In-plane torsional strength of slab

I think you are on the right track to check shear along a perimeter based on the ACI punching shear guidelines. I am not sure your second failure scenario is realistic.

DaveAtkins

I am trying to visualize the loading and resisting system ..

-Consider a SOG similar to a spread ftg and supporting a column subjected and concentrated torque in its own plane ,in this case the torque will be resisted by friction btw ftg and the soil and a similar model with classical torsion analysis may be developed for this case,

-If consider , a flat slab system having several columns in both directions and one of the plates subjected to concentrated torque ( by a conc .pedestal planted on the slab ), The torque will be resisted by other columns which could be modelled with STM..

- I would develop a FEM and see the shear force developing around the column for this specific case.

My opinion..






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1) I don't feel that punching shear provisions make sense for this. The punching shear provisions are all about vertical shearing stresses whereas the torsional shearing stresses will be horizontal in nature. Apples and oranges.

2) As with most things, I feel that the stiffest mechanism of resistance will be the best one. To that end, I would propose that this works as shown in the sketch below wherein in plane forces in the slab resist the torsion. Those forces would themselves generate in plane slab shear in most cases but, then, that would be distributed over long lengths of slab and thus tend to be non-critical.

3) My gut feel is that the critical part of the design for something like this would not be the blob of torqued concrete beneath the column but, rather, making shear friction work for the torque at the interface between the column and the top of the slab.