Structural panel bulkhead joining question.

That pretty much sums it up I’d say.

However, the effects are different, as I’m sure you’re aware. It all depends upon how you wish the structure to perform and what is surrounding it and of course what, if any, factors of safety you wish to add.

1) Continuous laminate

The hull skin is structurally sound, and performs the job as designed. Whatever that may be…we don’t know, only groper does.
What this means is that the WTB can, as I have previously mentioned, this joint can be designed to be flexible. When you have flexibility at the joint, in the structural sense, there is no stiffness, and the loads pass through it unhindered as reaction loads. Thus in simple language it is like a hinged or pin joint. There is no bending only rotation, transversely it can be considered infinity stiff.

Should the joint fail, the hull remains intact.

2) None continuous inner Laminate

This WTB-hull joint is now one large joint. The load paths rely heavily upon the joint geometry not creating a high stress concentration. Since the whole joint is now the structure and now very stiff, there is no flexibility of this joint. Thus the ‘weak’ area is now at the extreme ends of the overlaminate on the hull, i.e. where the laminate just becomes hull again, and of course the fillet radius (other than the E too).

WTB are by their nature stiff compared to the surrounding structure, therefore all the load shall be attracted to this location to transfer any load transversely. If this joint is stiff as well, then the hull structure immediately either side of this joint must be able to take this increase in load. It shall be an increase over and beyond that which the hull skins are ‘nominally’ designed to take.

Should the joint fail..the hull structural integrity has been compromised.

So you need to establish the load paths, and if there is sufficient shear area and stiffness longitudinally and transversely to accept the higher load that has been attracted to this region owing to the now stiffer joint. Also to ensure that nothing is attached or located in this region which is not designed to have high stress at its foundation.

It’s back to the EI again, and has groper done coupon testing to establish consistent E of the layup in all the planes, x-y and z?

 

Exactly, Samsam seems to be one of few people that actually understood the problem...

In the initial drawing, i only defined the hull panels, BHD and tabbing... i gave no detail as to the composition of each panel and what theyre made of besides a foam core FRP sandwich... the edges could have been decored and reinforced in the join area, or god knows how many other ways of solving the problem. But the problem, simply related to whether the hull inside skin needs to be continous under the BHD...

The solution, will be to join the panel first (glass both sides), then offer it upto the BHD as i dont feel comfortable otherwise, considering everyones advice... The logistical problem has been solved allowing me to do so. In another area with a similar problem, i have opted to make a fibre flange on the BHD edge first, then bond the 2 panel edges onto it, followed by the normal tabbing thereafter. So in essence, the fibres are again under the BHD, although still secondary bonded...

 

I wanted to revisit this as im still not convinced, despite having already completed the building of the part in question as per my previous comment.

So, look at a typical concrete overpass road bridge, i see it as the same engineering problem analogy to this topic here.

If you take the Upright pillars - which hold up the roadway- as the BHD.

And take the roadway spanning beams- as the hull panels - you arrive at the same problem.

Now take a look at the typical solution to the problem, solved by civil engineers whom are tasked to design these bridges...
Due to logistical and ecomonmic building constraints, typically the upright pillars are errected, then the prefabbed spanning beams are dropped in place via cranes forming each section. An expansion joint is incorporated directly over each upright pillar to allow for thermal movement of each section, some sections are even mounted on rollers to allow for this. In effect, each spanning section is completely isolated from the adjacent section.

Not withstanding windloadings, the typical load definition is a maximum traffic situation acting downwards on each span section - which aligns nicely with a water pressure acting on our hull panels.

So why is the "status quo" in boat building, to continue the fibre under these sections, when each hull panel - located between BHD`s - can be treated as a simply supported beam at both ends???

 

get on with it ansd stop all this messing around .

No!!! the bulkhead tabing is just that !!
the joining of the hull skin is something else . Do the skin join first and then tab the bulkhead over it !!
Whats you problem ??.
Enough of the idiotic theories just get on with it . One step at a time !! Do it !!.

 

On the one hand yes, but on the other no.

This appears the same but it is not. Have you come across the theorem of 3 moments or Clapeyron’s equations? This is used in such loading scenarios of bridges and support. The key being that it is a continuous beam..i.e. the same properties, which a hull is not. Since on the conditions required to solving thuse equations is that the slope at one support is the same but opposite in direction to the other. Which a changing hull shape, and hence the structure owing to its shape this condition is not met. There is also the bending moment at each support and the resulting reaction load. The pillar on a bridge is very stiff and torsionaly stiff too in all 3 planes, whereas a WTB is only stiff in one plane, it cannot prevent rotation about its support from a bending moment. Which is why these joints must be able to take rotation i.e. be flexible.

