mat foudation 3

[khatar]

Thank u for having me in ur forum

I USED TO CALCULATE MAT FOUDATION BY HAND USING RIGID METHOD.
I CALCULATE THE THICKNESS BY COVERING THE PUNCHING SHEAR REQUIRMENTS .( IT GOVERNS MOST OF THE TIME)

WHEN I TRIED TO USE A SOFTWARE USING FINITE ELEMENT METHOD,
THE THICKNESS WENT UP TO APPROXIMENTLY 1.5 TIMES THE THICKNESS (RIGID METHOD) IN SOME ISOLATED POINTS IN THE SLAB AS THEY CALCULATE THE ONE WAY SHEAR REQUIRMENTS,I FOUND ALSO THE SAME THING FOR BEARING CAPACITY.

I APPRECIATE TO HEAR A FELLOW OPPINION

 

[FixedEarth]

Kh;

You seem to have a lot of experience with Mats using manual methods. Have you tried putting your Rigid method in a spreadsheet format? You can easily catch if there is an error, which you would be able to catch it, when you see the output.

The bearing capacity (you mean footing pressure) would ofcourse be dependent on loadings, foundation geometry and your subgrade modulus, k, but is not heavily dependent on the k value.

With your analyis spreadsheet, you can then take your max. shear and max. moment and then create another worksheet to do the the structural design once factored.

It is difficult to know let alone verify what the computer programmer and his engineer assumed during the software creation.

 

[mtu1972]

We had a purchased software for combined/continuous footings (not quite mats) that had a similar difference with my hand and spreadsheet calculations. Finally figured they were using shear at the face of the columns rather than at "d" from the face - because the load was applied to the bottom of the "flexural member".

A literal interpretation of the Code, but not realizing that it could be treated like an upside down beam. Without knowing what exactly is happening in your software, I'd be somewhat skeptical as you are.

The bearing capacity could well have more localized distributions than made with an overall P/A + Mc/I calculation.

gjc

 

[mconti]

you mean geotexiles with the term mat?

 

[JoshPlumSE]

Verify the accuracy of your software (or hand calcs) first by changing your analysis assumptions for soil stiffness or concrete stiffness to approximate your hand calc assumptions of a rigid mat.

Once you are sure that the two methods produce similar results for a rigid mat, then you can investigate what types of local flexure is causing the original analysis to give you higher shear demand and higher bearing pressures.

Personally, I like to look at bearing pressure or deflection contours to understand this behavior best. But, also realize that these could be cases of very LOCALIZED fore or pressure results. A finite element analysis can sometimes highlight localized stresses more than you would like. In which case, you might have to exercise some engineering judgement on interpreting the software results.

 

[Mccoy]

you mean geotexiles with the term mat?

mconti, I believe that the term 'mat foundation' is synonymous with 'slab foundation', a large and laterally extended RC foundation with a small thickness/width ratio.

 

[mconti]

right, it was a my misleading
thank