Tower leg foundation

[Polecat]

I am analyzing the foundation for the legs of a lattice steel tower. It has a 5x5 angle embedded in a 18" square pier which goes below grade for 8 feet and terminates into a 9'x 9' x 2' mat pad. I am trying to determine the bending strength of the pier, which is also reinforced with eight #5 bars.
Question is: Would one consider the strength of the pier to be primarily a steel angle which is protected and stiffened by an 18" sqr concrete pedestal, or is it primarily a concrete cantilever beam which has both rebar and a steel angle as reinforcing?
I am using a stress-stain compatibility method to compute the nominal moment.

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[hokie66]

Are you sure the reinforcing bars are developed in the footing? If so, the concrete pier will be much stronger in bending that the angle.

 

[shin25]

The bending resistance of your pier will mainly come from the reinforcing. I do not think that the angles goes thru the piers and embed in the footing enough to develop them? Select the thickness of the footing so that the rebars in piers will have enough space to develop.

 

[jike]

Another way to look at it is that the axial stress stays in the angle and the pier provides lateral confinement and support to the angle.

 

[Polecat]

To Hokie66 & Shin25:

Thanks for the response.

Yes, the rebar and the angle penetrate the mat fully. The rebar is fully hooked and the angle has an 90 deg. cross member welded to it in the mat that is a 4x3 angle x 1.5 ft long.

I have calculated the moment using the area of the angle plus the rebar, and of course it is larger than a fully braced angle taken by itself, but my concern now is how to calculate the ultimate shear capacity. The pier has very few ties in it (6 pieces of 3/8 in the entire length) so their contribution to Vs is nil. But intuively, I feel that one cannot ignore the effect of the shear resisitance of that angle even though it is primarily treated as longitudinal reinforcing. It still has a leg that is perpendicular and runs in the same direction as the ties. If so, the shear resistance of that leg is huge and it seems to me that its value cannot be ignored even though ACI doesn't exactly have a way of treating it.

Your thoughts???

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[shin25]

Polecat-

I believe, the size and spacing of the ties for your case satisfy ACI-318 requirement? If so, use equation 11-4 (ACI-318) to calculate the shear strength of the piers, ignoring the angles. If you are low on shear capacity, then only consider the contribution from the angles.

 

[hokie66]

I disagree with shin25, and consider that you are correct in using the shear capacity of the angle. Don't worry about ACI shear, it is a steel member, so analyze it as such for shear.

In summary, the angle takes the axial load and the shear, and the reinforced concrete takes the bending.

 

[csd72]

Just playing devils advocate, but without shear in the concrete how do you get the bending? Isnt one the result of the other?

 

[shin25]

The shear force has to travel thru the concrete to get to the steel angles. If the the concrete shear capacity to the actual shear is too high, then large cracks will form in the piers.

 

[Polecat]

Good thoughts, all.

I like the idea of having the angle take the axial load and the shear, and to let the rebar take the bending. Shin25 does have a point in saying that the angle won't see any shear force until it propagates cracks thru the concrete. But this would be true only as long as the external load was placed on the concrete.

In this case, however, the loading on the pier comes first into the angle, not the concrete pedestal. The shear is diminished as you go down by the reaction of the soil on the pier, but the load that causes bending in the pier is initially placed on the angle. It is this fact that leads me to want to use the angle's shear capacity without considering the ACI formulas.

I'm just trying to develop a warm, fuzzy feeling about doing so. Am still waiting for that solid logic to kick in.

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[csd72]

The stiffer element will take the shear, the concrete pedestal would have to completely fail before the more flexible angle takes the shear.

Shear causes bending, no shear no bending. You cant just pick and choose one member to take bending and one to take shear, it doesnt work that way.

csd

 

[miecz]

csd makes a good point.

I would proportion the load to the angle and the concrete by their relative stiffness, including shear and bending flexibiltiy for cracked concrete. Axial load can be proportioned by area of steel. Then, check the angle and the concrete pier separately for thier portions of the loads. If either one fails, distribute the excess force to the other element.

 

[Polecat]

I certainly agree with csd72 and jmiec about the relative stiffnesses between the two members, which incidentally and not surprisingly, shows that the concrete pier is about 100 times stiffer than the angle.

But what if, in this case, the load is applied to the angle at a point just above where it enters the concrete? At that point the angle is seeing all of the shear and doesn't dissapate it until it gets fully bonded into the concrete. How would that load then be apportioned relative to stiffness?

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[miecz]

I would check two sections. One, at the base of the pier, where I would assign all the load to the reinforced concrete section. Two, at the top of the pier, where I would assign all the load to the angle. O.K., so there is a zone where the load is shared by the angle and the pier. To cover that zone, check the angle for shear and bending at some distance, say 6", below the top of the pier.

 

[risa2d]

The shear is at the top of the pier and the moment used to design the pier is taken at the top of the footing which is equal to the shear times the height of pier. Knowing that this is a tower foundation, The leg embedded into the pier and footing is to provide resistance against tension.

 

[civilperson]

The angle is not effective in resisting shear since it is parallel to the mid span shear cracks. The ties and the concrete are the only element resisting shear from the moment.