Case Study

Cable Stay Bridge for the Dee River Crossing

Much of the structural analysis work for the new Deeside road link and river crossing in North Wales carrying the A548 Shotton to Connah's Quay by-pass was conducted by UK-based Gifford and Partners using LUSAS Bridge. Pont Sir Y Fflint is believed to be the largest asymmetric cable stayed bridge in Britain.

By promoting the Deeside road link the client Flintshire County Council were interested in regenerating the region around the Deeside industrial park and in relieving the congestion around Shotton and Connor's Quay. Peter Curran, Gifford's project engineer for the bridge explains, "Flintshire County Council were, quite rightly, very sensible about environmental and aesthetic issues so we tried to design a bridge that was both cost-effective and aesthetically pleasing. The Royal Fine Art Commission has commented favourably on the design". The attractive asymmetric cable stayed design carries a dual carriageway across the Dee river. The 118m tall A-shaped tower supports an asymmetric 294m long main span with a cast-insitu concrete deck.

LUSAS was used for two areas of the overall structural analysis. The design team conducted a 3D linear stress analysis using shell and beam elements to check the stress levels under various dead and live load combinations. They also ran an eigenvalue analysis to look at the modes of vibration for subsequent use in wind tunnel tests. As Peter Curran explained, "It is extremely important to ensure that the vertical and torsional vibration modes are fully separated - otherwise instability will result and you get the Tacoma Narrows effect". A seismic analysis was also conducted to look at the expected response of the bridge during an earthquake.