I wasn't actually visualizing a T stiffener, but rather just a web without a flange fitted, which is permissible by ABS provided it meets the shape limits. Note that the ABS calcs for cones and cylinders assume a flange which is symmetrical about the stiffener web. Even in conical shells, the stiffener flanges need to be parallel to the cone axis, and not to the conical surface, in order to use the ABS equations. Conical stiffener flanges would require a dedicated analysis from scratch.
Stiffening of heads is outside the scope of the ABS Rules, so this would require a dedicated from-scratch analysis under all circumstances, but it seems logical to me that circumferential stiffeners would have some manner of strengthening effect - if you look at this on a per-bay basis, it is almost the same thing as a conical shell with a taper angle interpolated between diameters. The problem will be the effect on overall buckling of the head, and this may come in to play quite quickly, rendering the stiffening scheme impractical. My S.W.A.G. is that this is probably applicable within the 60 degree half-apex angle allowed by ABS (considering the surface tangent angle at the stiffener location), with diminishing returns at increasing angles, but I'd need to look at a few FEA examples to provide an informed commentary.
Interesting to note that the ABS equations for conical shells are similar to those for cylindrical shells, if you use the instantaneous curvature of the conical surface in a plane perpendicular to the surface, instead of perpendicular to the cone axis.
-Sean
On Nov 2, 2009, Cliff Redus <cliffordredus@sbcglobal.net> wrote:
I am referring to Sean comment on parallel circumferential inner stiffeners. I was visualizing standard T stiffeners. The web should be cut with plasma cutter and also the flange but the flange would have to be rolled into a cone shape. All of this would have to be positioned correctly and each of these circumferential stiffeners would be a different circumferential length. Doable but for me at least, a bit harder to fabricate.
Cliff