Davit Design

Discussion in 'Class Societies' started by Hamish_A, Mar 6, 2013.

  1. Hamish_A
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    Hamish_A Member

    I am looking to retrofit a davit for a 40 man life boat to the side of a ship.

    It is ABS Classed, dose anyone know which part of their regulations are relevant for the stress in the ship structure cause by this davit? Im looking for the allowables values as they give different ones in lifting aplience and ship rules?

    Thanks
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    If i recall..its in SOLAS. I think, from memory, it is a 2.2 times design load as allowable stress.
     
  3. Hamish_A
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    Hamish_A Member

    I have has a quick look through SOLAS and i cant find any reference to anything similar to this I have also looked through LSA seems to be a hard topic to find reference to.
     
  4. TANSL
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    TANSL Senior Member

    I fear that this has nothing to do with SOLAS, so look no further in SOLAS. Looking like you were doing, in the rules of any classification society (CS). An allowable stress of 140 N/mm2 for steel may be more or less correct. This value also depends on you to do a direct calculation or you apply the CS formulas. The answer is in the CS.
     
  5. Hamish_A
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    Hamish_A Member

    Yeah it is definatly not in SOLAS been through that. but ABS rules also dosent have any reference to this and it has a lot of different allowable stresses for use via direct calculation. Cant seem to find a just simple allowable stress for this sort of thing

    i would imagine its the 0.8 times yeild like you said TANSL
     
  6. gonzo
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    gonzo Senior Member

    It is not different from calculating the load for a cleat, bollard or mast. You have a footprint from the flange on the davit and it is either welded or bolted. Calculate the bending moment at maximum load and that is the stress or force applied to the deck. Basically, the davits and the deck structure are two different calculations and certifications.
     
  7. TANSL
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    TANSL Senior Member

    Gonzo, after reading many of your posts (you are one of the most active members of this forum) I almost consider myself your friend. So if I may, with all affection and respect, would convey some of my thoughts: I do not want you to hate me, but you say some things that, to me, makes no sense. Either my English is so bad that I can not understand you either, as you do not have clear concepts, you can not explain your self. And that could lead to people reading our comments to generate very incorrect conclusions.
    Now I will explain why I say this, following your words "Calculate the bending moment at maximum load and that is the stress or force applied to the deck" : The bending moment is one thing and the tension is supported by a davit another. By the way, to calculate a davit must take into account some other stresses acting or may act on it. The davit may be subject to different types of moments, bending or torsional, and various forces.
    Returning now to the bending moment induced stresses: bending moment has a value that does not depend on the davit. The stress to which it subjects the davit said time-dependent SI Boom form, its modulus, its moment of inertia and material. I mean that the same moment, acting on different objects, will generate different value for the stress in each. Therefore, in my opinion, to say that "Calculate the bending moment at maximum .......... " is totally incorrect.
    Otherwise, the davit structure and cover profiles can be calculated similarly, using the same methods as beams, the plates need, indeed, other methods of calculation, plate theory.
    Greetings
    Ignacio López (TANSL)
     
  8. gonzo
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    gonzo Senior Member

    I meant the bending load the davit base imposes on the deck. The poster is asking about how to calculate the deck structure. I think he can use the davit with the maximum load to calculate the bending load. It depends on the distance between the base and the end of the davit and the total weight plus accelerations. In that respect is like a cleat or bollard. I am still not making sense?
     
  9. TANSL
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    TANSL Senior Member

    Increasingly better understand what you say. Anyway: "Calculate the bending moment at maximum load and That is the stress or force Applied to the deck" is NOT correct. In addition, force and stress are different things.
     
  10. Petros
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    Petros Senior Member

    Gonzo,

    The OP did not ask how to calculate the loads, he asked for the relevant regulation.

    Your post is not really helpful.
     
  11. Hamish_A
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    Hamish_A Member

    As Petros state i was after the regulation as i am aware of how to perform the calculation and it is meerly the allowable to use for 'pass/fail' of a design.

    I have since heard from ABS and the said that the allowable should be taken as UTS/4.5 as this is from SOLAS. Has anyone heard of this before? I have gone through SOLAS again and am unable to find reference to this?

    Any help would be a great help again.
     
  12. TANSL
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    TANSL Senior Member

    I recommend you, again, to forget SOLAS. If I interpret well what you say, you know how to perform the calculation and the only thing you would need to know is the maximum sigma that ABS will admit for such calculation. I do not know more than what I said in a previous post (post # 4). Ask the ABS representing in your area (or the classification society you want to use), and let this discussion that is not solving anything.
     

  13. gonzo
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    gonzo Senior Member

    Hamish_A: sorry if I misunderstood your question. Sometimes it is hard to interpret the rules, particularly with ABS.
     
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