Transverse frame calculation

Lyndon did a very good of explaining, yet for some reason you wish to ignore what he is trying to tell you. Whilst your simplistic approach to what you ‘feel’ is a simple problem does not hold water under scrutiny.

So,
1st Question, do you understand what is a singularity?

2nd question, what is the relationship between a singularity and symmetry in your question?

Mike did not get it wrong, he just did the numbers quickly in his head, to satisfy the “I need a colour plot” for those that do not understand structural behaviour and structural analysis. Since the real analytical solutions derived mathematically appear not to satisfy. Every calculation must be checked, not because they don’t know what they are doing, it is professional procedural QA to ensure a simple error has not crept in which one has become blind too. This is why all drawings and calculations and documents are independently checked, in the professional working environment. Anyone who is a professional engineer, understands this, those that are not, see it as criticism. (There is nowt personal nor emotive about checking and requesting validation/clrification).

As for Mike’s answer I got 347cm3, so both our rough quick checks validate each others, independently. (I just assumed he made a minor error which he would notice upon review, no biggy)

This thread has been all about monocoque structures. It is not a question of “support” it is about understanding how to analysis these structures and the implications of being a monocoque compared to a frames structure. Cleary many, like you, do not appear to understand this. I implore you to read the entire thread, if you have, then you still do not understand, just like Brent. There is nothing I or Mike, or Lyndon et al, can do about that. No amount of pretty pictures with ever satisfy those that don’t understand structures. That has been proven several times on this thread already, judging by the responses. If you have read and understood, then you should understand and know why.

If you wanted to see something concrete, then you should have posted a something mimicking reality as a model, rather than a pointless shape that does not enhance yours or anyone else’s understanding of the basics of structures and how buckling and elastic instability is one of the 4 modes of failure one must address when analysing structures.

They are simplistic, to provide simple understandings (to those that wish the learn the basics) of the complex mechanisms involved. If you wish to understand more, may I suggest you read several books on structural analysis and elastic stability, then after reading them come back with questions for clarification. Since any more than a simplistic explanation by those that already have attempted to provide an understanding has gone over your head.

 

This is so true.

 

The simplistic treating of the very complex shapes of a hull and decks, as if it is a flat surface, standing alone, is another example.

 

Rick

I stand by my estimate for the 3m head, it's going to be close to 18mm.

I thought initially you were just after a weight comparison of methods. Then the errors all scaled similarly but the comparison was valid. I can tell you unframed 5 or 6mm plate will be a poor performer in that form.

 

Again, you are claiming, that which has worked extremely well for decades, a couple hundred yachts, and tens of thousands of miles of cruising, often in some of the roughest conditions anywhere , and in extreme torture test conditions, won't work. You are like the aircraft designer, who claimed that, since theoretically bees can't fly , then they really cant.

 

We are testing your claims. Because you have been stating that the frameless method could be scaled to 60 feet. The engineering basis for these claims is based on some dangerous presumptions and gut feelings which by now you should be able to see are going to mislead people into building very inadequate boats.

You have been told about the Bumble bee myth before. You are desperate for any angle, any spin that sounds like logic, and you don’t care what you say so long as it sounds plausible to the greater audience.

See post 56 Click on the link [http://www.boatdesign.net/forums/metal-boat-building/realistic-scantilings-20941-4.html#post205762]

And have a look here [http://www.boatdesign.net/forums/boat-design/noahs-ark-11077-2.html#post82477]

To dish up a fallacy for the sake of spin is dishonest. It also implies you will do this with structural arguments, like quoting ultimate tensile strengths, pretending buckling doesn’t exist and making false claims about pre-stressed longitudinal framing.

When you are giving cold hard verifiable facts you jump off sideways and start talking rubbish.

The fact is that you made dangerous assumptions in the presumed strength of your construction method. Those assumptions are wrong.

We haven’t analysed your 36 footer yet you can save all the flaming and rhetoric for that. What head of water do you think would collapse the fore-part of your 36 footer ?
How about a 60 footer ?
Why is this important ?
Do you know ?

