Bearing pressure under eccentic loading and moment 1

[t888]

what is the correct formula for maximum pressure for a eccentrically loaded foundation subject to a large download and moment simultaneously? i see a lot of examples for one but not both.
see the attached example

Thanks a lot

 

[IDS]

Find the moment due to the eccentricity, add the applied moment, then divide by the axial load.

That will give you an effective eccentricity and you can use the standard formula.

For combined loads as shown the eccentricity will be outside the edge of the foundation, so it won't work without piles or some sort of anchor.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[dik]

If your force is outside the kern, you will have tension. Your system, as shown, is incorrect and the load would have to be at the third point for that distribution assuming the moment is due to the eccentricity. With the moment shown, if in addition, the situation is worse.

There are several solutions to footings with eccentric loadings (wild ones even).

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[HTURKAK]

If the sum of additional eccentricity ( ead= M/P ) and ecc. of N is outside the footing area , the ftg will topple ..

Check your case for the following alternatives.. if e=M/P > (L/2) , the ftg will overturn ..

I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[phamENG]

This is a handy little practice article for this topic: Structure Magazine

 

[t888]

So, to clarify, the effective eccentricity, e is (N * e₁ + M*)/(N+Wf) ?

e₁ = dist of eccentricity to centre of footing (centre of wall to centre of footing)
wf = weight of footing

if e > b/6 then uplift

phamENG - your example link appears to be moment and concentric load

thanks

 

[dik]

...as I noted earlier, "With the moment shown, if in addition, the situation is worse."

"if e > b/6 then uplift" not necessarily so, if the moment is in addition to the eccentricity.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

That's easy to fix... just apply a load of +ve and -ve value at the location of where you want the load to be applied. the use the one at that location and add the moment caused by the force x the distance of the concentric load to the location of where you want the load... just remember to keep track of the sign of the moment... it may increase, or decrease the original applied moment.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

These three conditions are identical... showing translation of a point load to point 'a'.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JLNJ]

Draw your free-body diagram. The resultant from the bearing stress needs to be equal and opposite to the applied force.