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# Modeling a raft foundation

A raft or mat foundation is a solid reinforced concrete slab that sits directly on top of soil and spreads the load of the structure it supports over a large area. It is typically used when soil has a low bearing capacity or where the soil is unstable (i.e., soft soil). It is also useful in cases where structures need a uniform, stable base, like a multi-story and industrial building. It is possible to model a raft foundation supported on soil in both Frame and Sumo, however, the workflow of each module differs.

In Frame:

When going about designing supports to behave like soil in Frame, you’ll need to implement gap supports with spring elements. Two sets of nodes that coincide is required. Spring elements are defined “between” the coinciding nodes. The “top” set of nodes coincided with the slab nodes and the “bottom” set of nodes are assigned gap supports.

Figure 1: Graphical representation of how a typical raft foundation support is defined

To allow for movement in the y direction and not in z and x direction, you’ll need to specify a suitable Ky (stiffness value) in kN/m. For Kx and Kz, a large enough stiffness value needs to be used, to essentially ‘fix’ the spring in these directions. Another thing to remember is to fix rotation at the supports as well. Lastly, you need to specify a positive gap (‘+’ Gap) in the y direction (under the Support tab). Thus, you are left with a support that allows for uplift in the positive y direction. A second order or non-linear analysis is required.

In Sumo:

When designing a raft foundation in Sumo, you’ll use a Plane Area support and specify a subgrade modulus. The program will automatically calculate the elastic stiffness that must be applied based on the subgrade modulus and size of the shells in the connected element. You have the option to either allow or ignore tension in the elastic support. If tension is ignored, the support will allow uplift (gap support) and a second order analysis is required.