Case Study
Dee River Crossing
At the time of its construction Pont Sir Y Fflint
was believed to be the largest asymmetric cable stayed bridge in Britain.
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.
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.
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