This then makes the beam not longer continuous too.

The ends are just being supported for reaction loads only in this case, no bending. The bending movement between supports is taken by the “beam”. Your hull cannot do this, as the WTB is not sufficiently stiff in all 3 axes, and how are the reactions loads transferred and how is the rotation allowed for when this joint is asymmetric in stiffness of the main “beam” member…it can’t.

 

hmm, ok let me define the problem further...

The initial problem related to jointing of "vertical topside panels" or plates, i didnt make this clear in my initial post.... these panels/plates are separated from the deck and bilge panels, which were also fabricated separately. The hull is a very narrow catamaran hull.

It is my understanding that a slender catamaran hull is treated as a longitudinal beam, which puts the topside panels in global shear from hull hogging/sagging, and local bending from hydrostatic forces + wave slamming etc etc.

In the global load case, the shear is "in plane" with the laminate... so provided there is sufficient bracing area (panel area in contact with deck and bilge edges) we should be able to design for it like a trussed beam - each brace between BHD`s... in this load case, it doesnt matter whether the panel is a sandwich or not, only the fibre quantity and orientation, the speration between skins serves no purpose.

In the local hydrostatic load case, we go back to our simply supported beam analogy, and simply has to handle inward bending pressure. Provided the BHD is sufficiently sturdy to support the topside panel ends in inward bending. In this case we want the sandwich to give maximum separation between skins to increase the stiffness in this plane.

Now for the 3 moments problem, i can see a problem where there is uneven loading on the topsides from a wave impact, etc, which would cause the BHD rotation problem. However, doesnt this problem translate into a deck shear problem? So the attachment to the deck and bilges limit the lateral rotation of the BHD... if its a large BHD section, then could not the stringers be double tasked and designed to limit the bulkhead buckling midway between deck and bilge and not just hull panel stiffeners?

So ill agree that an entire hull is not structurally the same a bridge, but why can we not treat a single topside panel, supported between BHD`s like individual simply supported beams and thus not require inside laminate continuity?

 

If this were a very long and I mean over 50m, then yes this would be true. In ‘smaller’ catamarans, this is not so. Since the longitudinal bending moment is minor compared to the transverse.

Not true. If the construction is single skin, then yes, but in a sandwich, no. The core provides the shear force carrying capability. Without a link between the two skins, inner and outer, the thinner skins separated by a distance to get the stiffness which takes the bending, has no load path between the 2 skins, therefore it cannot transmit the shear force as there is no longer any load path..

If you image an I-beam that is spanning 2 supports some distance apart. Apply a load. The reason why it bends is that the flanges take the tensile/compressive loads and the web, takes the shear load. If you now remove the web, the flanges are no longer connected, such that the flange in contact with the applied load is all that is resisting the load. Thus the stiffness of the whole structure has been reduced from an I-beam to that of a single flange. The flange is being asked to carry the bending and shear loads. But it is significantly less stiff and unable to do so.

This is true if the WTB is a support only, and thus takes no bending. But in order to do so, the “beam”, must be out of plane from the WTB, and thus the stiffness of the WTB has no bearing on the hull load; other than out of plane support. Therefore the hull (beam) must have properties that are unaffected by the WTBs support. This means there is no bending about the support, it is ostensibly a simple support case.

If you image a pivot or hinge at the joint, and the hull (beams) can rotate either side of the pivot and the pivot is just supporting the reaction load, nothing else. Thus the rotation and flexibility becomes important to maintain in order for this joint to act “like” a pivot.

No.

You can, that’s basically what you need to do, it terms of the loading. BUT…like in the I-Beam example above, if you remove one flange, you have no strength. The only way to get this stiffness back is by making the I-beam part of the support, in other words your inner flange, or skin in this case, must become part of the WTB to change its stiffness from a single skin to that of 2 skins. But this then becomes a typical TT3 load scenario, and the WTB is now having to take moments, which they are not designed to do. Unless of course you calculate these moments and make the WTB satisfy this load scenario.

But in doing so, the pivot or hinged joint is now “locked”, so this is a very stiff joint. This shall attract greater loads and is no longer flexible. Thus the inner rad’s and the material properties become very important in this method. You may also find that the resulting moments and strains just at the location where the additional stiffness ends and becomes “hull laminate” only again, is greater than the hull laminate is designed to take from a normal sea pressure load.

Stiffness is required to improve the EI of any structure. But you need to understand and follow through the load paths, to fully appreciate the effects of stiffness on any structure. As it may end up being that in doing so it becomes deleterious to make the structure/joint, too stiff.