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 one-way spring support at each node 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.




Alternatively, the effect of soil can be modelled using a spring support and compression-only member. The following image shows how this can 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 FRRRRR at the top. This connects it rigidly to the spring node at the bottom, while allowing the footing to slide and rotate where it connects to the top node of the compression-only member. This ensures that no moment is transferred between the support node and footing.



The two approaches described above provide no horizontal restraint to the footing.It is important that the footing has some restraint for horizontal translation, otherwise the model may produce an instability error.


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.