300 ton crane: Why are dock cranes so massive?

  1. rollingstein

    rollingstein 621
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    I recently saw a massive new crane at a ship-repair yard. Something like this one:

    [​IMG]

    [​IMG]

    What surprised me was that for such a massive construction the load limit said "only" 300 tons.

    I seem to recall much smaller road moving cranes (say Demag) with a 300 ton lifting capacity.

    [​IMG]

    So what makes these dock cranes special & so massive (I guess expensive too!) Is it the reach, i.e. the moment? What do they use them for that cannot be accomplished with a mobile 300 ton crane?
     
  2. jcsd
  3. SteamKing

    SteamKing 9,913
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    A shipyard crane must be able to handle not only heavy objects, like the parts of a ship, but it must handle objects which are physically quite large in terms of length, height, and width. These objects tend to be so massive that it is impractical to lift them using one hook without damage to the object being lifted.

    The mobile crane in your picture can lift heavy objects if it is safe to do so with a single hook. However, the heaviest loads which such a crane can lift can only be done safely when the boom makes a large angle with the horizontal. If you wished to use a 300-ton mobile crane in a shipyard, for example, in order to reach over the edge of a dock and place an object many meters away from the crane would severely limit the maximum load which this type of crane could handle.

    Take a look at the brochure for a 300-ton mobile crane at this link:
    http://www.anthonycraneusa.com/loadcharts/Demag/Hydro Truck Crane/AC 300 Demag 2.pdf

    On page 2 you will find a lifting chart for this crane. This crane is able to lift 300 tons only when the boom is almost completely withdrawn to its shortest length. When the boom is extended to maximum length of 59 meters, the maximum load which can be safely handled is 28 tons at maximum elevation, which drops to only 7 tons when the elevation of the boom is lowered so that the maximum reach, or distance from the crane's axis of rotation, is obtained.
     
  4. SteamKing

    SteamKing 9,913
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    The pictures below will give you some idea of how massive some of the parts are which are assembled to make an aircraft carrier:

    [​IMG]

    In the picture above, the forward end of the carrier is being lifted. It takes 3 sets of lifting points to handle such a large structure. In the lower right hand corner of this image, you will see a teeny tiny man standing on the ground. This gives you a sense of size for this piece. The lifting points on the deck are each about as tall as the man on the ground.

    [​IMG]

    This is the part of the bow of the carrier which fits under the piece shown in the first image. You can see shipyard workers standing on the rest of the ship in the background, watching this lift take place.

    This article shows some more photos of how an aircraft carrier is assembled during construction:
    http://science.howstuffworks.com/aircraft-carrier.htm

    See p. 3 of the article above for more construction photos.
     
  5. SteamKing

    SteamKing 9,913
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  6. rollingstein

    rollingstein 621
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    But they use spreader beams for that limitation, right?

    Your other point convinces me. Reach matters. Placing 300 tons 150 feet away is a different ball game than placing it 20 feet away.
     
  7. SteamKing

    SteamKing 9,913
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    Even sailors who make heavy lifts for a living are not immune to mishaps:

    http://www.cargolaw.com/2007nightmare_jumbo_challe.html

    Scroll down to the bottom of the page for photos of the aftermath.

    The ship is listing the the starboard side because it was using ballast to counteract the weight of the cargo being lifted over the port side. When the cargo dropped to the dock, the weight of the ballast on the starboard side of the ship cause the vessel to roll to that side.

    The lesson here is safety is always first when planning and executing heavy lifts.
     
  8. SteamKing

    SteamKing 9,913
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    Yes, spreader beams are used so that too much load is not concentrated in one spot on the lift.
     
  9. rollingstein

    rollingstein 621
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    A related question:

    If you observe the yellow vertical columns for the shipyard crane photo I posted, they get wide at top & narrower at the base.

    Does this make intuitive sense? What stress / moment is it that needs greater cross section at top of column but lower at bottom?
     
  10. SteamKing

    SteamKing 9,913
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    I think what you are seeing in the first photo is not representative of the entire leg structure.

    Look at this view, for instance:

    [​IMG]

    On the upper right hand side, the leg's width is quite broad because there is no clearance problem there. In you look at the bottom, you can just see that the crane travels on a rail to allow it to move back and forth along the dock. At the bottom, the leg is lengthened to allow it to spread its load onto the rail traveler. The structure of the leg as a whole is 'twisted' as it moves from the bottom to the top to allow for a relatively lengthy portion at the bottom to accommodate the rail mechanism which transitions to a relatively wide and short connection to the cross beam at the top. There is structure inside the leg which cannot be seen from outside, and I'm sure that there is more metal supporting the crane at the bottom than at the top.
     
  11. Baluncore

    Baluncore 3,019
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    The dock crane must span the dock. That requires it have a long beam.
    The beam needs to be deep or it will fail under compression of the top flange.

    Where a section like an RSJ is used for a crane beam the top flange must be stabilised to prevent that buckling failure. Buckling failure is a catastrophic process because once any plastic deformation of the top flange begins it directs more force to the deformed area.

    A mobile crane has a very delicate boom. If anything touches the boom it can catastrophically fail and collapse. The heaviest loads can only be lifted close to the vehicle, with a steep boom. For large objects that has a high probability of contacting the boom.
     
  12. rollingstein

    rollingstein 621
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    Interesting. Are the two columns non identical or is that just perspective playing tricks.

    One looks like a split inverted-V-frame whereas the other looks a single structure.

    Other cranes look similar too. That's be non-intuitive too: I'd have expected overall symmetry. The second (both split inverted-V frames) photo shows what config. I mean by "symmetry".

    e.g.

    [​IMG]

    [​IMG]
     
    Last edited: Sep 26, 2013
  13. rollingstein

    rollingstein 621
    Gold Member

    It doesn't seem a trick of vision. Found another shot of the same crane and it is indeed non symmetric. I'm still not sure why.

    PS. I'm not trying to second guess or fault the designers just imagining post hoc, what went through their minds.

    [​IMG]
     
  14. Baluncore

    Baluncore 3,019
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    The more solid column controls the cross dock stability. The slender column carries vertical forces only and flexes to match the rail separation as the load and rail track varies. This forms a stable structure that has sufficient flexibility to adapt to variations in it's footings without over stressing the structure.
     
  15. rollingstein

    rollingstein 621
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    Thanks! When you say "stability" in structural mechanics terms what mode of failure / analysis are we looking at. I'm curious.

    Is the the degree of flexing in response to a cross load? Or an overturning moment?

    Since it's only one column that's more solid it isn't the compressive load or a shear stress / moment induced by the slung load I assume. Nor buckling. Since all that would equally impact both columns.

    Sorry, if I am picking your brain too much. Maybe I'm naive because I cannot intuitively visualize any other smaller / more typical structure that was stabilized by an asymmetrical design of it's twin support columns.
     
  16. SteamKing

    SteamKing 9,913
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    Well, look at it this way. The crane must travel on rails as it moves up and down the dock. If all goes well, both sides of the crane, independently driven, will move in perfect alignment as it travels. IF, however, there is a failure of some sort in the driving mechanism, the heavier leg will keep the whole crane from possibly getting too far out of alignment with itself such that it racks and collapses or is seriously damaged.
     
  17. Baluncore

    Baluncore 3,019
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    If you load the centre of the beam then any sag of the beam will result in a misalignment of the end connections to the columns. The survival of that distortion is why one column must be rigid and one flexible.

    A crane should not have side load. However, both changes in the velocity of the load and wind pressure will apply lateral forces. The crane will not be used in high wind speeds. The crane relies on it's weight and the spread of its carriage to survive high wind loads.
     
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