Structural Integrity Steel in Two-Way Slabs

[wcfrobert]

Hello all.

I have a question regarding bottom integrity steel provision for two way slab design.
CSA 23.3 specifies a minimum integrity steel area at the four faces of a column (Asb = 2V / fy).

In the case of a garage roof with 1-m of soil on top, If I were to design per CSA, I would need around 5 to 6 20M bars through the column core; which is quite a significant amount. Can this steel be used to reduce the number of bottom bars in the column strips? It's interesting because then the middle strip bar spacing would be smaller than that of the column strip.


ACI doesn't have any formula for integrity steel and only mentions that at least two bottom bars have to extend into the column core with full tension lap. That's quite the difference. Thoughts?

 

[rapt]

My understanding is that the integrity reinforcement does not have to be extra reinforcement. It is simply an area of reinforcement that has to be continuous through the column zone in the bottom of the slab. How you combine it with the remainder of the reinforcement is up to you.

I would work the other way in your case and make the bottom reinforcement the same for column and middle strips. And then work out how many of the column strip bars are contributing to the integrity requirements and then add extra to make up the difference. Then all of those bars in the column integrity zone need to be made continuous.

 

[KootK]

Can this steel be used to reduce the number of bottom bars in the column strips?

It can. Watch out that you're accounting for different effective depths etc which can be a thing if you're integrity bars are of a different size and on a different layer from your flexural steel.

ACI doesn't have any formula for integrity steel and only mentions that at least two bottom bars have to extend into the column core with full tension lap. That's quite the difference. Thoughts?

ACI's got more or less the same provision, it's just not in 318. I think it appears in the monolithic slab-column joint standard.

My preferred method is to not use the integrity bars flexurally and not extend them from column to column. I just toss in whatever is needed @ 2Ld each size of the column and let that be added as the uppermost later of the bottom bars, on top of the usual layers.. Easier to detail and easier to build in my opinion, even if it's a bit more steel overall.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

Kootk,

It is in ACI318 now in 4.10 which directs to 8.4.7 and other places.

But as usual they have changed it again. For RC flat slabs, all column strip bottom reinforcement now has to be fully continuous. There is no longer a special calculation for an area of bottom reinforcement to be continuous in the column zone. Not sure what you are supposed to do where there are different reinforcing patterns in adjoining spans!

For PT slabs, there is a special rule.