FEM - Stress Concentration around opening in concrete slabs

[Lawrence B]

Hello Everyone,

I have a general question regarding your approach of assessing stress concentration around the opening in concrete slab. In the attached example I have modelled a 200mm circular opening in a 200mm thick concrete slab. I have a light uniform load applied to the shell surface.

I am trying to simulate an opening coring in an existing slab. Normally my approach is to average such a peak (over a reasonable distance) and validate shear design using plain concrete only as I believe it is unreasonable to rely on any steel contribution without having correct trimming and closing rebar details for a typical opening.

The Qyy results are generating a high concentration of shear at two points around the opening. Using a Eurocode equation for shear strength validation of plain concrete (VRd,c = [0.035 k^3/2 fck^1/2] bw d), taking bw as 100mm I find that the force exceeds my capacity by some margin.

I would appreciate any comments regarding your opinion of my reading of the FEM results and approach undertaken for design validation.

Kind Regards,
Lawrence

 

[EireChch]

no attachment

 

[jayrod12]

Are they flexible enough to miss the reinforcing when coring the hole? If so, I can't imagine a hole that size is even an issue.

Even if they have to cut a single bar, unless the slab is running at 99% I'm sure you could find a way to lose a bar and still calc it out

 

[bkal]

On what basis have you taken bw as 100mm?

 

[KootK]

For a uniform load some distance from any supports, I'd be fairly aggressive in choosing the distance over which you're averaging your peak stresses. Consider that, when we do classic strip design, we typically average the shear over the width of the entire strip. Granted, the current trend seems to be looking at smaller strips...