Case Study

Slab Reinforcement Design for Milton Keynes Theatre

  • irregular shaped slabs and openings
  • automatic meshing techniques
  • wood armer reinforcement calculations

Milton Keynes Theatre (image by LCE andrzej blonski Architects )

Consulting engineers Whitby Bird and Partners use LUSAS Civil & Structural for a variety of engineering analysis work including reinforced concrete slab design, frequency analysis of grandstands, and bridge design. A number of recent projects have made use of the Wood-Armer reinforcement facilities in LUSAS to optimise slab thicknesses and the calculation of reinforcement quantities.

Overview

The 22 million Milton Keynes Theatre is a typical example of how analysis using LUSAS can produce savings in design time and give more efficient reinforcement arrangements. A number of slabs in the theatre were designed with the aid of LUSAS. The complex geometry and awkward shaped openings of the 300mm thick suspended ground floor slab provided an ideal oppportunity for using the automatic meshing facilty. With this feature, triangular, quadrilateral or irregular meshing can be used - all the engineer need specify is the number of default elements along each slab boundary. The 53m x 30m ground floor slab was modelled using thick plate elements each of a thickness to suit the slab depth at that location. Pinned supports were provided at the locations of walls and other supports. Dead and live loading, point loads from columns and onerous loads caused by moveable seating tiers were also specified. A simple linear static analysis gave results for post-processing use.

Automatic meshing of slab

Slab bending moments for bottom reinforcement

Load combinations and enveloping facilities in LUSAS allowed maximum effects to be calculated for use in Wood Armer reinforcement calculations. By using the Wood-Armer facilities in LUSAS accurate reinforcement quanties can be obtained. Reinforcement moments can be calculated for primary and secondary layers in the top and bottom faces of slabs for any direction of reinforcement.

Contour plots showing design areas of reinforcement quantities were plotted. The minimum contour value was set to represent the moment of resistance of 12mm diameter high yield steel bars at 150mm centres which were required primarily for cracking purposes. Plotting contours for bending moments in the top and bottom of the slab then showed in a straightforward manner the extent and size of the additional reinforcement required to resist the bending moments induced. By using the graphing facilities in LUSAS, localised bending moment diagrams were created to further aid the detailing process.

Henry Woodlock, the engineer who used LUSAS for the slab design of the Milton Keynes Theatre said: 'Hand calculations for this type of work are not straightforward. By using the Wood Armer facilities in LUSAS it is easy to view critical and non-critical areas of slabs and provide a far more accurate reinforcement arrangement".

Slab reinforcement design for the Faculty of Divinity, Cambridge

In another example, the ground, first and second floor slabs of the Faculty of Divinity at the University of Cambridge were also analysed using LUSAS. Using DXF data a model of the ground floor slab was created and then subsequently modified to create the first and second floor models. Reactions, deflections and ultimate moments were obtained for the floor slab and in this situation the results obtained enabled the concrete slab thickness to be reduced by 50mm.

Faculty of Divinity, University of Cambridge

Wood Armer moments in slab

'Hand calculations for this type of work are not straightforward. By using the Wood Armer facilities in LUSAS it is easy to view critical and non-critical areas of slabs and provide a far more accurate reinforcement arrangement."

Henry Woodlock, Whitby Bird

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