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Mat Foundation Design
(We call it Raft Foundations in South Africa)

I have to design mat foundations for low cost housing for the previously
disadvantaged communities in South Africa, the poorest of the poor.
A typical mat size is 6mx7m.
If one has to design one small mat like this and you over-design, the
implications are not too serious, but if 2000 houses are to be built then
one is actually wasting vast amounts of money that could have been used
elsewhere on the house. These houses are very rudimentary and every cent we
can save by not over-designing is very important. "Deemed-to-satisfy" rules
are this case not very satisfactory, in my opinion (because one is dealing
with large numbers of units, the approach should be a bit sharper than
Designing for heave is a problem for me because:
1. The effect of the structure on the soil is small in the case of lightly
loaded structures like these but the effect of the soil on the structure in
the case of heave is critical.
2. I don't seem to be able to get a grip on understanding the
soil-foundation interaction in this case
3. Some proposals have been made in the past by e.g. RL Lytton in 1972
during the Proceedings of the 3rd Inter-American Conference on Materials
Technology. I can not find his complete lecture but what it boils down to is
that he gives a formula for the shape of the idealised dome (inverted
saucer) effect of the soil below the structure. He also gives some formulas
for the bending moments that develop in the mat foundation due to this.
Account is taken of modulus of subgrade reaction, dimension of structure,
moment of inertia of the mat and depth of active layer.
I find it rather unsatisfactory to work with such "old" information that is
not backed up by field studies etc and where I just have to accept the
formulas and equations without understanding the underlying assumptions,
principles and theory.
4. Some designers apparently use this idealised dome shape to establish what
length of the mat foundation will be unsupported at the extremities and the
design for these cantilevers bending moments.
5. I don't seem to be able to find authoritative, practical literature on
the subject.
6. In finite analysis the upward pressure can be idealised as springs acting
at selected nodes. This is a very rough approximation because the shear
strength of the soil is not taken into account.
7. Some designers apparently use the beam on elastic support theory. Clearly
such a two-minensional
analysis is not the answer
I realise this is a complex issue but is there anybody that can give me some
sort of substantiated advise, point me in the direction of where to find
literature, propose some design software etc?

Any help will be greatly appreciated.

(Since writing this I have discovered a document "Criteria for Selection and
Design of Residential Slabs-on Ground" 1968 on the National Academy Press
(NAP) web site - can anybody tell me if it is authoritative. I have also
come across a demo download of design software at MLAW's web site,
unfortunately is all in imperial units whereas we work in SI units)

Thys Cilliers
Pr. Eng

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