“The unusual fact about this bridge is that it is all tied together with bolted splices,” says Murphy.

“I would say design of all strengthening around the bolted splices has been the challenge of the design.

“The vast majority of all these splices had to be strengthened as well. Trying to increase the capacity on the live structure has required large amounts of lateral thinking. The easy way to strengthen a splice in theory is to add more bolts to it. If it is already jam-packed and there isn’t any room for more bolts, you then have to start splicing in alternative load paths on the top of it.”

“Trying to increase the capacity on the live structure has required large amounts of lateral thinking”

Chris Murphy

“To do this bolts from the existing splice need to be removed, the new strengthening plate added and then the whole lot tied down using new bolts. This operation is similar to the magician who whips away the tablecloth from the dinner table, leaving the bone china intact. Strength check calculations were done to show that a maximum of two bolts could be removed at any one time.

All the structural gymnastics are done in the office to make sure that site work can go as smoothly as possible.

“With strengthening, practicality is king,” says Murphy.

“First you have to devise possible strengthening arrangements that can be applied on site and then apply the design calculations onto the practical options. In most instances, the practical option is often the hardest to design as practicalities are king on these jobs, the engineering challenge is trying to get the practical option to work.”

WEST GATE LESSONS

Key link: The West Gate Bridge carries traffic acrosst the Yarra to the central business district

Two years into the original West Gate bridge construction programme tragedy struck on 15 October 1970 when the 112m span between piers 10 and 11 collapsed and fell 50m to the ground and water below.

Many of the 35 workers who were killed were on lunch break beneath the structure and the others were working on and inside the girder.

A Royal Commission set up by the Victorian State Government to investigate the collapse attributed the failure of the bridge to two causes; the structural design by designers Freeman Fox & Partners and the unusual method of erection adopted by World Services & Construction, the original contractors of the project.

The commission found that on the day of the tragedy there was a difference in camber of 114mm between two half girders at the west end of the span which needed to be joined. It was proposed that the higher one be weighted down with 10, 8t concrete blocks.

The weight of these blocks caused the span to buckle, which was a sign of structural failure.

Bolts inside the girder were loosened to help with the alignment effort, but these caused the bridge to snap back and the span collapsed.

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