Method Led Design - A249 Sheppey Crossing, Kent

This is a 1.2km long viaduct constructed at high level over an environmentally-vulnerable tidal estuary that was ideally-suited to the installation of a primarily offsite-manufactured steel composite bridge deck using launching. The steelwork was designed to sustain the effects of the launching process that was conducted in three separate zoned phases in order to accommodate a launch path following curves in both the vertical and horizontal planes. At the main span over the Swale the launch freestanding cantilever was 93m long and landed well within the tolerances predicted by our analysis. In addition to the launch analysis our design remit was developed to suit the construction method as follows:

• Pile layouts and pile-cap proportions were arranged to minimise excavation in soft estuary ground
• Twin-column piers were designed to withstand frictional effects of the steelwork launch
• Columns were shaped to include permanent ledges for mounting temporary crossheads at the top
• We designed temporary steel cross-heads for mounting of the rocking skid support stools and side guides