Reinforcing concrete shell

[hyper]

hi, I would like to know if my proceeding is ok. It seems accurate enough.

I have F11,F22,F12 (internal forces of the shell) (Ultimate-str.; NOT service loads)
then I found the principal forces Fmax, Fmin. and design:

1. Tension As = P/(phi*fy) phi=0,9
2. Compression "check P" with phi [0,85 *f'c(Ag-Ast)+Ast*fy] phi=0,7
but simplify Ast*fy ---verify thickness---
3. check flexural/torsion zones


What method can I use to check cracking? Is it ok the Lipnitski method (used for silos)

Thanks
Regards.

 

[ishvaaag]

A shell should not be reinforced with meshes of separations bigger than twice the thickness; at this separations of rebar one first attempt to evaluate the crack width is to adjudicate to every single crack the elongation corresponding to the mesh module. If reinforcement is equal in both directions the mechanical capactity is equal to the value in each direction of the rebar for whatever the direction.

A traditional way of limiting the crack width is to limit the service level tensile stresses in the rebar to 1000 or 1200 kgf/cm2.

It will be in any case interesting to evaluate if the shear or tensile stress causes the concrete to be entirely cracked; for this cases you may consider you have some set of cables with concrete hanged of the rebar, and maybe an alternative design is better.

In more than that, many things made with shells need notwithstanding proper additional waterproofing, be the construction of, say, roofs or tanks.

You can also use prestress to help against cracks and tensile forces.

In your named checks I find missing the checks related to the buckling of the shells, that for many of them are paramount, especially vaults and domes.

The formulations used for what I remember are quite consistent with the elastic buckling of the shells for the proper case and shape, except that very adequately, a degraded modulus of deformation of the concrete (Young) is used in the checks, to prevent that with age the structure may buckle.

Presently this calculation can be directly approximated in the FEM analysis by inclusion of P-Delta effects caring of the geometrical nonlinearities- or iterating for the sucessive deformed geometries- whilst using the degraded modulus of Young. Doing so you will have proven that your structure is stable at old age as long as akin to the model.