Concrete footing design

Because the code requirements for raft footings are essentially the same as for slabs, you can use the concrete slab design module to design or check raft footings. The main difference with raft footings is that the soil interaction underneath the footing must be correctly taken into account.


Modelling the effect of the soil requires a spring support underneath each node and a compression-only member between the spring and the footing so that downward forces are resisted but uplift can occur. Because the meshing of the footing generally results in plate elements of various shapes and sizes, calculating the tributary area of each node and hence its spring stiffness is not easy to do by hand. A "Plate spring supports" calculation tool is therefore available that automates this process. It not only calculates each node's spring stiffness but also generates the spring restraint and inserts a compression-only member between each spring support and the footing.





The following image shows how a typical spring support and compression-only member should be modelled. The FSFFFF node restraint (or FFSFFF for Z vertical) with its associated spring stiffness models the soil elasticity and enables the node to remain stable even if the compression-only member above it is disabled in some load cases. The compression-only member has a fixity of FFFFFF at the bottom and FFFRRR at the top. This connects it rigidly to the spring node at the bottom and provides a pinned connection to the footing node at the top. The purpose of the pinned connection is so that no moment is transferred from the support node into the footing. If you use the "Plate spring supports" generation tool then all of this is set up automatically for you.




Once the raft footing model has been created then its design or checking procedure is the same as for a concrete slab.


A bearing pressure contour diagram can also be displayed via the plate contour button on the side toolbar.



A wizard for generating standard raft footings is also available via the "Structure wizard". The footing wizard generates the fully meshed model, including all compression-only spring restraints and pattern loading.