Slab shear

MIStructE_IRE · Dec 9, 2020

Hi,

I have a short one way spanning transfer slab which supports a line load from a concrete wall above. This wall supports 3 storeys above and so the line load is significant resulting in high shear forces.

An outline sketch is included below.

The wall (line load) is not close enough to the support to benefit from shear enhancement.

The slab is supported by walls rather than columns so I have no punching shear issues, just slab shear. I have no issues with bending etc but the slab currently fails in shear.

My usual approach to slab shear is if it fails - its too thin! However in this case I have no option to thicken it due to architectural constraints. Has anyone ever put shear reinforcement into a slab? Personally I never have! Any thoughts would be great.

KootK · Dec 9, 2020

MIStructE_IRE said:
I have no option to thicken it due to architectural constraints. Has anyone ever put shear reinforcement into a slab?

Sure:

1) Stud rails all over the place. That's usually a two way shear thing but can certainly be done one way. Not cheap of course.

2) Overt shear reinforcement done similar to beam stirrups as individual candy cane ties, saw toothed bent bars etc. They make a light gauge bend strap thing for this too.

What's best probably comes down to what's commonly available and preferred by contractors in your area.

KootK · Dec 9, 2020

Humor me with some proportions. I've done plenty of arching worthy of Bikram in my time.

r13 · Dec 9, 2020

Has anyone ever put shear reinforcement into a slab?

Yes. Nat before increase fc'.

phamENG · Dec 9, 2020

I'm wrestling with some one way shear hot spots in a 2-way slab model right now and convincing myself of an acceptable average width, so I sort of feel your pain.

How about some old school draped reinforcement that crosses the shear failure plain? I suspect that's what KootK is getting at with #2, though done with independent bars rather than playing with the flexural steel.

KootK · Dec 9, 2020

Similar to r13's comment, if you're close you might get over the hump with using your 56 day f'c or a locally poured strip of stronger concrete. Unfortunately, that square root in the formula keeps gains pretty modest.

MIStructE_IRE · Dec 9, 2020

Thanks Koot,

Yes, I’m considering candy cane bars peppered throughout the slab. If it works - it works right!

I’ve looked at the strut & tie models for your sketch suggestion and it is justifiable in one area but not others... To answer your dimensions anyway:

A - 250mm in one area (fine) but 2000mm in another... (doh!)

B - 450mmm

C - 250mm

D - 215mm

Thanks Pham,

Yes, I considered the old skool cranked bars also but I’m leaning towards links... Essentially designing the slab as a 10m wide x 450mm deep beam! Cost is what it is, this just has to work.

I’ve just never done shear reinforcement (for this reason) in a slab...

KootK · Dec 9, 2020

I've seen people do this for basement walls but, at 450 depth, I like ties too or bent saw tooth depending on your confidence with QC.

KootK · Dec 9, 2020

OP said:
A - 250mm in one area (fine) but 2000mm in another... (doh!)

Agreed. A shear span of 4.5 is getting to long for arching.

r13 · Dec 9, 2020

Yes, I considered the old skool cranked bars also but I’m leaning towards links...

The former is easier to place and more stable.

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