Minimum Flexure and S&T Reinforcement for Slab on Grade/Footing

[Blackfairy]

Hi,

I am checking a 30'x30'x2' footing/slab on grade for a transformer and basically the designer gave me a calc. The minimum S&T 0.0018Ag, he distributed it on half on top and half on bottom which is correct, but since there is Overturning moment due to the center of gravity of the transformer (which is above the top of footing) and seismic, the slab/footing will experience a moment and will need flexure reinforcement. Now regarding minimum flex reinforcement, it is also 0.0018Ag distributed near the tension face, which is at the bottom, this will double the reinforcement on what he has right now. Will this be the right step?

I guess another question i have is, what is the S&T for? I think it is very rare for a footing to not experience moment, and as soon as they have moment the minimum flexure kicks in already which is the same as the S&T, so you can put all the rebars at the bottom and also call it S&T reinforcement because they are the same ratio of 0.0018 minimum.

 

[driftLimiter]

The tension face reinforcement essentially doubles. However its not based on Ag its based on the width x d. (d is the distance to the from extreme compression face to the centroid of the flexural reinforcement).

There are types of footings that dont need flexure. For example a continuous wall footing. The load from a stemwall can spread at 45 degrees through the depth of the footing, if the resulting width at the bottom of the footing is enough to resist bearing on the soil, then the footing is not in flexure and 0.0018Ag is used and can be spread as needed from top and bottom side.

 

[JoshPlumSE]

Now regarding minimum flex reinforcement, it is also 0.0018Ag distributed near the tension face, which is at the bottom, this will double the reinforcement on what he has right now. Will this be the right step?

Yup! It used to be that we would check the 0.0009Ag against 4/3 * As Required and use the larger of the two. But, the newer codes (for whatever reason) do not allow that.

There are some thread on this forum (or the ACI one) where I've complained about that in the past. However, to my knowledge, there is no movement in ACI to allow this liberalization back into the code.

 

[JLNJ]

If it works unreinforced, then don't worry about the reinforcement. It's a footing not an exposed concrete beam. I'm not even sure "shrinkage and temperature" theory applies here.