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Into the valley

Rheola - A new bowstring arch bridge symbolises the change that a relief road should bring to a former Welsh mining town, says Andrew Mylius.

Sarah Chapman, owner of Porth's thriving Cafe Hot, is reserving judgment on the bowstring Rheola Bridge, the signature structure of the Welsh valley town's new relief road. 'I can see it from my bedroom window. It dominates the view, ' she says. 'Did it have to be so big, I wonder?' Well, yes, it did, according to the client for the road, Rhondda Cynon Taf County Borough Council, and design-build contractor Costain.

The 150m-span structure crosses two rivers - the Rhondda Fach and the Rhondda Fawr - the Cardiff to Treherbert railway line, and the busy A4233. There is no clear ground between the bridge's abutments for intermediate piers. The client's purse precluded the use of a cable stayed or suspension bridge solution. In any case, 'they would have been over the top for this location', says Costain highways director Darren James.

A heavier, common-or-garden bridge solution with a deep box girder or truss could have been used, avoiding the need for arches, James admits. But Rheola Bridge soars close to the beer garden of the Rheola pub. Other buildings press hard up against the abutments in this steep-sided valley. Client and contractor wanted to give locals something of beauty rather than a bog-standard highway bridge.

'The Rhondda Fach has become a lot more attractive in the two or three decades since coal mining ceased, ' says James. Vegetation has greened the mountainous spoil heaps.

'But on a wet day it can still be pretty grim.' Rheola Bridge is quite a bit more elegant than it was at scheme design stage.

Rhondda Cynon Taf with designer Glamorgan Engineering Consultancy (GEC) had xed an alignment for the 5km long relief road and decided on a bowstring arch for the route's main bridge.

But, says the client's head of construction Mark Adams, the scheme was not affordable.

Costain was initially taken on pretender and pre-planning stage to look for savings. It managed to shave £7.1M off the outturn cost by tweaking the road's vertical alignment, reducing the number of retaining walls from 50 to 25.

Eliminating the number of bridges from 13 to 11 by adjusting the trace lopped off another £2.9M, redesigning and rephasing roadworks saved another £1.9M, and £3.8M of 'miscellaneous' savings were made. Alterations to the route's structures, including the Rheola Bridge, delivered £2.2M. Value engineering brought total costs down from nearly £103M to £85M, of which construction represents £60M.

James says that with Arup and steelwork fabricator Faireld Mabey, his team removed cantilevered edges from the deck. The grillage of beams supporting the deck was simplied by getting rid of secondary longitudinal members.

Composite decking would be laid on top of transverse beams spanning between the bridge's two longitudinal beams, which were themselves made lighter.

Transverse steelwork would be in weathering steel to avoid the need for steel preparation and painting. Hanger connections to arch and deck were simplied, cross-members between the arches were stripped out and the bridge's abutments were redesigned, with reinforced earth ramps replacing reinforced concrete. Savings on the bridge alone amounted to £660,000.

'We had to submit our reworked version of the bridge to the Design Commission for Wales for approval. It commended [the design] for improving on the original with a 'simpler design language', ' says Ian Walsh, GEC client representative.

Building the bridge in such a tightly constrained environment dictated that it had to be erected insitu. Assembling it off line and then craning it into place was a technical non-starter. Constructing it on one of the approach ramps and push launching was also quickly discounted. 'Temporary intermediate support, which you'd need for a push launch, would have to have been designed to resist lateral loading, ' says James. 'As it is, we decided to erect it insitu. The trestles propping the deck are simple supports.' Even though the bridge has been pieced together from relatively light, manoeuvrable elements, lifting them into place has been a tense experience, notes Costain works manager George Black. 'When we were placing the arch sections people came to watch. There was a crowd drinking in the pub garden, ' he says.

Each arch was assembled from three pieces that were supported on temporary steel towers until connections had been welded. Unusually, the arch box sections and longitudinal deck beams were delivered to site pre-painted, leaving Costain the task of blasting and painting only the welds insitu. Conventionally the entire structure would have to be painted once complete.

'There were concerns that the patch painting we'd carry out would be conspicuous, ' says James. Paint tests convinced all parties that the new paint would blend seamlessly with the rest, though.

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