Bulkheads

These are a couple of points I have always wondered about and have never been able to get a straight answer from anyone on..

I have designed quite a few boat structures in steel/aluminium and recently have been trying out composites to the ISO rule. The issue I am having is trying to compare structural bulkheads, one using a plywood core and the other using a foam core (both with eglass skins). This is in the slamming area of a boat doing 40 knots, so shear forces are pretty much determining the structure.

The first issue I have is shear force, I have been assuming that for a sandwich bulkhead on edge the shear is all carried by the eglass skins (whether the core is foam or ply) so all the core is doing is preventing buckling of the skins. This essentially means that the skins will be the same in both cases, but the core will be thicker in the case of foam as it's less structurally effective (ISO rule gives general minimum ratios for web thickness/height ratios etc.) Is this correct or should I perhaps be working out shear modulus of the whole sandwich and then multiply by least shear strain to failure to get the shear failure stress?

The second issue is when does a bulkhead need to be determined by using a frame calculation? Calculating a normal ring frame or structural floor is fine, it has a known height and can be input into the calculation procedure in the normal way, but a full bulkhead has a very large height, which is not necessarily all effective in supporting the plate - in the past I have used a t/16 ratio, i.e assuming any material above t/16 above the plate is ineffective and can be ignored. If it is a full bulkhead which is strong enough to be considered 'watertight' does that mean it is automatically strong enough to support the attached plating/any intersecting stiffeners? I guess by virtue of being watertight it remains fairly effective across its entire span...

 

No, the slamming pressure load and the frame spacing and stiffener spacing, or distance between min and max support is dictating your scantlings.

Incorrect.

The core caries the shear load.

It is the same for any structural member, composite or otherwise.

The skins, or flanges, take the tensile loads, and the web, or core, takes the shear load.

You just to ensure you have enough shear load path from the core, which carries the shear to the skins i.e. the secondary bond shear stress. Since if there is insufficient load path, the core will break free, owing to lack of load path, or simply put, shear area!

A WTB is simply the hydrostatic pressure dictated by the head. Then the support is max beam and depth, to principal stiffening members. There is no magic to it. Thus if a sandwich structure, the beam is from inboard side to outboard side of the hull. However, half the beam is supported by the outer side and half by the inner side.Depth is self explanatory.

 

Bulkheads, like Ad hoc says, are structures that are in tension and compression at the same time. They will wrinkle like fabric when it is tensioned.