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Re: Fw: MAT FOUNDATION DESIGN : ANY HELP?

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Use PCA MATS.
It is a user friendly software for both the US and SI Systems.
The program has only one pitfall the punching shear needs
to be calculated by hand.

Hope this helps.

Desi J. Kiss, P.E.




On Mon, 02 Apr 2001 08:30:59 +0200 Thys Cilliers <cillmc(--nospam--at)mweb.co.za>
writes:
> 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
> that?).
> 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
> cillmc(--nospam--at)mweb.co.za.
> 
> 
> 
> 
> 
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