Deck loading

Discussion in 'Class Societies' started by nikezz, Nov 14, 2010.

  1. nikezz
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    nikezz Junior Member

    Hi,
    I have a question on deck loading. If a deck load say 5 tonnes/metre squared, a 10 x 10 m area will hold up to a max distributed load of 500 tonnes, right?

    Some person told me there is a thing as "load transfer" to a lower deck, so that loads can somehow transfer to a lower deck to distribute out the load i.e. main deck takes load of 10 tonnes on a 1x1 m area, then transfers it to a lower deck of 3x3 m, therefore it is 10 divided by 9. I have never heard of something like that before.
     
  2. MikeJohns
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    MikeJohns Senior Member

    All the weight is ultimately supported by pressure acting on the hull plate. Look at where and how the loads and forces are transferred through the structure. The deck can load via pillars or partial bulkheads any other structure such as a subdeck or directly down into the plate supporting grillage . You design as a whole to get that load sensibly through the structure with a factor of safety.

    There’s no magic bullet, you transfer loads onto parts of the vessel that are in turn designed to carry those loads. If you load a subdeck from a deck above then that becomes an additional subdeck load that you design for.
     
  3. Landlubber
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    Landlubber Senior Member

    ...yes, all loads in a boat are transferred throughout the boat, one way or another...have a look for instance when a collision has occurred, say portside midships, you also have to see the transferred damage to the stb side bulkheads, as they will be punched out too. Same principle applies to vertical loads.
     
  4. nikezz
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    nikezz Junior Member

    I apologize for not being clear. Perhaps this will be clearer. If you look at a deckload of 2 tons/m2, would you drive a 3 ton vehicle with a foot print area of 1m2 (3 tons/m2) onto the deck?
     
  5. Landlubber
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    Landlubber Senior Member

    ...woa......now that is a different story, i will leave that to one of the NA types we have here.....
     
  6. MikeJohns
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    MikeJohns Senior Member

    You don't say whether this is a live vehicle deck subject to traffic or a deck cargo scenario. Class rules have guides to vehicle decks that you should read.

    You look at the wheel loading of the vehicle and you need to consider the tyre print. Make sure your local panel is stiff enough and strong enough. As a guide do the bending moment based stress calc for the smallest stiffener with the largest wheel load mid span and with 25t of attached plating is a good start. Keep that under your fatigue stress limit for the heaviest wheel. Then check the deflection too.
     
    Last edited: Nov 18, 2010
  7. Petros
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    Petros Senior Member

    Typically for such concentrated loads you would design for both conditions, and the worst case controls the design. For example on highway bridge decks or parking garages there is both a uniformly distributed load to apply to the deck surface, and a single point load on a small area that represents a tire contact patch. And a design engineer has to examine both conditions for the worst case loading, one or the other controls the design. You typically end up with deck and beams designed to the point loads, and girders, supporting walls, and columns designed to the uniform load condition.

    In your example is seems very unlikely a structure can handle that kind of concentrated loading, but it would depend on the details of the structural design. There is no simple rule for such a design condition. Each load condition has to be analyzed against the full load path and see if the structure is adequate for the new load conditions. It seldom is, but usually all it will take is reinforcing those particular elements of the load path that are inadequate.

    If this question is because you want to park a vehicle on a surface designed for a uniform load, I would advise against it until you have someone look at it that knows what they are doing. It might be okay, but it might not. Sometimes you can do it if you align the wheels or tracks over a main supporting girder. Or it might only take building up the floor thickness, or make a floor pad, to spread the load out to the adjoining supporting members. In your example if you make a pallet or pad that was 1.5 m2 to put your 3 ton vehicle on top, it would be safe. Make sure you make the pallet strong enough to distribute the load more or less evenly over the floor area.
     
  8. gonzo
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    gonzo Senior Member

    Localized loads is why heavy road vehicles use the "bridge formula". It allows a maximum load per wheel or axle. A thin panel may hold 10 tonnes on a meter square but puncture with a localized load. They are two different specifications on deck loading. Also, the maximum load is a different specification. It is related to other structures and stability.
     
  9. nikezz
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    nikezz Junior Member

    Hi,
    Can I have a link to class rules on deck loading? Most of the time, I google search results that require me to buy a standard. I just need to understand the rationale behind deck loading for class societies. Thank you.

    Edit: Typically, from what I have seen, the drawings would show the type of vehicle, pneumatic tyre area, the side view of said vehicle with a load in tonnes on every section of wheels.
     
  10. Petros
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    Petros Senior Member

    You can down load the German lloyd rules in for free.
     
  11. MikeJohns
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    MikeJohns Senior Member


  12. nikezz
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    nikezz Junior Member

    Hi,
    Can someone point me to the section involving deckloading for the ABS rules? In which section do I find it. I can't seem to find it in the Rules for Building and Classing Steel Vessel Rules (2011).
     
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