Mat Foundation Design

[ChiEngr]

Hello,

I am performing a mat foundation design using RISAFoundation. This "mat" is essentially a thickened concrete slab near grade elevation to support a heavy, long folding door for an airport hanger. The geotechnical report provided a subgrade modulus as well as a net allowable bearing pressure. I am sure I am missing something, but is the design as simple as ensuring the soil subgrade modulus is input properly into the program, with the resulting soil pressures compared against the allowable value given by the geotechnical report?

 

[TLHS]

The complexity of a mat foundation design is really dependent on the actual mat and how aggressive it is, and as with anything you should know what it reasonably looks like before it goes into a computer.

Is it a mat because that's just the most practical configuration and the utilization is tiny? Is there a rigidity that you need to provide? Is the soil high deformation? Does the subgrade modulus actually account for everything you need to worry about from a deflection standpoint?

Subgrade modulus is a really hand-wavy variable and design can sometimes be really sensitive to the value of it. At a minimum, you should know what happens if that number goes up or down by even a large amount. You may decide that it isn't realistic or that you're okay with that, but you should understand the sensitivity of your system to that value.

It sounds like you haven't done this before. You should have at least an approximate hand method that you're doing to confirm whatever the computer is saying. For a mat that might be taking your applied load and spreading it manually as a reaction to the bottom of the footing based on a reasonable assumed distribution (potentially across the whole footing or maybe across some distance near the loads if the ground is really stiff). You could also do a more simplified beam on elastic foundation style analysis as a check.

You may have to make some assumptions as well regarding the stiffness of the mat. This could depend on whether things crack with a bunch of bending stress or if they're closer to a pure bearing into the soil.