Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations waross on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

concrete slab - shear reinforcement - better option? 4

Status
Not open for further replies.

mats12

Geotechnical
Dec 17, 2016
181
Hi, Im reinforcin a RC slab (200 mm thick). Since shear forces are still larger than slab shar capacity, I have to add shear rebars.

Blue line in the picture is a virtual crack line - so no matter which option I choose, shear reinforcement (rebars) has to cross this line.
I like 2nd option better but option one is much easier to make since I have only 200 mm between walls.

Which option would you suggest based on your experience and why?

SLABSHEAR_igzm17.png
 
Replies continue below

Recommended for you

Option 1 is not constructible because after you deduct your rebar cover, you don't have enough space to bend the stirrups as shown. Option 2 is better using a shear friction concept, just make sure you can develop the bars on both sides of the crack
 
If you need shear reinforcement in a slab, it is too thin!
 
I agree with Rapt, typically shear reinforcement in a slab isn't cost effective at this point. Option 2 (if memory serves) is better at providing shear resistance. It's just not done as much anymore because it (also) is no longer cost effective.

 
I am not just suggesting it is too thin from a cost perspective.

For a short cantilever transfer like this one, the slab thickness should be based on the minimum depth to satisfy shear without requiring shear reinforcement, and I would make sure I am not near the limit. There can be no logical justification to try to do it with a 200mm slab and then add shear reinforcement to make it work.

And I would make sure there is some fully developed bottom steel in the cantilever as well.
 
As far as reinforcement strategies go, I definitely prefer #2 as well as #1 would lead to a lot of congestion and finicky placement issues. That said, I also agree that it would be far better to be able to make shear work without reinforcement here. The stakes are pretty high and, in a continuous application you'll have to install a ton of those inclined ties to get the job done. I've used a similar strategy on occasion to shear reinforce deep basement walls. I've felt better about that though for the simple fact that I mostly don't believe in monster retained earth pressures coming to pass against basement walls. But that's a subject for another thread.

Based on your previous thread, I suspect that you're pretty close to making it work without shear reinforcement. Is that correct? If so, perhaps an economical strategy would be to use higher strength concrete. It's diminishing returns because of the square root business but, if you don't need much, you might be able to get it that way. Another strategy, in a similar vein, might be to keep the cantilever propped until the concrete reaches it's 56 day strength which, presumably, would allow you to take advantage of an f'c that's a bit higher than the conventional 28 day strength.

MC said:
Option 2 is better using a shear friction concept

It is my opinion that shear friction is not a valid strategy for resolving diagonal tension issues. In fact, I reject it for any application where the resisted displacement has a significant component perpendicular to the assumed shear friction plane. I see this a lot with folks trying to circumvent appendix D breakout issues as well. They get tempted by the notion that the resisted displacement can be conceived of as having a component parallel to an inclined shear friction slip plane. When you drill down on it conceptually as I've done below, however, I think that the logical fallacy of it becomes apparent. There really is not any tendency towards slip parallel to the shear plane. And that neuters the strategy.

Capture_v9baof.png






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.
 
Agree with rapt. But, although I don't use them myself, I am surprised no one has suggested studrails.
 
Agree with Kootk on shear friction, it has nothing to do with this.

Though it is close to a strut/tie / deep beam situation, so make sure the cantilever top flexural reinforcement is fully developed at the end of the cantilever.

I doubt that any design code would allow you to use the 56 day concrete strength for shear, no matter when it is loaded! All design codes I know say to use the 28 day Characteristic strength!
 
Make it thicker.

Also how did you calculate your shear forces?
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor