Torsional Moment - Footings
Torsional Moment - Footings
(OP)
Just wondering on a Sunday morning, if there are procedures for designing a spread footing for torsional moment?
Imagine those cantilevered signs on the sides of roads. They have biaxial moment (due to gravity in one direction and wind in the other). Also wind will produce a torsional moment.
It appears that the footings can develop torsional resistance from lateral bearing on the sides. Will the coefficient of friction at the bottom provide a restraint to torsion? Are there ways to quantify this?
Imagine those cantilevered signs on the sides of roads. They have biaxial moment (due to gravity in one direction and wind in the other). Also wind will produce a torsional moment.
It appears that the footings can develop torsional resistance from lateral bearing on the sides. Will the coefficient of friction at the bottom provide a restraint to torsion? Are there ways to quantify this?






RE: Torsional Moment - Footings
I have, however used passive pressure on the corners to counteract a torsional moment.
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Torsional Moment - Footings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
I'm attaching a sketch that shows my approach. Of course this assumes torsion only and no direct shear. The direct shear complicates the matter and will change the soil pressure distributions.
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
I'm with you if it's a square, but, like you said, it's not as simple for a rectangle.
RE: Torsional Moment - Footings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Torsional Moment - Footings
Obviously, this would not work for a drilled shaft footing.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
The difference I see is that the passive pressure is limiting value. If two retaining walls move, they both develop the active pressure. I've never heard anyone try to calculate something other than the active or passive based on actual movements.
I agree with the statement, in general, but I think from a design standpoint, any movement develops the passive pressure.
RE: Torsional Moment - Footings
BA
RE: Torsional Moment - Footings
You could apportion the relative forces of the short and long sides based on a geometric ratio of the relative rotational deflections.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Torsional Moment - Footings
So the short sides will be taking a lesser pressure than the long sides.
I agree with Lion06 regarding the base friction, it is probably too complicated to accurately asess so I would tend to ignore it also.
RE: Torsional Moment - Footings
Would one bother to design reinforcing for the torsion?
Interesting topic.
RE: Torsional Moment - Footings
Yes, I've just "checked" the already designed footing for torsion. I've never added reinforcement for it, either.
RE: Torsional Moment - Footings
RE: Torsional Moment - Footings
Those road signs, traffic lights, etc. on the interstate and toll roads are supported by drilled shaft reinforced concrete piles, and torsion is a factor and must be designed for. My experience was it is more of a geotech problem, spiral stirrups usually take care of the torsion. This was discussed recently on another thread, if you need that type of foundation FDOT has a free Mathcad program on their website.