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Re: MAT FOUNDATION DESIGN : ANY HELP?

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Dear Thys:

You can look at ACI 's Guide to Residential Cast-In-Place Concrete
Construction, reported by ACI Committee 332. It is very important the type
of soil your placing the slab on. Considering the size of your slab, you
need reinforcement if you're pouring it all at once. I suggest welded wire
mesh reinforcement. You can get high fy values for wwm (welded wire mesh)
which will save you some reinforcement and ductility wouldn't be an issue in
my humble opinion. If you have a stiff soil what controls design is the
shrinkage. When the slab tries to shrink, friction with the ground generates
a tension force that needs to be supported by the wwm, this is also
proportional to the contact area and weight of the slab, so smaller thinner
slabs have need less reinforcement. If you find a way to let the slab shrink
and expand freely, you might need no reinforcement at all. We have not found
in the literature (neither the list) the principles that govern the minimum
amount of steel for temperature control, it's more like a practice obtained
number, which I assume, covers a lot of different experiences, so you could
use less if you expect less contraction forces. For example, in Peru, the
difference between the maximum and minimum anual temperatures is around 18
degrees Celcius, so it's not a lot. I have used a 3 inch polypropilene fiber
reinforced slab as a foundation for a small concrete house, also for poor
people (I used fibers for the concrete walls too)and it has worked fine.
Very little shrinkage cracking, mostly stress cracking, where we expected in
our FEA analysis, but since there is no steel to get rusted, there is no
problem. I used steel for the roof, to get a ductile failure if it was the
case.

A 100 mm thick slab with 4 mm diameter bars @ 150 mm wwm in each direction
should be enough for some places where ground may undergo small movements,
caused by changes in soil moisture.

I also runned some finite element analyses modeling the soil as springs with
the subgrade modulus K, something like CBR for pavements, which is how much
force is needed per area unit to deform the soil one unit. That's why it is
expressed in force/length^3. When you mesh your slab you get each spring
stiffness multiplying K by the tributary area of each node. This will help
you find the bending moments for different load scenarios and check if the
amount of wwf is enough.

Finally, if the house is built and the sun light doesn't go directly to the
slab (because of the house itself), the temperature effect will not be
important.

You could save some concrete using a concept used in whitetopping
technology, the micro slab concept. You have a lot of little slabs that act
independently using many contraction joints. You have to measure the cost of
sawed or hand tooled contraction joints (hand tooled should be cheaper as it
is here) and do many of them trying to get squared slabs. Use fiber and
avoid steel to get longer life avoiding corrosion, and have a very well
compacted soil under the slab.

Hope this helps,

Walter Sheen

----- Original Message -----
From: "Thys Cilliers" <cillmc(--nospam--at)mweb.co.za>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, April 02, 2001 1:30 AM
Subject: Fw: MAT FOUNDATION DESIGN : ANY HELP?


> 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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