metal frame detail

Discussion in 'Metal Boat Building' started by Guest62110524, Jul 14, 2009.

  1. Guest62110524

    Guest62110524 Previous Member

    in al al we usually detail like this
    It is accepted by Lloyds and other codes
    the blue plate, which is welded all round ties the flange on the tee, , the rest of the flange where it overlaps the floor, is trimmed off, the web is welded all arount too, the floors are a continous weld to the bottom plate

    The blue plate I would leave off in a steel build
     

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    Last edited: Jun 21, 2010
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    For those reading and looking at this poor detailing, I shall summaries as follows (Whoosh as usual shall ignore):

    1 The joint is not ideal it has made no attempt to reduce the stress concentration of the side frame with the bottom frame joint. As can be seen in the ground breaking paper on structural details (publish by IIW “Fatigue strength of Hold Frame Ends in Ship Hulls”, by Iida & Matoba). The SCF is around 4, in attached image from paper.

    In other words whatever stress you have designed the frames for, at the base, iwo of the toe it is 4 times higher. This is significant and is extremely poor detailing, and just indicates a total lack of understanding of structural principals/design.

    2 So, to reduce this SCF what has been done? A thick cover plate has been added. Well this attracts more problems than it solves, or tries too.

    Firstly. The corner joint is hard, ie not a smooth radius. Therefore, given that is says 12mm plate, the minimum bend radius is 3, but better to use 4, for ‘safety’. This gives a radius of 48mm. That plate has not bent into a smooth radius it has been pressed. As such the SCF remains. In addition, this 12mm plate that has been pressed and has yielded locally; the inside corner is too tight (another high SCF) and consequently the other face (on the rider side) has yielded plastically, ie it has already cracked, microscopically. All it needs now, is an exciting force for it to propagate.

    No doubt the welder will try and over come this by gap filling and or have poor access to the joint. As such this will just encouagre lack of penetration/fusion on the weld. Again, andother futher reduce of 50%, see attached, ref: D.Kosteas.

    Secondly, endless tests have been conducted to show that “cover plates” are a disaster for fatigue. See attached image by Maddox in his excellent work “Review of fatigue assessment procedure for welded aluminium structures, by the TWI. The stress range reduces from 120MPa to around 20MPa.

    Thirdly. The plate itself. The weld, as noted in TWI report creates extremely high stress concentrations. It also makes the weld fial in shear not tension. If the plate were in-plane, the failure would be tensile, ie higher strength. But being on top, out of plane, it is in shear, which is slower strength. Then looking at the weld shape itself. See attached, again excellent work by D. Kosteas “Fatigue Behaviour and Analysis”.

    This can be seen in the very basic concept of SCF at weld toes, again by the authors above in various other papers as well as E.Niemi “Stress Determination for fatigue Analysis of Welded Components” shows reduction in the order of 50%.

    So looking at the joint, it has a SCF of 4, it has already cracked during fabrication, the allowable design stress is now around 20MPa and the fatigue strength has been reduced by some 80% from all the items noted above. In other words, the joint has a very poor load carry capacity and will fail very quickly under any high load.

    I also find it extremely hard to believe that LR would endorse such a detail, especially considering their guidance notes, see attached.

    But that is just a very rough summary, without going into too much detail of fracture mechanics, stress intensity factors, crack propagation, manufacturing details etc. For those that wish to learn a bit more than...."ive been in this industry for 40 years, can't tell me, you know nothing...".
     

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    Last edited: Jul 18, 2009
    oataru likes this.
  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    oh, forgot to add. Those that think pressing small plates into place is fine, see attached. Designed a composite grill with small spacers in between for waterjet intake grill. The fabricator, being a lazyjack, pressed the corner, and not as the drawing showed, with a smooth radius. It failed within 2~3weeks of service.

    Once it was taken out and a new one put in place with the correct spec's...no problems. As far as I'm aware no problems for almost 10 years.

    Quality = pride, dedication, conscientiousness and consistency.
    Not "i've been...blah blah blah..", you know the story!

    Pity you didn't show the picture of that joint/detail as you did on the other post. Ther was plenty wrong with the whole frame, not just the corner joint.
     

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  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Oh...no other comments on this 'delightful' piece of detail design?

    I did try to keep it brief....oh well :(
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Can you please provide the LR approval stamp/email/letter for this type of joint. You claim this to be LR approved, but i see no reaosn why this will be accepted. The silence from everyone suggests the same too.

    So, as you say, you always back up your claims, please do so.

    I shall be very interested to know who passed it and at which office.....if indeed it has been.
     
  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    While we all wait for those certs from whoosh (again)....don't hold your breath..

    attached are 2 very quick simple solutions.

    One a monohull, showing the frame with a large smooth radius in the chine with the sole butting into it, not the other way around. Therby removing the shockingly poor detail shown above by whoosh.

    And another, a catamaran where the side frame butts directly onto a tank top, like the picture shown by whoosh, but done correctly with smooth brackets at the intersections, not a small pressed bit of plate that just makes matters worse.

    2 very simple quick solutions. The images don't show too much, this is owing to confidentiality reasons.
     

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  7. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Just wondering if you're ever going to produce your claim above that this is approved by LR.

    Since you have not done so in over 1 week, I take it as a "no" then. As I know how much you like to support your claims...when questioning others.
     
  8. apex1

    apex1 Guest

    Hmmm,
    and I was soo sure we will see whoosh answering immediately. Because HE is the real professional, helping all the people around here, and WE are only picking holes...........................
    here...

    na, at least it is a consolation to know his PC is full of thankful mails......
     
  9. dskira

    dskira Previous Member

    Letter to Woosh

    "The only thing I know is that I know nothing." This is a quote by Socrates
    Whoosh.
    I made this quote mine, and thanks to the forum I started learning humility, and the respect of other. Never too late to learn! Let me tell you, I learn the hard way. But it worth it.
    Please, post positive thought, post what you know best, and I am sure you know about something.
    But you are so against or very pompous that it is difficult to know exactly your scope of knowledge.
    I read in the forum a lot of posts from highly regarded professional, and if I want to know something I just ask.
    Post what you know, post to help, post to share. post your little brick so the house of knowledge can be built every time bigger.
    You always try to take of the brick under the edifice.
    Built Woosh, built, do not destroy.
    I am of topic, but on the other thread about Richard's vessel, he is humble enough to ask our opinion about his OWN boat, lets respect him, it is a sign of great professionalism. Help him if you can with your real knowledge not rambling wild guess. He never asked to agree with him, he wants opinions with a backing of experience or knoweldge, or just honnest and genuine thought. Plaine aand simple.
    You know that Ad Hoc is a very highly trained professional, read all his posts to be convinced, ask him a question, you will learn a lot from him.
    You know, one day Richard's vessel will be launched, and looking at the picture of his vessel I will be saying in myself: you see this minuscule little bolt at the aft end of the hull?, that bolt perhaps is my contribution. And that Woosh it's what it is about. Contribution. Contribution. Contribution.
    Please elevate yourself, with constructive thought, and read the posts.
    Thank you
    Daniel
     
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  10. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    "...and I was soo sure we will see whoosh answering immediately..."
    Well, you know the old Chinese proverb, an empty can makes a lot of noise...

    "..Contribution. Contribution. Contribution..."
    exactly, exactly, exactly..

    Still not seen that "...accepted by Lloyds and other codes..", such a simple thing to post given the constant "I've been building for 40 years can't tell me anything" or the "look in my gallery" ...maybe the LR stamp dwg is in the gallery and i cannot see it?
     
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