Tower leg foundation
Tower leg foundation
(OP)
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.
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.






RE: Tower leg foundation
RE: Tower leg foundation
RE: Tower leg foundation
RE: Tower leg foundation
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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RE: Tower leg foundation
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.
RE: Tower leg foundation
In summary, the angle takes the axial load and the shear, and the reinforced concrete takes the bending.
RE: Tower leg foundation
RE: Tower leg foundation
RE: Tower leg foundation
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.
http://www.spiraleng.com
RE: Tower leg foundation
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
RE: Tower leg foundation
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.
RE: Tower leg foundation
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?
http://www.spiraleng.com
RE: Tower leg foundation
RE: Tower leg foundation
RE: Tower leg foundation