Geo/structural

[miko20001999]

I design a square footing 24in by 24in for a power generator W=220lbs the dimensions are 61*61*40 (in) and stands 3-4 ft of the ground but I’m not sure if I’m going to have problems with overturning or sliding? How can I check if my 24in by 24in square footing will be ok? Also the depth of height of my footing is 6 in.
Thank you

 

[BigH]

Pretty much statics - do you have overturning moment or horizontal component? If only vertical, if the force is in the middle third of the footing, overturning shouldn't be a problem. With no horizontal force or moment, what would cause the sliding? Of course, you need also check that the permissible bearing pressure isn't exceeded. 6 inches embedment is rather shallow in the practical world.

 

[miko20001999]

I do have static wind pressure of 21 psf

 

[miko20001999]

Thanks for the help BigH and any one else that replies

 

[msquared48]

That's not what he meant by statics... Statics is the summation of the forces and moments operating on the structural system, to include vertical and horizontal forces.

The vertical forces are the weight of the equipment and footing.

The horizontal forces are wind and seismic induced. Wnether or not the generator is attached to the footing will affect these numbers.

Mike McCann
MMC Engineering

 

[csd72]

check:

overturning moment is less than resisting moment(divided by safety factor of 1.5)

Sliding force is less than base friction plus passive resistance (both divided by factor of safety).

The analysis of these two is exactly the same as that for a retaining wall (I suggest you google it).

 

[JAE]

Most codes don't allow 6" footings -

 

[BigH]

I think our friend JAE meant embedment of 6 inches (150 mm).

 

[msquared48]

No, BigH. 7" is the minimum I have seen, but I never use less than 8".

Mike McCann
MMC Engineering

 

[JAE]

Right - many codes limit the thickness of footings. However, now that I look at ACI section 15.7 I see that a 6" thick footing is permitted (I thought initially it was 8" minimum).

I would guess that the Canadian code has something similar.