Shear key design for base slab to resist uplift in caisson 2

[satheshkumar]

To resist the hydraulic uplift for a caisson, I thought of providing shear pockets (say 400x400x75mm) on the caisson walls at 1m centres horizontally so that the base wall can be cast into the shear pockets which would give good shear capacity against the uplift. Also, I was thinking to provide some dowel bars within the shear pockets.What will be the design for this dowels? Can anyone help on this?

 

[ishvaaag]

Please provide a sketch to clarify your design intent.

 

[hawkaz]

Interesting idea- How would this be constructed? I gather someone would need to drop into the caisson to hand dig these shear pockets and tie the rebar

 

[satheshkumar]

While casting, polystyrene will be placed at the pockets which can be removed later to form the shear pockets. There will be a concrete plug to stop the water. People can go down and weld the dowels to the caisson reinforcement.

 

[ishvaaag]

Well, lacking the sketch, one can muse that the design of dowels crossing the interface between concrete in the caisson and concrete in the plug should be dimensioned according to shear-friction principles, or, more classically as single groups of inclined rebar against shear. Strut and tie schemes can also be brought to usefulness... so one must ensure a maximum shear stress is respected as limited by shear strength of the concrete, compression strut maximum stress and tie recommended maximum stress.

Once the shear in the concrete is approaching its ultimate value, the strut and tie scheme will be representing the situation, in spite of the fact that at lower solicitations and on the key action the steel may not be much stressed.