In what was thought to be the one of the largest falsework contracts ever awarded in the UK, RMD Kwikform supplied over 52,000m3 of Rapidshor support system to the Deeside Road Link contract in North Wales.

The contract, being carried out by a joint venture comprising Norwest Holst, Kier Construction and Campernon Bernard SGE, covers the construction of approach roads and viaducts, leading to an ëA' frame, single leg cable stay bridge over the river Dee.

At the core of the scheme design is the 117.4m high precast ëA' frame support for the cable stayed bridge deck. This unique design is made up of 88 precast hollow concrete segments, 30 units up each leg of the ëA' and 28 to the top of the spire, all sitting on barrette pile foundations.

The 3.985m x 5.515m concrete segments are manufactured in an on-site casting yard, by the JV using RMD Kwikform wall formwork, with a combination of Alform Beams and Super Slim Soldier sections. The segments are 2m high, with an average concrete thickness of 150mm and are being crane lifted into position.

The temporary works supplied by RMD Kwikform were broken down into 3 specific areas:

•	The support and formwork for the two anchor spans to the intersection with the ëA' frame support
•	The Access Scaffolding up each leg of the ëA' frame, to allow accurate positioning and placing of each precast concrete segment
•	Support for the tie beam and bridge deck, between the two legs of the ëA' frame

The bridge deck is 27.86m wide and 1.50m deep, with an average propping height of approximately 16.3m. It will be cast in four pours, in a length totalling 113m ñ including its intersection with the ëA' frame. The total length of the cable stayed bridge is 294m, with 194m being actual cross river span.

The JV specified that on the ëA' frame, any access scaffolding must allow for the location and placement of the precast units and should incorporate a support system for a 3m deep tie beam, which is to be cast in situ between the ëA' frame precast legs. This tie beam, on which the road sections of the bridge deck will sit, will be cast in 3 sections, comprising the central core and two stitching pieces tied into the legs of the ëA' frame. This beam is 3m wide and approximately 33m long.

The Rapidshor system supporting the beam is generally arranged on a grid of 1.2m x 1.2m with a SWL of 80kN. The access scaffold around the legs is based around 2.4m x 1.2m grid. Where the scaffold is being used to support the approach road deck, it has been specially designed so that when the time comes for cable insertion, the outer leg of the support framework can be removed, allowing the cables to be passed through the deck and bolted into position.

The original tower construction scheme involved the excavation of an 8.4m x 68m x 6m deep cofferdam to site the foundations of the support legs. This was then to be backfilled allowing both the falsework for the deck and tie beam and the access scaffolding to be lifted from ground level. After consultations, it was decided that progress could be speeded up significantly if the support system started from the base of the cofferdam, allowing the filling to be done at a later stage.

This meant that the overall support system would have to span the 7, 700mm diameter temporary props supporting the driven steel sheet piling of the cofferdam walls. These props were spaced at 6m centres and had to be bridged using a split system featuring a short Super Slim beam and twin support leg.

Another difficulty for the scaffold designers was the fact that the bridge deck was planned to be cast through any support system utilised. RMD Kwikform's solution was to use 4 H33 girders, which clear span the bridge deck at a height of 36m, and are incorporated into the access scaffolding around the ëA' frame legs.

The H33 girders are 30.4m long and 2m deep. They consist of two girders braced together on either side of the ëA' frame. Tied with tubes, a Super Slim platform is positioned across the top. The advantage of this system is that it allows the access scaffold for the placing of the concrete ring segments to continue to be erected on top of the girders themselves.

The gap in the ëA' frame at bridge deck level also had to be sufficient to allow a travelling formwork gantry (21m wide x 8.4m high) to pass through. This would facilitate the cantilevered pours for the formation of the bridge deck over the river.

The major benefit of this type of support system is its flexibility. The use of Rapidshor in the lower levels of the cofferdam and in forming the arch through which the cantilever forming gantry can pass shortened scheduled construction programme time.

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MAGNA COMMUNICATIONS LIMITED
150 Magna Road Bournemouth Dorset BH11 9NB England
Tel: +44 (0) 1202 581756 Fax: +44 (0) 1202 582177
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