Shear links in flat slabs - Is it only necessary to hook them around the tension reinforcement?

[HermanLJ]

SABS code says you only need to have it fixed to the tension reinforcement (i.e. the top reinforcement in 99.99% of the cases). However my logic disagrees and I normally detail mine as follows:

http://imgur.com/Bu2PMKp.

Would it be okay to just have it hooked around the top as per code? How do you guys normally do yours?

 

[hokie66]

Normally? I have never used shear reinforcement in a flat slab.

 

[HermanLJ]

You've never had to deal with punching shear? I refuse to believe that!

How do you get away with it?

 

[hokie66]

Of course I have had to deal with punching shear. In a flat slab, I would make the drop panel deeper.

 

[EngineeringEric]

Hokie can get away anything he likes in my honest opinion. But I am curious as well for an elevated concrete slab how one accounts for local shear or if the top/btm reinf. can be shown as sufficient in 99% of the cases... For instance a 6" slab is too shallow for any type of vertical tie and where would you locate it, over the entire slab or just one area?

 

[HermanLJ]

Sorry, I thought maybe you meant drop panels or column heads. Drop panels/column heads however often go against the architect's vision for the building so we hardly ever get to use them.

 

[hokie66]

Well, then you need to get the definition right. A flat slab by definition has drop panels, while a flat plate doesn't. The thickness design of a flat plate is usually controlled by the depth required to resist punching shear.

 

[HermanLJ]

@EngineeringEric, no I don't think you can assume that. I've had a few cases where my calcs showed that the local shear was more than the capacity of the slab. I would just design it like one would a beam, and then also check for local punching as well if it was a concentrated force and not one resulting from shear at supports. Then I would over design my reinforcement by a pretty large factor just to be safe (lets be honest, shear reinforcement is negligible cost wise).

 

[HermanLJ]

Apologies hokie. We use different terminology here so I think it was just a simple mix-up, no offense intended.

 

[bookowski]

I agree with your logic - I would not just hook the top and leave a straight bar on the bottom (I think that's what you are saying). We typically use proprietary products - like studrails or lenton shear reinf etc., however I have used stirrups like you show for thicker slabs and have always assumed that you need to anchor both ends.

 

[HermanLJ]

@boolowski, but even the free end of the studrails aren't anchored any more than what a bent bar would offer? And the thing about the code is it doesn't say that the end has to be a bent bar, the free end of the bar could be straight and still conform to code...

 

[hokie66]

If your code says that, it is simply wrong. Neither the ACI or the Australian Code would permit straight ends on shear stirrups.

Studrails are anchored at both ends. On the bottom by welding to the rail, and on the top by the head of the stud. I don't use them or support their use, but that is the way they are supposed to work.

 

[rapt]

I agree with you Hokie, extra concrete should be the solution. Unfortunately everywhere else in the world uses flat plates and punching shear reinforcement. Some codes limit the minimum depth in this case (BS8110 and I think EC2 limit to 200mm minimum). I do not believe that is low enough either and I think a lot of it works simply because the design methods are conservative and loadings never reach the critical values.

If using ties, they must be anchored at both ends around the flexural reinforcement.