Reversing the direction of deck beams, transverse frames, bottom floors ...

Discussion in 'Class Societies' started by ldigas, Jan 23, 2012.

  1. ldigas
    Joined: Feb 2011
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    ldigas Senior Member

    Let's say the beams are frames are L or holland profiles.

    Why are deck beams/transverse frames/floors ... reversed in direction, on the fore and on the aft part of the boat?

    What I mean? For example, on the fore ... deck beams are towards the bow, frames are towards the stern and so on.
    On the stern, the deck beams are towards the stern, the frames are towards the bow ... and so on.

    Why is this done?

    Since neither of them are elements of longitudinal strength, it is not the reason. They participate in transverse strength, but in that case, their orientation is irrelevant - they have the same geometrical characteristics both ways.

    So why?

    This is something I've seen as common practice, only never heard a reason why (apart from technological reasons, but for that they could stay oriented the same way along the whole length).
     
  2. cestes
    Joined: Apr 2002
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    cestes Junior Member

    I can't answer with absolute authority, but one common reason for this is if the structural members are all made of flanged plate and the builder wants to lap the side frames to the deck beams and bottom floors, instead of butting them.

    This is a very common scheme in steel fishboat construction in the US. This lap supplies a very effective end connection and, in most cases alleviates the need for brackets.

    To explain the forward and aft question, think of a steel frame as being really (say 25mm) thick. Now go way up in the bow and put a flanged plate transverse frame against the shell plate. you will note that the frame can be no larger than the intersection of the forward face of the frame with the shell, so the forward face becomes the "molded" surface and all the material that touches the shell (deck beam, side frame, and bottom floor) "throws" aft. This is slightly complicated in the case of lapped members discussed above. In practice, the frame's members are considerably thinner than 25mm, but the principle still applies.

    It is just the opposite in the aft portion, where the hull gets larger as it progresses forward toward midship. In that case, the molded surface is the aft face and the frame's material "throws" forward.

    Another way to remember it is that the face away from midship is molded and the material "throws" toward midship.
     
  3. jehardiman
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    jehardiman Senior Member

    The answer is very simple, it increases stick clearence and reduces edge prep like Cestes alluded to.

    Think about it this way, sections are getting smaller towards the bow and stern so the shell plating is rolling in. If the flange on the floor is pointed towards the end, the space for the welding rod is closing out (i.e. for the weld space, the opening is narrower than the bottom), making welding harder. If the flange points towards midships, the opening is larger and more room is available for the rod. Furthermore if the floor is placed so the flat face is on the frame line, the web would not need to be edge preped because the shell plating would be opening up the gap between the floor and the web. This means that all floors/frames can be cut to size and installed directly without having to bevel (for small <1/2" stuff or partial penetration joints)

    Decks on the other hard are welded flat downhand then picked up, inverted, and installed so it really doesnt matter which way they face. If the two flat faces are set on the frameline, the floor towards midships and the beam towards the ends, the two web flats meet face to face with no snipes or edge prep required.

    Notice that this is just the opposite of wooden construction, where the frames/floors are placed on the side of the frameline away from midships. This is done again to minimize work because the bevel does not have to be developed for the frame back and can be cut with the frame siding on the shipsaw with only minor dubbing at planking.
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The image, which is distorted in order to make it clearer case, attempts to explain why, at least in metal construction, the thickness of the transverse elements are located fore or aft of the line path, depending on its position on amidship.
    If not placed in this way, the contour of the part (floor for example) should have a chamfer, very expensive and difficult to achieve.
     

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  5. ldigas
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    ldigas Senior Member

    Hello TANSL,
    sorry about the late reply.

    But, if one were to put them like in your picture, wouldn't that make welding of the rightmost one very difficult.

    I didn't catch the part about the chamfer and the floor.
     

  6. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    When welding from one side it is very difficult, you can use welding techniques on one side: full penetratiom, backing ceramic, welding in inert gas,...
     

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