Class Acceptance of Square Tube Framing

Hi All,
1) Do any/all of the classification societies accept square (or similar 'closed' shape) framing?
2) Are they just and variation on standard ring frame dimensions?
3) As an example, is a 3 3/4" X 1/2" frame with a 1" X 1/4" flange the same as a 4" X 1" X 1/4" channel ('open' end against the plating)?
4) If the above is true, what about a square section? Would a square of the same dimensions (4" X 1" X 1/4") be used for expedience, when the outboard 1/4" thickness is unnecessary?
I know Dave Gerr, in Boat Strenght, say "Water can get into or behind these shapes where corrosion can take place, but you can't ever see it to inspect."
Any other thoughts though? Have you ever used them? What benefit(s) does it give?
Thank you.

 

Welcome to the forum.

You can use any shape you like, so long as the modulus requirements are satisfied.

However, you must bear in mind in-service inspection and maintenance and of course ease of repair.

The "environment" the scantling is in also affects the choice. For example, RHS/Tubes etc are not allowed as stiffening inside tanks, especially fuel tanks. Since if they corrode they loose their strength and the stiffness gained by the said scantling has been lost; which could be a serious issue.

Good luck.

 

I appreciate that clear and helpful reply. Thanks Ad Hoc.

 

You have in mind that a part of the profile does not contribute almost nothing to increase the modulus of the reinforcement but it increases, rather, its weight.

 

Good point.

 

I appreciate your contribution TANSL; thank you. In my example of a 4" High X 1" Wide (OD) square tube with a 1/4" Thick wall, I do expect that the continuous outboard 1" Wide X 1/4" Thick face attached to the plating, wouldn't contribute almost anything but weight. Since I've already pointed that out, I'm not sure which other part of the profile you're referring to, that contributes almost nothing but weight. Or maybe I just wasn't clear enough. Again, thank you.

 

You're right, the face attached to the plating contributes little to the module and the two sides perpendicular to the plate collaborate less still. That is, the face directly attached to plate and one of the faces perpendicular to the plate could also be deleted. From this point of view, an angle unequal sides is much more effective than a rectangular tube.
Probably you will have to compensate for the slight loss modulus with an increase in the height of the profile.
Furthermore, the bending operation, or forming, of an open profile is easier than with a closed one.

 

Thanks TANSL. That's exactly the kind of discussion that's of interest right now. I wonder if you might have missed that in my example, the original web was *1/2" Thick*. The square tube substitute has two perpendicular sides 4" High X *1/4"* Thick. Are those two things indeed equal from a scantling point of view? Also, I get that the 1" Wide X 1/4" Thick 'flange' against the plating is extra. I was wondering though, if builders who use square tubes have a special extrusion or process that mitigates that extra weight, or do they just ignore that extra weight?

 

From the point of view of getting strongest structure as light as possible, use a tube makes no sense. I have never seen use tubes as frames. The tube is very useful as a pillar.
In metal buildings I'd only use the tube if it were impossible to find other profile. Also, as AdHoc says, in tanks are prohibited, even for pillars.
In GRP boats is quite normal to use omega-shaped profiles but this is due to the construction process of the stiffener.
The profile called "flat bulb", widely used in shipbuilding, tries to optimize the weight as it gets a section with high modulus and very little cross-sectional area (light weight).

 

Hi, I don't think it makes much sense, besides the effectiveness in structure already mentioned, the mating surface to the shell plating would not fit up to both sides in a vessel with some shape & only in a flat bottomed/sided barge. In this case would also the mating surfaces become a lap joint & suffer corrosion issues between if not seal welded all round plus also to RHS ends?
Jeff.

PS: I seem to remember a steel trawler amateur built at a yard I worked at in the eighties not gaining survey on this basis... also lthe welding to this vessel may have been the cause..

 

You don't like tubes for framing. Most architects and builders don't. I get that. Thanks. Does anyone know if those two example have equal web strength, or why some rare builders *do* choose to use RHS for framing? PS I like you gallery TANSL.

 

Hi waikikin. Mating an RHS to plating is also something I've been wondering about. So they don't have any sneaky tricks you know of? They just weld it to the closest corner of the RHS extrusion?

I believe I understand the basics of the conventional wisdom on this subject. I try not to make or take to many assumptions. So I'm trying to fully understand the cons *and the pros*. Since Ad Hoc answered my only question that was 'class' related, I think I'll re-phrase and continue in the more appropriate forum of design. Thank you all.

 

Since you're now digressing.

There are no sneaky tricks

As noted above, being an RHS, you have a "flat" section on the 'base'. It requires another flat surface to mate correctly. If you are using an RHS on flat deck plating or flat side plating or superstructure/deckhouse plating which is also flat, then you can do this without any difficulty. If there is curvature, you first have to consider, how are you going to bend the RHS without fracturing it. Bearing in mind, the main reason for choosing an RHS, is its stiffness, ergo not easy to bend!

However, any other 'reasoning' is made by the designer. Does it add unnecessary weight, is it easy to join etc etc....whatever the pro's and con's of these can only be measured against your SOR.

Since if the overall weight was say 20% higher than if "conventionally" stiffened, but the finished boat still performs as designed, so what?

There are very few "absolutes" in design. The purpose of design to solve the question. The question being here is the SOR (Statement of Requirements); can you design something to satisfy the SOR.

How you achieve it, is totally up to you and for your to justify. The technical merits of each "solution" generally dictate which way to proceed. Do not be prejudiced by "other" design solutions, make the technical case for each, so you know what will and won't work but importantly, why.

In this simplistic case, you may find that 20% increase in weight is unacceptable. Thus you select a different section. However, what if the outcome were that you saved 20% in labour costs and construction time, because no welding/bending of a rider bar to a conventional frame web? Thus may be heavier, but cheaper to build. Until you investigate, you will be constantly guessing.

Thus, never start with a prejudiced mindset when designing. Let the "facts" speak to you...not the other way around.

As I have said many times before, if you want to design a boat made out of Switz cheese, go ahead, so long as you can satisfy yourself that it "passes" the requirements that either you require or listed in the SOR.

 

The applications I have used it for is for trailers, transport & storage cradles, laminating table frames, steel "horses" for workshop, also in the strongback for mold station & frame set up, once I used some stainless RHS in the framing to a cabin clad with a plastic sheet product- this was for a mooring barge & the cabin sides in one plane with no curves, my building shed also uses Duragal SHS in it's columns & trusses 9 http://www.boatdesign.net/gallery/showphoto.php/photo/20458/size/big/cat//ppuser/12438 ) RHS/SHS is terrific material to fabricate things quickly, esp as the corner radius is kind to humans & also generally sets up as weld prep for component intersections.
Jeff.

PS: I also once used it for cambered cabin deck beams in a small(25') timber houseboat, in this configuration it added a couple of inches of headroom over using timber beams.

 

Thanks everyone. I like the way you said that Ad Hoc. I'm thinking of RHS to save labour. I only hear about it's negatives, but a few are using it. Before I dismiss it outright, and certainly before I seriously consider it, I want to keep an open mind and give it a chance. That means, as I said, understanding it's pros and cons. Just bought (the only) three current books on aluminum boat building I could find. Just hope they discuss 'alternative' kinds of materials and methods. Wish me luck.

PS Anybody have suggestions on where to find out about 'state of the art' aluminum, or even hybrid materials and methods?