Guidelines for the design of hull supporting structures for deck fittings

Discussion in 'Class Societies' started by Vinassman, Apr 7, 2011.

  1. Vinassman
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    Vinassman Junior Member

    Hi everybody,

    Do you know if there is a guideline for the design of hull supporting structures (such as carling, deck reinforcement, ...) for deck fittings (bollards, mooring winches, capstan, panama chock and so on)?

    There are a few pages in ABS rules but I don't really get how to apply the design load since it should imply bending moment around several axis in addition of normal and sheer stress, isn't it?

    Otherwise, if you know any -good :D- structural and/or mechanical books which might help me on this matter, I would appreciate as well.

    Thanks a lot.

    VM
     
  2. gonzo
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    gonzo Senior Member

    You have to first determine what rule you are designing for.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    The guidelines, from Class, are always just "general". In the case of say ABS, as you quoted, for small boats you can find in Part3. Sec.14 of theirHigh-Speed Craft rules.

    You would not find what you're looking for in terms of ....use force XXX..an a bollard. Or use moment YYY on davit etc.

    Why?

    Well..what size davit...what reach, and max capacity?...there are more commericially available davits, off the shelf, than you can shake a stick at. It is pointless making a rule for each.

    In the case of a davit, you look at the max loadings provided by the manufacturer. Then take into account the location on the boat and then the motions of the vessel on the davits load, and hence foundations as an additional dynamic load. Same for a bollard..what displacement is your boat...how many bollards will be used, where is you primary axis of loading? etc etc

    Then design your structure, as you have already implied, to take said loads/moments etc.

    If you are unable to perform this type of simple analysis/design, you should give the task to someone who can. As this is basic structural design for any naval architect/engineer.
     
  4. gonzo
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    gonzo Senior Member

    If you hire someone, make sure you accurate specify the intended use of the vessel.
     
  5. Vinassman
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    Vinassman Junior Member

    @Ad hoc: do you have any reference book/material for this kind of calculation? I don't really understand how to apply the load on the supporting structures: bending moment? Sheer stress? Normal stress?

    I tried to look in a lot of books but cannot find anything... FYI I'm talking about deck fittings used on 15,000 DWT bulk carrier.

    Thanks anyway for the information to both fo you.

    VM
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    I think any standard mechanical engineering book or structures related book would provide some guidance.

    All you need to do is, say in the case of a crane. Find the feet/attachment details, supplied by the manufacturer. The feet details will be a series of bolts, in a set arrangement or pattern, or similar. The technical data sheet of the crane will tell you what is the max load experienced at each location, i.e. each bolt. You also need to take into account its load cycle, for fatigue and also the ship motions..and of course adding a factor of safety, to arrive at a SWL.

    You then lay the feet arrangement onto the deck, where the structure that supports the crane is being located either a deck (main deck or otherwise) and you make a pedestal/seating for it, to sit on the deck.

    You establish the load paths from the bolts, into the deck or pedestal, and work out the scantlings required, from the loads supplied and how it affects the surrounding structure. And make changes accordingly.

    If this is beyond your ability, you need someone in your office to show you how to do it.
     
  7. Vinassman
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    Vinassman Junior Member

    @Ad Hoc,

    Thanks for the explanation, it's very helpful! However, I'm not sure to understand how to establish the load paths from the bolt into the deck or pedestal... And sadly, nobody can help me overhere.

    If we start again with the crane example, let's say we are using a skid. Then the bolts are tighten to this skid and this one is welded on the main deck. Since the bolts and the skid are connected, I suppose we can use the same loading to assess the strength of the skid (if the supplier gives you for example 10 kN in normal stress and 5kN in transverse, then you can make the scantling). However, to pass to the deck plate and the stiffeners/girders below main deck is where I am lost.

    Sorry if these questions are stupid but I really try to do it by myself and to understand it.

    VM
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Vinassman

    No need to be sorry, we must all start somewhere.

    However, what you are asking cannot be answerd in a simple one liner or a single post. You really need to seek advise from someone who can tutor face to face...otherwise this shall end up having more pages than War & Peace! The amount of instructions and questions you shall ask...requires personal guidance.

    In the graduate naval architects i had under me...to explain such things, was not a 5min job...so you have a long way to go yet im affraid.
     

  9. Vinassman
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    Vinassman Junior Member

    @Ad hoc: thanks for your time and explanation. I will try to get some helps, I have to admit I'm quite "light" when it comes about structures and this question is actually to try to fill one of the numerous holes in my knowledge. Last question: do you have any reference I can look for which might help me on that matter?

    VM
     
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