Case Study

Slab design on the St George Wharf Development

  • Irregular shaped in-situ floor slabs
  • Reinforced concrete slab design wizard used to produce reinforcement bar contours
  • Detailing process improved

White Young Green is provided civil and structural design for the 2nd-5th phases of the St George Wharf development in London. LUSAS Civil & Structural was used to assist with the detailed design of numerous irregular-shaped in-situ reinforced concrete floor slabs in the residential tower blocks. The LUSAS RC slab design facility proved particularly useful in providing accurate contour plots of reinforcement bar diameters to aid with the detailing of each unique floor slab arrangement.

Overview

St George Wharf is a mixed use riverside development of offices, shops, a hotel and residential, situated on the South bank of the River Thames next to Vauxhall Bridge in London. The residential part of the scheme comprises a number of multi-storey tower blocks having a reinforced concrete frame built on piled foundations. Floors are of in-situ reinforced concrete. Pre-cast balcony slabs and wall panels help reduce construction time to the minimum. Floor layouts are different all the way up the towers requiring each to be modelled separately in LUSAS. Numerous holes and irregular slab shapes meant that using the simplified design methods for flat slabs in British Standard BS8110 was just not possible - hence the use of LUSAS and its RC slab design facility to produce the results in the time available.

St George Wharf

   

Floor modelling

Slab geometry for each floor comes from Autocad via a DXF file. Tim Dodd of White Young Green explains the process involved: "We strip-out all irrelevant geometry from the Autocad file and create a DXF file for import into LUSAS. Then we apply a regular mesh arrangement, add supports and loading, and analyse and process the results using the RC slab design facility."

 

   
Large-scale line or colour-filled contour plots are produced from LUSAS to enable direct measurement and scaling of extents of bars. If bar contour plots from LUSAS are set to show, for example, T12’s at 150 centres across the whole slab, areas where additional steel is required can be easily identified. Tim Dodd adds: "The main benefit of using LUSAS on this project is that it handles all the complicated flat slab geometry and holes, produces bar reinforcement contours, and enables us to design slabs almost impossible by hand."

Contours of reinforcement bar diameter

A conservative approach to modelling the column supports results in two models being required for each floor level. In one model simply supported column supports are used to give worst-case span moments, and in the other, restraints at column positions give worst-case hogging moments over supports. This method avoids the high peak hogging moments that can sometimes result when a single support is used to represent a column position.  Potential slab deflections, and early striking of concrete was also investigated with LUSAS. On-site measurements of slab deflections compared very favourably with LUSAS predicted values.

White Young Green was also involved in the design of a commercial office facility and a hotel on the site as well as providing initial design advice to its client, St George plc, for a proposed 181m high, 49 storey residential tower to the south of the residential blocks. For the tower a 3D LUSAS model was used to evaluate lateral deflections of shear walls.

"The main benefit of using LUSAS on this project is that it handles all the complicated flat slab geometry and holes, produces bar reinforcement contours, and enables us to design slabs almost impossible by hand."

Tim Dodd, Project Engineer, White Young Green

 

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