 

Mike
What I was after was an FEA of the structure using appropriate failure criteria to see what pressure it fails at. It is a complex analysis that specialist still struggle to do with modern design tools.

Rick W

 

Rick
Since I have the flu and it's raining I'll look at it now. It might help illustrate how inuition can be dangerous

I'll do one at 5mm plate thickness and on at 18mm, there will be some slightly different pre-collapse patterns but post collapse they will both be a single crease.

By the way Rhino is a good solids modeller .

 

Mike
Sorry about the flu. Look forward to the results though.

Up here it is a good day for the garage.

Rick

 

Ok, while Mike is wiping his noise, taking his paracetomols and tapping away with the instability issues, I’ll look at the fabrication aspects and their influences. Since no designer is worth his money if s/he doesn’t take note of the effects of fabrication on the static and fatigue life of the design and factor them in to final design.

So, from the other thread, a “yagoo” hull shell expansion was given. Since non can be found provided by Brent, this shall suffice.



The main areas to consider are those in red, green and blue.

Lets deal with the easy one first, the area highlighted in green.

As can be seen, when this is ‘pulled’ together to make its shape, the seam welds and butt welds shall become coincident. In other words, 4 welds all joint in one location. Stress raisers such as this went out with the ark. This is such poor practice, since if one of those welds fail, then all 4 shall fail. This is given by IACS too, the governing body of all the Classification societies. They recommend a positive gap, see attached. That’s just the start of it….but this is covered more by the red areas.



Those areas in red, these are welds, which suddenly stop, in the middle of the plate. Hmm!!....ok, so the welder runs along and then…er…um..stops. Ok, how does s/he stop…where is the run off plate going to go…er..it can’t. So, the welder shall do one of the following. At the end of this run, s/he stop, create a sink, and a crater will form (and possibly a pore will form too), or they’ll just put too much heat in and burn through, or if luckily, shall create an over sized weld bead.

Lets looks at those, and their effects. The TWI has the following on each of these, see attached.



What does this mean, apart from making an already difficult weld for a pro, even harder for an amateur boat builder to produce perfectly?

The effects are seen in the fatigue life and strength of the joint.



The 3 arrows from the left indicate 1) virginal plate, 2nd butt welded and machined cleaned, and 3rd, butt welded and left with over sized weld bead. Since as soon as any metal is welded, it instantly has introduced flaws, which are affected by fatigue loadings.

The fatigue strength goes from 250MPa down to 150MPa. A loss of 40%. I don’t think I would be happy with a welded joint losing that much strength with no structural redundancy available, since there are no frames to shirk the loads to!

Additionally, if the pore or crack has not been identified; I can’t image too many bothering with dye-pen or x-rays once completed; the small hole has the following effect on the localised stress concentration. The stress concentration goes up to roughly 3 times the surrounding. So whatever stress is in the plate, around that hole, its 3 times. Not good when you’ve just lost 40% of the strength available. And of course we are told these boats are all pulled together to “compress”, this is what is termed internal residual stress. So, in the fatigue environment, again, this is cumulative to the already 40% loss, we have a crack of half depth giving a SCF of 3, and up to 15 before the crack/pore passes right through the weld.



As for an over sized weld, again looking at the fatigue curves, but it has also been shown that weld flank angle and the radius of the bead can reduce fatigue life by up to 50% too. Again, this is all cumulative.

Then circled in blue, we have the centre-line welded joint.

So, this centre line weld...what is supporting it??...well the arguments goes, the other plate. Ok, lets assume this, for now, is correct (since Brent doesn’t accept any of the previous analysis examples), what occurs further aft, well, its flat! Since it is flat, what is supporting this welded joint when the vessel is slamming and flexing? It must flex since the vessel is subjected to external loads. These loads create strains, for their to be strain, there is stress. Otherwise, the vessel is not at sea, but out of the water on the hard.

