How does uplift reinforcement work? 1

[Lindsey]

Maybe some of you can help out a young EIT. I am designing metal building footings where the uplift condition far outweighs the compressive force from the column load (in Florida). My dillema is that I do not have any background in how to design the reinforcement for the top mat of the square spread footing for this uplift situation. Common sense told me to size the footing to have enough weight to overcome the manufacturer's given uplift reaction. But, as far as the top mat design, I have some intuitive hunches, but I want to do this right.

 

[ishvaaag]

In your view that I think is basically correct you are counteracting pulling action by weight, assuming a tensile action on say the whole column. Then of course anchors need be proper (give equilibrium) to the worst forces at the base, and, provided the base metal setup is well solved, you have then only basically a problem of anchoring the pulling force to a concrete mass. Then the ordinary provisions for embeddment length in a mass of concrete, or the equivalent ones for anchors of whatever kind, will ensure that you have passed the tensile action to the concrete weight, upon which case no special provision of top face reinforcement in the footing would be ordinarily required; in any case if you want to make a whole cage for the footing do then in accord with some minimumn geometrical requirements, but that would be rarely seen practice. More common is to see passing the anchoring bars till the botton face and then angling outwards to ensure maximuadherence; but every anchor type has its particularities.

 

[JAE]

ishvaaag - correct, but we normally provide a top mat of nominal reinforcing in a spread footing that is resisting uplift. The footing will also be held down by any soil above the top of the footing so there will be some level of minimal moment. Granted - its really two-way and difficult to determine exactly, but some one-way approximations can be made if necessary.

 

[whymrg]

In addition to this you may consider Factor of Safety of 1.5 against uplift specified by manufacturer (similar to 1611.6 UBC-97).

 

[ishvaaag]

OK, good practice; I would surely put top reinforcement if, say, there was a sole "mat" footing receiving 4 legs of a transmission tower. If there were 4 footings for the same only more rarely, I think.

 

[engcomp]

Elementary, Watson - you have a clump of concrete being torn out of the adhering ground by an uplift force, which presumably is anchored adequately into the clump. What can happen? The clump can break in two along one axis or the other. How do you prevent it? By putting reinforcement in the top of the clump. How much reinforcement? Something like the weight of concrete and soil on one side of a possible break line times their lever arms divided by a distance related to the clump and a stress related to the reinforcement. Add a bit (like 1.25/0.8) for good measure.

Helmut Schmidhofer
engcomp@pbq.com.au

http://www.engs-comp.com/

 

[austim]

Don't forget the possibility of a local shear failure round the column base. Check in accordance with your local code requirement for 'punching shear' in an upwards direction. You basically need to ensure that there is sufficient concrete shear capacity to prevent a square cone being pulled out of the footing. If in any doubt, then add local shear bars (inclined down from the upper surface, anchored at the underside).

 

[dik]

further to austim... and your vertical pier steel and anchorage into the footing... (don't you just hate tension across a cold joint <G>).

 

[Lindsey]

Thanks for the help, guys, but I am already ahead of you. Maybe I should have explained my question better. I already have the footings sized for uplift and have sufficient anchor bolt depth to anchor to the mass of concrete. The footings, in this case, have a top surface that is at floor elevation. In other words, these footings will become part of the floor slab (when it is poured at a later date). Therefore, there are no piers and no soil over the top of the footing to resist. This was not by my choice, this is the situation that was given to me. What I am trying to prevent is cracking along the top surface of the concrete, since this will eventually be part of the finished floor. Since I have sized the footings to resist the given uplift, and it is adequately anchored, they will never actually lift from the ground. But, what I am trying to prevent is the cracking that may occur from a high uplift load causing tension in the top face of the footing. Essentially, the way that I perceive it, it is equivalent to viewing the footing as trying to fold under by its own weight (these footings are large enough to do so). So, that is my problem, how to prevent the compromise of the surface of the concrete.

 

[engcomp]

lindsay - read my answer above!

Is there any point in addressing questions on this forum when the answers are ignored?

Helmut
engcomp@pbq.com.au

http://www.engs-comp.com/

 

[ishvaaag]

It is easy, maye a joint around your column for the whole standing pour.

 

[gglass]

Lindsey

As you write, you are on the right track. Since your footing does not rest below grade it would not resist soil weight. I assume from your description that the slab and footing are monolithic. That being the case, a worst case design would be when uplift nearly overcomes the shear resistance of the slab at the perimeter of the footing. Therefore, I would check for dead weight of the footing and a line load equivalent to the shear capacity of the slab around the perimeter of the footing. If that does not result in an excessive top mat of reinforcing, I would use it. To cover your concern about floor cracking, you may design for an ultimate moment 30% higher than ACI 318. This is the means used in ACI 350 for treatment plants to limit crack width. In any case, your reinforcement ratio should not be less than the usual .0033.