When designing slabs, what size patches should I use?
Not version-specific
Tekla Structural Designer
Environment
Not environment-specific
Question
When designing slabs, what size patches should I use?Answer
The purpose of patches within Tekla Structural Designer is principally to allow the engineer to "ring fence" areas of high moment in order to produce a more economic design. For patches with all Strips set to Design Force = Maximum setting, then you can place patches of whatever size you see fit, since the maximum moment within the patch will always be designed for. However this is not the case for the Average setting. The larger the patch size and hence its Strip widths, the lower (more often then not) the average moment will be. If the width of strips is exessive then the moment could be unrealistically low.Image
Unfortunately there are no hard and fast rules for sizing patches but a good starting point is using a column strip size ly/2 from Figure I.1 of EC2-1
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The above gives us a good starting point but should not be considered the finished article. The design procedure within the code is based on rectilinear slabs with regular bays. For slabs with unusual geometry the moment diagrams will vary so caution is advised.
Once the patch has been placed we would then recommend that you review the moment contour diagrams within the patch area. If in doubt always go smaller.
Another approach for sizing patches can be found within Concrete Center guide TR43 Post Tensioned Concrete Floors Design Handbook. The below image is taken from Section 5.7.3 and the guide advises that the location of zero shear either side of the column (w1 & w2) should be found. The width of the design strip (patch) can then be taken as 0.4(w1 + w2).
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Suggested reading
Concrete Center Guide: How to design reinforced concrete flat slabs using Finite Element Analysis, O BrookerConcrete Society Technical report: TR43 - Post-tensioned concrete floors design handbook