Foundation Pier for steel frame

[FiniteElemet]

Are we correct to neglect the uplift on concrete piers due to steel column in frames building.

Also, am I correct to design pier as stand alone pedestal for reinforcement purposes?

 

[JedClampett]

Kind of vague. I think you should rephrase.
But if there's uplift, you don't neglect it. You design for it. Now maybe the weight of the pier is enough to resist it, but you need to check.

 

[FiniteElemet]

Thanks Jed,

These piers will be part of a 100x100 ft building. Uplift on columns is not a lot and can be omitted?

In another problem, I have this 48 ft diameter circular slab 24" thick. Slab will support liquid retained tank and sit concrete columns. I ran my analysis and have plate moments and plate shear values. I have modeled this slab as plates. However, if I want to verify whether concrete beams are required, which ACI check shall I look into?

 

[JAE]

You can't just decide to omit uplift forces. They need to be considered in any design. Will they control the design? That depends on the nature of the connections and details.

On your circular plate model, beams would be required if a flat slab/mat doesn't work under all provisions of ACI for moment, shear, deflection, etc.

Your question is pretty vague and if you absolutely need an answer - then the answer will be vague - i.e. ALL the ACI 318 provisions must be met.

 

[FiniteElemet]

Regarding the uplift question, piers are integral with perimeter foundation walls. For max uplift of 20 kips per column, downward weight of individual pier plus foundation pad will resist the load. Piers being integral with wall, walls will contribute to resist the uplift such that vertical reinforcement in the piers is tied to the longitudinal rebara within the wall. Additional longitudinal bars will be provided at pier location with lesser spacing for transfer of uplift forces from steel column moving down into pier. Spacing of these piers are generally smaller at pier top.

Regarding the circular slab, moment capacity and shear capacity of the plates need is satisfied. Just wanted to read opinions of various approaches.

 

[msquared48]

If you have enough dead load in the walls and piers to resist the uplift generated by the columns, and show that fact on your calculations, then you have, in effect, negated the uplift. But you still have to show the BO how you got there...ON PAPER! Point being, it is NOT "ignored".

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com