Concrete two-way slab, localized shear stresses
Concrete two-way slab, localized shear stresses
(OP)
Working on a small two-way slab with an opening and slightly non-parallel edges. Created a quick finite element model to get a reasonable approximation of the moments in the slab for flexural design. There's a small re-entrant corner on one side of the slab which causes a extremely high localized shear stress. In the past, I've generally provided additional reinforcement in these areas but am really wondering how "real" these stresses are? Obviously they exist, but once a crack forms the stress will be relieved and distributed out along the width of the slab, correct? Essentially, you can't shear a "portion" of the slab, you have to shear it from one edge to another, correct?
Please see the attached picture for the slab and stresses.
Also, I am aware that a one-way shear failure can occur between the re-entrant corner and the manhole opening. This will obviously be designed for.
Please see the attached picture for the slab and stresses.
Also, I am aware that a one-way shear failure can occur between the re-entrant corner and the manhole opening. This will obviously be designed for.
Maine EIT, Civil/Structural.






RE: Concrete two-way slab, localized shear stresses
I hold similar beliefs. Moreover, I feel that there needs to be some compatibility between the moment at a given location and the shear there. Once a flexural crack opens up, your shear will start to redistribute in plan too.
That being said, it's tough to know how to take advantage of that redistribution in a conservative way. So I don't. I would treat this as a punching shear situation and add stud rails if required.
The issue arises partly result of modelling inaccuracies. Because your walls fix the slab vertically, and do so a foot or two away from the corner, the FEM simulates almost perfect fixity at the corner and therefore draws a ton of shear. It's a bit reminiscent of the back stay effect for tall shear walls with basements.
If some of your wall supports beyond the corner could be omitted or turned into springs, it might yield more accurate results and lower shears. But again, who knows how to do that accurately and, more importantly, who has the time.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Concrete two-way slab, localized shear stresses
I'd love to see your final rebar layout. She's small but tricky.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Concrete two-way slab, localized shear stresses
Actually, in the interest of comparison, and because I have a light workload today, I've done just that. See the attached image for the difference.
...Yeah, this reduced it to what is clearly a modeling stress concentration and I'm no longer concerned that this was a "real" stress. I'm still curious how other people treat this sort of modeled stress, though.
As for the rebar, it's a 8 ft span for the long direction, H-20 load rated cast-in-place concrete cover. Very small job so I'm mostly going to be conservative on the rebar when I present this to my boss for approval. I'll probably do much of the reinforcement based on conservative combination of the FEA moments and shears combined with the strip design method. I'll post a quick sketch of my rebar arrangement if I have time later today.
Maine EIT, Civil/Structural.
RE: Concrete two-way slab, localized shear stresses
Maine EIT, Civil/Structural.
RE: Concrete two-way slab, localized shear stresses
Maine EIT, Civil/Structural.
RE: Concrete two-way slab, localized shear stresses
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Concrete two-way slab, localized shear stresses
Maine EIT, Civil/Structural.