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RHYMNEY SOLDIERS

Building a reinforced concrete arch to support a road deck 24m above the valley floor in Mid Glamorgan, South Wales needed more than really tall scaffolding. Jessica Rowson and Ruby Kitching report.

When the River Rhymney Bridge opens in June it will form part of the £25M Angel Way, a new single carriageway road that will connect communities and businesses in Caerphilly, South Wales.

The new road runs through the recently designated Bargoed Country Park via a 140m span concrete arch bridge over the River Rhymney. The arched bridge design was chosen by Caerphilly Council to suit the contours of the surrounding area.

Joint venture contractor Hochtief Griffiths started on site in March 2007 and the last remaining elements of formwork will be dismantled in the next few months.


Click here for diagram

The arch soars 24m above the river at its crown, but achieving this height wasn't the main problem to overcome during construction.

"Normally we would be dealing with wall loads or slab loads," explains SGB Formwork's engineering director, Stuart Bamford. "Here we are dealing with interaction of the vertical and horizontal – that in itself is challenging. The arch and sloping soffit throw tricky loads into the formwork."

SGB Formwork worked closely with Hochtief Griffiths to derive a method of work that allowed SGB to take full advantage of the arch/formwork interaction.

By looking at the proposed pour sequence, SGB devised a method that involved tying the formwork in initial pours so that the weight of later pours would not pull the formwork arch out of shape.

"The load from one pour would cause uplift in an area not yet poured," explains Bamford. "We had to anchor it down to maintain shape as loads from one side would squash the other."

When the wet concrete is poured onto the arch formwork, it exerts both a horizontal and vertical load on the formwork.

The vertical load could be taken down to the ground with a standard aluminium shoring system. However, SGB had to be more inventive and create a system to resist the horizontal loads.

It introduced horizontal props in the formwork structure at four points on the arch, symmetrical about the apex. The lower level of horizontal props were tied to the arch and designed to be cast into early concrete pours. So when the horizontal forces were transferred to these members, they were transferred back into the arch in tension.

The upper horizontal support points were provided by a single continuous member, which spanned the width of the arch.

SGB then propped the curved soffit of the arch off these horizontal members with diagonal "soldier" elements. The vertical component of the load was transmitted down the soldiers to the standard shoring system and the horizontal load was transmitted via the soldiers to the horizontal members.

SGB used finite element analysis software, Robot.

"For every pour, we ran a frame analysis," says Bamford. We had to make sure the load going back into the arch at each stage was acceptable."

WHO'S WHO
- Client Caerphilly County Borough Council
- Structural engineer Capita Symonds
- Main Contractor Joint Venture Hochtief Griffiths
- Formwork supplier SGB Formwork
- Funding body Welsh Assembly Government

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