Concrete foundations 4

[mshall]

What is the formula for calculating the thickness of a concrete foundation from a predetermined Soil Bearing load? With a Soil Bearing load of 2,000psi, what thickness would my foundation need to be, in order to have a foundation strength of 3,000 psi. The purpose is to find out how many yards of concrete will be required, For cost estimating purposes.<br>
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Regards<br>
Mhall

 

[jfmann]

First.......hopefully you mean psf (pounds per square foot) as opposed to psi ??<br>
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Question is a bit confusing........but, generally for an allowable bearing pressure, the thickness of concrete required depends on the loads to be supported, both vertical, horizontal and, if applicable, moment......other loading conditions such as seismic, wind or vibration need to be considered as well.......<br>
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The footing should generally be reinforced concrete, designed to resist the forces.......<br>
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Of course the allowable bearing pressure should be determined by a qualified engineer using soil boring data or at least some information about the soil......<br>
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If you can provide more specific information, it will be easier to provide a better answer........

 

[merbas]

if it is industrial ground supported slab I offer you as a turkish way at least BS25 concrete for 20 cm thichness.<br>
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according to soil situation, weak or strong, you should thing fiber reinforced concrete.<br>
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if ground not hard enough use 50 mm from bottom wiremesh and 1,8kg/m3 fibermesh.<br>
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if ground good enough use 1,8kg/m3 fibermesh alone.<br>
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if ground perfect use 0.9kg/m3 fibermesh.<br>
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the main idea : ----------U---------- surface<br>
joint 1/3 20<br>
wiremesh 50mm --0--0--0--0--0-- cm <br>
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///\\\///\\\///\\\///\\ground<br>
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<br>

 

[Wilworth]

As the first response stated, your question is not very specific. There are a lot of factors which influence the thick of the concrete: type of sub-grade, usage, strength, etc. There is a small booklet entitled (" Concrete on Floors", Second Edition, PCA). I think it could be very helpful in clarifying some of what you are asking. <br>
Address: PCA, 5420 Old Orchard Road, Skokie, Illinois 60077-1083<br>
It contains some charts that you can read off the slab thickness based on the design loads of the column. I think that could be very helpful for your cost estimation.<br>

 

[ishvaaag]

Respect the first question what I think it is that for proper forecast of the convenient foundation, and hence any likely cost estimate, the concurrence of a structural designer is a necessity, if none of the parties gathers the required abilities.

Respect the second question, excavation in general, you can consult for example

Moving the Earth 4th Ed.
Nichols and Day
Mc Graw Hill

Standard Handbook of Heavy Construction 3d ed.
O' Brienm Havers, Stubbs Jr.
Mc Graw Hill

Respect concrete, in developed countries it is unlikely that for structural purposes reinforced concretes of less than specified compressive strength 25 MPa be now used, since cement contents -correlative to strength- is very significant in the protection against corrosion. However for poor cement content and mass concrete boulder foundations, or preparation layers, lower strength concretes are used, maybe up to a minimum of 10 to 7 MPa, but more typically in the 10 to 15 MPa range.

 

[Jim6758]

Wilworth gave you good advice, but, when extracting the numbers, don't forget to allow for less than perfect sub-grade work. I've seen floors in large commercial structures which were supposed to be at 4 inches get poured at ranges from 3 to 5 inches and worse. Also, be careful to consider construction loads when setting the thickness of the footings. It is virtually impossible, when setting concrete panels weighing up to 80,000 lbs, to set them perfectly on the footings. Point loads are invevitable. If you don't allow for this, you will overload your allowable soil bearing pressure. It doesn't help to claim that you designed for the finished condition and to make the Contractor responsible for means and methods to achieve this.