So, what happens to these strains, well, it must flex, so, again, what structure is there supporting this now flexing welded joint? None. Therefore, this welded join is also in a fatigue situation, like those circled in red. So, all class societies, including Brents fav, LR, require these locations to be stiffened to prevent flexing. Not only that, but the method of attachment and detailing itself, as before, has an affect. See the LR guidelines attached, for hull bottom and stems. Also note, the modes of failure, buckling!



I’ve also noted that the plates are in compression and tension, when the plate is curved, as has Lyndon and MikeJ. The retort is as always, it is in compression only. This is not the case at all, see my previous post with the stress distribution through a curved plate. Secondly, since the boat is pulled up, these compression and tensile stress shall not be small, that are required to pull and hold the shape. In tensile residual stress fields, crack propagation occurs even when subjected to compressive stress cycles. Despite the fact it is generally taken that fatigue cracks do not propagate during unloading in the compression phase of the cycle. Since the crack tips are displaced into the tensile regions. So, even when in compression, the stress concentration factor (SCF) can exist and increase, not decrease.

So, without dwelling too much, the simple philosophy that Brent has for his boats, is only applicable when 1) not in Class, and 2) when very small boats. It also reliese on the welder being very very good at his/her job, to not introduce all these unwanted flaws, which shall shorted the fatigue life of the vessel, notwithstanding the buckling issues already raised.

The size of the boat relative to the thickness, as I previously noted, as ‘slender’ falls fowl, when increasing in size. This change in sizes, seriously affects those issued I have noted above.

 

Elastic instability aka Buckling. Sorry Rick I had to cut your nose and tail off to analyze this

Here's Ricks geometrically perfect shape plated in 5mm 350 grade structural steel. It is subject to a nice even pressure ( with no other loads or moments) which was incremented gradually untill it buckled. The pictures are of the 90% load state before it collapsed into a toothpaste tube. A sudden load can have a different failure pattern and an earlier collapse which applies to this model..

Of interest the ends dimpled first.

Look at the stress plot, consider the yield strength is 350MPa , so we are no where near any traditional failure. In fact you could claim that we still have a Factor of safety of 4.7 over yield. The stresses are concentrated as we'd expect.

But of course and obviously it still fails.

Look at the dimples ( I scaled it up a bit to make it clearer ) they are just about to merge together, the last thing holding them from the downhill rush into full collapse are those ridges formed between them. A very small increase in pressure and there is an unrecoverable collapse and total failure.

The pressure was the equivalent of half a meter of water. Or 5 kPa or around 9 tons spread over each surface evenly.

 

Rick

It takes half an hour to do each run on that shape with full non linearity and that can increase to an hour when getting close to buckling with small gentle load increments. ! Having a fuzzy head doesn't help either .

I just tried 3m head for interest and 15mm plate buckled. I ran it on 18mm plate and it passed but then the rules of thumb are usually loaded on the pass side. I'll stop there.
In reality there should be a factor of safety too, allowance for geometric imperfection, deficiencies in welding etc.

On hulls there's more buckling loading; eccentric loads, applied moments from keel and rig and the spectre of collisions groundings and unexpected combinations of loads.

What all this should illustrate quite clearly is the role of decent well designed transverse framing.

 

Mike
That is great work. I appreciate the effort you have gone to.

I expected the 18mm plate to be conservative.

Now you have the model set up would it be possible to look at how long the shape would be if it was 6mm plate under the same load condition?

The model also give insight into where you would place bulkheads to greatest effect.


Rick W

 

Exactly how many 60-foot origami yachts have cruised tens of thousands miles, often in some of the roughest conditions anywhere, and in extreme torture test conditions?

 

Ad hoc
I cant realy see anything that different or difficult in welding up an origami vs any other method of building in metal, pretty much welding 101. As far as the recomended gapping the seams, that's just the recomended, doesn't mean it should be or the only way. Welding up the inside seams first ' inside corner to inside corner ' and back gouging on the outside . The end results are the same ? The distructive testing I have done the welds are considerably stronger than the aparent metal with ether method.
Tom