Shear force increase near hole in out-of-plane loaded slab

[maskarp]

Hello everyone,

I first tried these questions in the Finite Element Analysis forum without getting any replies. I hope it is okay to try here. It is the impact on concrete design that I am mainly interested in after all.

My questions are regarding shear forces near holes in FE analyses of slabs. To better illustrate the problem, I put together a small FE example (please see the attached picture).

FE EXAMPLE:
- a rectangular slab is simply supported at all edges
- the slab is loaded out of plane by a uniform surface load
- the slab is modelled using shell elements (i.e. 6 DOFs/node)
- results show the shear force variation along a section of the slab

QUESTIONS:
A) Why does the shear force increase in the region of the hole? Is this only a result of some singularity problem in the FE analysis, or is the increase to be expected in reality?
B) Why is the increase in shear force greater in the region of a small hole than a large?
C) Why would you / would you not design a concrete slab for these peak values in shear force?

 

[KootK]

A) I suspect that this is the result of load "flowing" around the hole.

B) I speculate that, when the hole is of larger radius relative to the plate, the stress has more time to sort of see the hole coming.

C) I wouldn't design for the shear spikes. Concrete has plenty of plastic redistribution capacity, even in shear. Taking advantage of that would yield a more reasonable reinforcing scheme.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

The hole is created at the point of greatest bending. KootK makes an interesting point that it could be "load" flow. I agree since your load orientation will change with bending causing a confluence of loading. Further, since this is the highest bending area, you have compression shear flow in the top and tension shear flow in the bottom.

 

[KootK]

You've established your reporting section at what is theoretically a line of zero shear, correct? If so, it may be the case that the effect of local discontinuities simply appear significant in comparison to the near zero values near by. I'd be curious to see what this would look like with the holes shifted to the 1/4 span position in one direction.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

...or, change to a rectangular section w 2:1 ratio and see how the stress pattern changes