# Connection between cross structure and demihull

Discussion in 'Multihulls' started by ktimg, Sep 20, 2016.

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### ktimgJunior Member

Again, another question about cross structure of a catamaran. I think now I realize how to calculate the scantlings of the members of the cross-structure(cross beams+plate thicknesses) from guideline of the ABS High Speed Craft Rule. But, I'm still unsure about the two things.
1. The connection joint of each demihull (ama) to the cross-structure (aka). I'm not sure how to design it. In ABS rules there's equation for calculating the bracket sizes to support structural members. ( in attached photo 2, bracket size 'x' is associated to the 'y' size of the member which is to be supported) In this case, too, can we assume that this joint is the bracket which is to support a cross deck beam and a transverse frame in the demihull.
2. In the attached photo 1, what are those holes called and how do we add them? I have a rough idea that they might be lightening holes. If so, how can we design them?
Thank you.

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### TANSLSenior Member

See attached an extract of "Bureau Veritas-Rules for Yatchs". All Classification Societies have instructions concerning manholes and large openings in web frames.

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### ktimgJunior Member

the height of the cut-out can be as high as 50% of the primary member, the rule says. So, doesn't a cut-out that big in the cross BEAM reduce the necessary Transverse Section Modulus of the cross-structure?? (value which has been calculated according to the bending moments in the beam(+shear forces+torsional moments))

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### Ad HocNaval Architect

ktimg

It is actually very straight forward.

For example, let us also imagine that the hull has a force applied at the half the draft. What happens to the hull? It bends. So what is required to prevent the bending..stiffness!

So look at your item 1, if the hull butted directing into the raft section like a square, is this a good connection? Obviously not…why??..because the butting hull into the rfat is a square connection, in other words it is a stress concentration. The applied load, lower down in the hull must be transferred to the raft structure. The way to achieve this without increasing the stress, is to add a smooth transition…a large radius. Just like that shown in no.1

However you don’t want a large panel of plate between the hull in-board side the main deck (raft), because 1) it adds weight and 2) it is prone to buckling. Solution…split it by adding a lightening hole.

As for no.2 These are called lightening holes. You can place them wherever you like, just make sure you have sufficient shear area available and that they don’t create any localised stress oncentrations.

Simple…

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### ktimgJunior Member

Thank you, Ad Hoc. I totally grasp the background concepts of both 1 and 2 now. But, I'm wondering if there's an equation or class guideline as to how much this radius should be?

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### TANSLSenior Member

The modulus of each cross section, necessary to withstand the stresses in that area, must be respected. Keep in mind that the stress in the area near the neutral axis is very small. It must also be sufficient area of the cross section to withstand shear stresses.
After applying the standard 50% of the section, the above should be checked, at least in areas of the beam where the bending moment is greatest.
Radius : check what the CS's rules you are applying say.

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### Ad HocNaval Architect

Nope.

What "looks" about right. Clearly you don't want a tight radius, not do you want such a large one it becomes difficult to fabricate and causes other internal issues.

Start off with, 300mm, then 500mm, then 750mm, then 1000mm...it all depends on the size of your hull,i.e. the freeboard to the wetdeck and if you also are adding a long.t BHD in the same location too and distance of joint off the hull centre line too.

Just play and see

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### ktimgJunior Member

Thank you both, Ad Hoc and TANSL. I'm totally clear about both things here.
(P.S. TANSL I have GL and ABS HSC rules in my hand, but can't find things in them either about the radius or the joint. Can you please help me find them, from any rule, if there's it? Thank you.)

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### rxcompositeSenior Member

ktimg,

Not the radius you are looking for but the proportions as shown from LR rules. The computations are shown in the rules.

The curved inboard side shell connecting to the crossbeam is called "haunch" and is usually curved (and reinforced) following the curve of the shear stress.

When the twin hull is hit by a transverse wave, one hull lifts up, the other displaces down and in effect creating a cantelever load diagram. Like a fishing rod with a fish caught in the other end, the reel side (connection side) is thick and reinforced.

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### TANSLSenior Member

ktimg, I searched the Lloyd's SSC and Bureau Veritas and have not found anything about this matter, other than the attachement in my previous post. I am sorry.

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### Ad HocNaval Architect

RX

Those are good images, but only serve to act as guides. Since as noted there is no rule min or max for such. It is left to the judgement of the designer. However what should be taken from those images, if not already understood, is that without such, a stress concentration occurs. And whether a simple beam to beam connection of two minor members, or a hull to raft connection, stress concentrations must always be avoided. And as the first image shows, the bracket, it does not state nor will it ever, the radius required. What is in effect saying is, you need a smooth transition to connect two structural members that butt together at right angles. That is what must be taken from those images.

Then the designer can investigate the effects of different sizes, as I noted above.

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### rxcompositeSenior Member

AH
The first two images are from LR. The third one is a teaser which would lead to analysis.

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### ktimgJunior Member

Thanks RX,
I suppose your images (viz. LR) give the bracket size to support two structural units, which is similar in concept to that given by the ABS in my picture 2. So, should I conclude that the radius should be larger than this bracket size? Please correct me, if I get it wrong.
[P.S- As I calculate the bracket size from the rule, the two structural units in this case would be Cross beam and the Side Frame of the hull.]

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### rxcompositeSenior Member

ktimg- Well, not exactly. If you look at the LR guidelines, the intersection of the length effective of a bracket or gusset is 1.5x the frame/beam and the center of the diagonal is much farther. Thus, the bracket is more than 1.5x the frame depth. the center of the diagonal coincides with the radius end. ABS is more precise.

Any major structure must be bracketed.

When a deck beam or crossbeam is uniformly loaded, the greatest shear is at the end connections. If the beam is sized correctly, a simple gusset works as an insurance but creates a stress point due to abrupt termination at ends. This what AH is saying. A curved or radiused bracket will ease the stress concentration.

In the case of a cantelever load where one end sinks vertically, the vertical shear stress is much greater as diagrammed. The shape is more parabolic rather than linear.

If you look at the diagram which has been subdivided into sections, you will see that it is the web that supports the vertical shear and the flanges receives the tensile/compressive stresses.

So what radius? It depends on the design (I am repeating AH again). The web, to be of the same thickness must be able to handle the vertical shear. The flanges, as it moves further away from the neutral axis receives less stress so it can be thinner. Max Stress= My/I. As long as the web and the flanges satisfies the designed stress for a given radius.

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### hump101Senior Member

The class rules aren't explicit because Class expects the designer to use a suitable first-principles method that accounts for the 3 dimensional loading at the joint and shows that the resulting structure meets the limiting stress requirements of the rules.

You need to account for the 3D loading, which includes the transverse bending noted above, plus water pressure loads, shear from racking, any local loads, etc. Before you think about specifying and analysing your structure you need to define and quantify all the loads.

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