Mast step beam material

Discussion in 'Sailboats' started by Roly, Aug 8, 2010.

  1. Roly
    Joined: Jul 2005
    Posts: 508
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    Location: NZ

    Roly Senior Member

    My mast step is required to bridge 4 floors @ 350mm cntrs
    I propose to use two pieces 100x50x6mm Stainless steel channel welded together
    to form 200x50mm with a plate welded to form the actual step.
    I will cut the flanges down in an arc from 50mm to 20mm from the plate
    (maststep) to the outer floor.
    Working load compression 6tonnes upwind @30deg heel/ 20knots wind.

    Is this an acceptable mast step- what do you think?
    Perhaps I should extend the maststep top plate to the middle floors?
    Overkill?
    Seems to me that the outer two frames will only start loading if the middle two deflect.
     

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  2. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member

    When your step spans four supports and is center loaded the outer spans have a reversed stress. You are trying to push the middle two floors down and push the outer two up. The beam should have the same section for it's full extent for it to work that way.

    The other approach is to make it stiffer than the floor hull combination and load the 4 floors together. The design for that will depend on the rest of the structure, and not just the step support beam in isolation.
     
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  3. Roly
    Joined: Jul 2005
    Posts: 508
    Likes: 23, Points: 18, Legacy Rep: 222
    Location: NZ

    Roly Senior Member

    When your step spans four supports and is center loaded the outer spans have a reversed stress. You are trying to push the middle two floors down and push the outer two up. The beam should have the same section for it's full extent for it to work that way.

    Thanks Mike.
    Does that mean then, that the load is spread any more by spanning 4 floors instead of two?
    The other scenario of calculating the stiffness of the hull through that region
    would be an approximation as it is at the keel junction. (Steel & wood)
     

  4. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 206, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    I sent you a PM.

    I'd really need to see some drawings to comment properly but otherwise assume a reasonably stiff layup and design the step accordingly to load 4 floors.
     
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