ower leg foundation

huangyhg

tower leg foundation
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.
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.
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.
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.
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   
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.
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   
just playing devils advocate, but without shear in the concrete how do you get the bending? isnt one the result of the other?
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.
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.
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