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Torsional Moment - Footings

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?

 

RE: Torsional Moment - Footings

I've never considered the friction at the bottom.  If you have moment such that the bearing pressure isn't uniform, it's seems like it would be unreliable, at best.  

I have, however used passive pressure on the corners to counteract a torsional moment.

RE: Torsional Moment - Footings

(OP)
You would use passive pressure on 2 sides (similar to a handrail post in grout?) + a skin friction on the other 2 sides ? Or just passive pressure on all 4 sides?

RE: Torsional Moment - Footings

I used passive pressure on all four sides (triangular distribution).

RE: Torsional Moment - Footings

I agree wth Lion, but the distribution would be triangular, with tne moment arms to resist the torsion at the CG of each triangle.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Torsional Moment - Footings

Each of the four sides would see soil pressure, triangularly distributed on half the length, not the whole length, due to the soil movement.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Torsional Moment - Footings

(OP)
Thanks guys. It makes sense.

RE: Torsional Moment - Footings

I think a different approach would be to use the maximum passive pressure to calculate an allowable torsion.  Without a starting point for pressures, how would you arrive at P1 and P2?

RE: Torsional Moment - Footings

(OP)
I think I started with a square footing. In that case determining P would be easy, since it would be 4*P*B/3 = Moment. Then I get P. Once I get P, I can figure out what passive pressure yields P (based on area of triangle). But I extended it to a rectangle, and it was not making sense.

RE: Torsional Moment - Footings

(OP)
Lion: Your approach makes more sense though.

RE: Torsional Moment - Footings

slick-
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

It's really no different with a rectangle, there is just more torsional capacity to develop with the longer sides.  The longer sides will take a larger portion of the torsion.  No worries.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Torsional Moment - Footings

Oh, but not to complicate the analysis, but the allowable lateral bearing will increase with the depth of the footing.  So you might want to analyze this on an average depth for thinner footings, or stepped blocks of 1 to 2 feet for thicker ones.  

Obviously, this would not work for a drilled shaft footing.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Torsional Moment - Footings

Good point on the passive pressure increasing with the depth.

RE: Torsional Moment - Footings

The foundation rotates as a rigid body, so the long sides will see a higher pressure at their corners, than the short sides will.  For a degree of rotation, the longer distance to a long side corner will cause more angular movement there than at a short side corner; thus the soil compression and passive pressures will be greater in direct proportion to these angular movements.  I would liken this to the rotation of a conc. retaining wall under loading, causing higher pressure under the toe of footing and a lesser, or no pressure under the heel.  And, I would compare the sum of these pressures, caused by your torsional moment, against the allowable for the given soil conditions.  I'll have to think on your exact formulas a bit more.  I'd just be sure to lock my steel column into the found. so it can transmit this torque.

RE: Torsional Moment - Footings

dh-

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

In addition to the passive pressure on each side, you have the coefficient of friction on the bottom of the footing which will also contribute to torsional resistance.

BA

RE: Torsional Moment - Footings

I agree dhengr - the lateral bearing tables in the IBC are set up based on 1" of lateral deflection.  

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

For a rectangular footing, I would attribute the bearing pressure to the sides in proportion to the amount of movement.

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

I'm guessing that this would be more of a "check" after designing the footing. I can't see this being a controlling factor, no?
Would one bother to design reinforcing for the torsion?
Interesting topic.  

RE: Torsional Moment - Footings

Toad,
Yes, I've just "checked" the already designed footing for torsion.  I've never added reinforcement for it, either.

RE: Torsional Moment - Footings

Well, as far as the footing is concerned it wouldnt really be torsion as such rather a series of radial compression struts akin to the spokes of a bicycle wheel. As long as the torsion is adequately transferred into the concrete and the soil resists this by passive resistance then I would think there is much else to look at.

RE: Torsional Moment - Footings

First of all Slick, on a Sunday AM in S Florida you really should not be wondering about torsion in 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.

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