Bridge analysis, design + assessment

Case Study

Pier Design on the Plashet School Footbridge

  • steel plate bridge pier
  • eigenvalue buckling analysis
  • optimised design

A 67m long footbridge links the two sides of Plashet School across Plashet Grove in Newham, London. Designed by structural engineers Techniker with architects Birds Portchmouth Russum for Newham Borough Council it is supported on sculptural steel piers that emerge as distorted fins from the ground and clamp the bridge on either side. According to Techniker, the sculptural expression of the piers begged to be formed from a single sheet of steel. LUSAS Bridge was used to verify the initial design assumptions and to try to reduce the plate thickness to justify the elimination of stiffeners.

Cost was the main driving force behind the final design, against which the appearance needed to be balanced. The options studied included solid steel plate, diaphragms, and as a compromise to examine the real cost difference - folded and stiffened plates. A diaphragm was quickly discarded as the cost of fabrication outweighed the saving in steelwork. The initial design suggested an unstiffened plate thickness of 40mm against which the integration of stiffeners into the design needed to be balanced.

A full 3D model of the pier structure was generated using 3D thick shell elements. A refined mesh was used at the pier base and around the top and also for the site-welded connection plates where the stress distribution was expected to be critical. Reactions for five different loadcases were derived from a linear bridge deck analysis. A crash load of 50kN was applied horizontally to the pier as a globally distributed line load, 3.0 m above ground level. The S-plan form of the bridge generates twist which needs to be taken out at the supports. Additional horizontal loads were applied to the connection plate surfaces to account for the construction tolerances and the thermal expansion of the bridge deck of +/- 50 mm in the transverse and longitudinal directions.

Two eigenvalue analyses were performed to extract the natural frequencies and the buckling load factors of the structure. Solutions for the first five modes were obtained using Subspace Jacobi Iterations. Close examination of the results and tweaking of the model to smooth out hot spots allowed Techniker to reduce the steel plate thickness to 32mm, thereby justifying the elimination of stiffeners.

The use of LUSAS Bridge on this project made it possible to create a unique piece of sculptural steelwork within a very tight budget.

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