Stirrups in One Way Slabs

[Leo Baldwin]

Hello,

I could swear I saw a reference in ACI-318 about shear stirrups allowed to project into the minimum required reinforcement cover. In other words the 3/4" cover would be only applicable to primary bars exempting #3 bars which would have less than 3/4"...

Can anyone help... I have spent so much time looking through 318 and I just can't find it...

Thank you!

 

[BAretired]

Stirrups are not effective in thin slabs. They may be suitable in thick slabs (10" or more) but encroaching on a 3/4" cover would not be prudent as it requires more precision in placement than most contractors are capable of providing. Better to increase the cover.

BA

 

[Leo Baldwin]

it is actually a 3" slab with 10" deep joists @ 24" o/c
(13" total concrete depth)

 

[jayrod12]

That's a different animal.

Generally the concrete joist systems I've seen have had a form of single legged stirrups.

In regards to the infringing on clear cover, I'd still be inclined to maintain at least 3/4" to any steel (main steel or stirrups).

 

[Leo Baldwin]

jayrod
Yeah - single leg S or Z shape...ACI allows one single bar to be bent into squared snake form along the joist where the horizontal legs (at top and bottom) simply develop into concrete - not having to tie into bottom or top bars of the T beam - kind of neat... Though logically Id like my stirrups to be anchored around primary steel.

 

[hokie66]

I have never used stirrups in ribbed slabs. Widening the ribs, or using tapered pans, is a better way to go.

 

[TehMightyEngineer]

I've yet to find a situation where thickening the slab (or otherwise getting more concrete shear strength) was more expensive than shear reinforcement in a one-way slab. Only time I've ever reinforced a one-way slab for shear was when I had a thickness limitation and it was a huge hassle for the contractor.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[hokie66]

TME,
In spite of the thread title, the OP is talking about pan joists.

 

[TehMightyEngineer]

Ah, didn't read enough. My bad.

Still, I would modify my point to agree with what you said hokie; widening the ribs or otherwise providing more concrete shear strength would appear to be cheaper unless you needed to save weight or depth.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[Leo Baldwin]

I cannot widen the ribs - the forms are what they are...
Appreciate everyone's input - all good solid advice!

 

[Archie264]

>>>I cannot widen the ribs<<<

Well then, can you shorten them? I.e., make the beams wider?

 

[Archie264]

...or tighten the joist spacing? (Just throwing out some thoughts.)

 

[Leo Baldwin]

nope... 24" o/c always. 4.5" wide joists always.
Depth can vary but i am already using the deepest form that creates 10" depth.
I notice using higher strength mix reduces shear but ever so little.
I wish I could find that place in 318 where it allows shear steel to protrude past primary rebar into 3/4" cover...

 

[hokie66]

3/4" is minimal cover, so you don't want to do that. If you use the continuous snake as described, you don't need to decrease cover.

 

[Ingenuity]

24" o/c always. 4.5" wide joists always.
Depth can vary but i am already using the deepest form that creates 10" depth.

Your rib width, depth and spacing complies with ACI 318-11 §8.13.2 on joist construction, so you are permitted under §8.13.8 to increase your V[sub]c[/sub] by 10%, if you have not done so already.

 

[KootK]

If complying with cover is the difference between a design that works and one that doesn't, then your design is already okay within a reasonable degree of engineering accuracy. No further effort required. I see this making, at most, a 5% difference in capacity.

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.

 

[BAretired]

It seems strange that your pans are so rigidly defined, i.e. no tapered pans and no permissible variation in joist width or spacing.

Perhaps you could consider using sections of deformed welded wire mesh (WWM) as shear reinforcement at the ends of each joist. They would be fairly simple to install and fabrication costs should be relatively low. You should not encroach into the 3/4" cover, however.

BA

 

[Leo Baldwin]

BA, the WWM idea is a good one. I dont think it needs to be deformed, 4x4 single strips that are 12" tall and the length of 8 feet should slip in just fine...