Box sections with attached plates are useful for supporting cavity brickwork and for supporting timber joists or precast concrete within the floor depth. Users often attempt to build up these sections from the individual plates using the Elements method but this does not produce a correct value for the torsion constant because the elements calculation does not `know' that the four plates forming the box are a `closed' section.
The solution is to input the box as one rectangular element corresponding to its outer surface and then deduct the inner void. This can be done by entering either the width or depth of the inner rectangle as a minus value as shown in the screenshot below. The illustration is for a 150 x 100 x 8 mm box with a 270 x 10 mm bottom plate. This solution is available in Analyse 3D version 3.64 and later and A3D MAX version 3.41 and later.
Please note that although frame analysis will produce correct deflection and rotation results, this type of section cannot be checked to BS 5950 using the current version of CADS Steelwork Member Designer (SWMD) because it does not handle non-standard section shapes or torsion moments. New software is in preparation to cover these points.
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