overturning of block foundation

[acolakoglu]

How can I calculate the over turning capacity of a concrete block in soil?

Concrete dimensions are approximately
12' long 5' wide and 7' deep.

Any reference or method?

Thanks in advance,
Ahmet

 

[jdonville]

acolakoglu,

I don't understand exactly what you're asking.

Please provide more information about the block: i.e., how it is oriented, embedment in the soil (if any), any applied loads on the block, etc.

When you say 'deep' do you mean 'high'?

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[acolakoglu]

Jeffrey, it is in the soil for 7'. Top of it is flush with top of soil. I have an overturning moment of 400 k.ft. The moment is trying to overturn the width, in other words the side bearing will be on the 12' side.

Thanks a lot,
Ahmet Colakoglu, PE
D&L

http://www.damianolong.com

 

[jdonville]

Ahmet,

It sounds like you want to determine the overturning factor of safety.

You will need to compare the driving and resisting moments about the toe of the block. Driving moment in this case will be due to the applied load. Resisting moment will be due to the self-weight of the block. The soil will not usually be considered to provide any resistance. However, your block geometry is unusually thick.

If you can ensure that the sum of the resisting moments divided by the sum of the driving moments is greater than 2.0 then you are OK. If you need a contribution from the soil then you should contact a geotechnical engineer to assist your determination of the forces the soil will exert on the block.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[1969grad]

I am not a civil engineer but it seems to me that the foundation will rotate around it's cg and be resisted by the allowable bearing capacity of the soil at the base. The soil loading would be triangluar in shape with the greatest resistance at the outer edge of the base and zero at the centerline of the foundation.

 

[jdonville]

1969grad,

There are several failure cases that are usually considered for foundations: 1) sliding or translation, 2) rotation about the toe or overturning, and 3) bearing capacity failure.

For each failure mode, it is usually assumed that the other two do not apply. However, you make a good point in that rotation may not necessarily take place at the toe of the structure.

For blocks, the overturning analysis will generally be a calculation of the eccentricity of the applied vertical load on the bearing soil from the design loads. The applied vertical load is then adjusted to account for the eccentricity and feeds into the bearing capacity analysis. Not enough allowable bearing capacity will indicate potential for rotation (due to bearing capacity failure) about some other point than the toe of the structure.

This situation is unusual in that the embedment of the element is deeper than is common for shallow foundations. Thus, a more careful analysis may be warranted.

Thanks for getting me to think more critically about my earlier advice.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com