Shear reinforcement - Foundation slab

[kostast88]

Can I have an opinion on whether these are contributing at all in shear resistance of a (piled) foundation slab?

On my construction site the designer said yes but I have doubts. In my designs I never considered them, nor I have seen a code reference (in any code) that says you can consider them.

I believe they simply support the upper reinforcement layers to build the base slab. Shear reinforcement should be provided ignoring these.

 

[HTURKAK]

My opinion is ;

-The sketch which you posted is chair for top reinf . However , it will have contribution to shear resistance of the foundation slab as long as located at the probable shear failure surfaces ..

- The slab foundation thickness should be sufficiently deep so that shear reinforcement is not required. In general ,the thickness of the base is often controlled by the design for shear..


- I will suggest you to look Clause 6.2.2 Members not requiring design shear reinforcement .






Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[bugbus]

If the short legs of the 90 degree bends are sitting above the bottom reinforcement mat, then I think it's potentially OK to consider this as shear reinforcement. Not sure what code you're using but I believe most (if not all) would not consider shear reinforcement with 90 degree hooks to be adequately anchored if the hooks are in the cover zone (and therefore liable to bursting out / unfolding).

You'd also either need to have additional bars on the inside of those bends (to consider them immediately anchored at the point of the bend) - or (more likely, assuming they don't want to go to the effort of threading a bunch of additional bars through), leave them as-is and consider a longer development length and adjust the usable height of these bars in contributing to the shear strength.

 

[OldDawgNewTricks]

You could consider the shear strength contibution of the standees if you want, but they are typically spaced too far apart to matter much.

 

[human909]

I would suggest yes. In fact it is a simple and effective way to achieve the vertical reinforcing while provided also providing chairing for the top reinforcement.

The slab foundation thickness should be sufficiently deep so that shear reinforcement is not required. In general ,the thickness of the base is often controlled by the design for shear.

Under AS/NZ structural codes thick slabs are required to have shear reinforcement. I believe this is due to more recent research showing that the shear resistance of the concrete is less predictable for deep slabs. Of course this might not be the case with other codes, but just something to consider...

Though my knowledge of concrete is relatively rudimentary so I'd welcome further discussion.

 

[kostast88]

Thank you for your responses.
Is there a code reference or similar in a guideline?

 

[centondollar]

The strut-and-tie model for shear reinforcement adopted in Eurocodes assumes that all tension from shear shall be taken by stirrups (concrete contribution in is ignored), which means that stirrups continuous (or spliced) along the depth must be used. If shear reinforcement is discontinuous across the depth and located far away from the probable diagonal tension shear crack, the tension at the failure surface will not be resisted by the stirrups.

The same argument applies to other codes. The stirrups must traverse the diagonal tension crack to be of any use at all.