Plate Results - Shear Near Top Edge - Elastic Model

[ATSE]

I'm getting unusual out-of-plane shear results for a simple plate model.
Model Description:
1) Program = RISA (linear elastic)
2) 30ft x 30ft x 2ft thick
3) Elements are 2ft thick (out of plane) x 1ft wide x 1 ft tall
4) top edge is pinned out of plane only, free otherwise
5) sides and bottom edge are fixed
6) loading is triangular, based on fluid: 0ksf at top, 2.73ksf at base
7) E (modulus) based on f'c = 4ksi


Problem:
a) Horizontal shear Qx along top edge is very high
b) The rest of the model results agree with solid mechanics theory, except the top.
c) Top reactions agree with theory.
d) Vertical shear Qy agrees with theory.
e) Attachment: pdf plots of Qx, Qy, Mx, My

So here's the question: Is there a good theoretical explanation why the out-of-plane shear along the top edge (i.e. beam action horizontally) is so high, or is this a modeling error?
[I have a similar post under RISA forum, but this is more of a FEM problem].

 

[ishvaaag]

Looks like a shear diagram for a beam reacting the actions in the horizontal plane. Give a look to the deformed elements, this may confirm that the center of the panel when bearing towards the top pins produce this effect. Curvature may be there bigger and, not there being there moment restraint it is up to the shear mechanism to cope with the relief it supposes respect the more rigid side and bottoms.

 

[cntw1953]

Yes, I would have a quick look on defomation as pointed out by ish.

Another question in mind, if the top edge is pinned, why My exists along top edge (assume my is bending about x-axis).

 

[ishvaaag]

I interpret the contoured diagram means there to be from -2.7 to 7.2 kip ft. It may then be 0.

 

[cntw1953]

You could be right.

 

[ATSE]

ishvaaag,
You interpretation of the contours is correct. There is no appreciable moment being reported at top edge.

Would it be correct to say that the My moment along top edge is simply thrown into the top slab, which has no problem resisting this shear (which is tension in the plane of the horiz slab)? That is, the result may be statically correct, but the design impact of the wall is null?

 

[cntw1953]

The other thing is you should do a quick check on nodal/joint forces along the top edge (in both directions) to see if they make sense.

 

[cntw1953]

Also, you can check your results against that tabled in BOR's monograph, if it is available to you.

 

[ishvaaag]

Normally the impact of the push of one level underground or alike is scarce or negligible on the slab at ground level; this can be checked, anyway. Even 5 cm thick minimally reinforced toppings attached to one way systems use to work well. 4 months ago I saw (again) an example of orthogonal steel beam connectors having to sustain the full thrust of several levels of cellar walls. With reinforced concrete practices this is less usual yet easily done if wanted.