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Steel stand construction?

  1. Mar 12, 2007 #1
    I am having trouble figuring out how "heavy" to build a stand for a large aquarium. I have tried calculating "moments of inertia" and "section modulus" values, but frankly find myself lacking in the ability to work out my problem, this is quite frustrating for me. :grumpy: I like to think of myself as reasonably inteligent, but I just can't work myself through this one. Perhaps soemone or "someones" can help me here.

    Here is some info about the problem...

    The tank (I guess it would be reffered to as the "load") has a flat bottom measuring 96" x 48".

    It weighs, fully loaded, about 5000 lbs.

    The weight will be pretty evenly spread across the whole surface.

    It must be supported pretty evenly across it's entire bottom surface.

    I have available to me (for free) rectangular steel tubing measuring 2" x 3" with a .089" wall thickness.

    I want to leave as much open space below the load as possible. Ideally, there would be legs around the periffery of the stand, a top frame with cross members, and a matching bottom frame, with completely open space under the load. I would like to have the "ends" of the stand open, and have three openings along both the "front" and "back" sides of the stand. I was thinking the top frame would consist of an outer "frame" and eight equally spaced cross members traversing the short axis of the stand, on top of this frame I would have a sheet of 1" plywood to spread the load between the rails and and spread out the forces inflicted on the glass it supports.

    This top frame is the first problem I was trying to tackle, how do I determine how much (if any) deflection there will be in the top frame assuming the legs are sufficient to support the load? I want to eliminate (or at least minimize) and kind of distortion in the top frame as this could lead to failure of the tank's bottom, a very unwelcome occurance as you might imagine.

    Anybody care to take a crack at this? :confused:

    Input would be greatly apreciated,
    -Dave
     
  2. jcsd
  3. Mar 13, 2007 #2

    AlephZero

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    Some fairly random thoughts:

    First off, what can you compare this with? You have got something a bit smaller than a compact car and about three times the mass. If you look at typical devices to support a car (jacks, axle stands, wheel ramps) you will soon see that you don't need much steelwork to actually support the load. For example your 2 by 3 tube has an perimeter of 10 inches and an area of 0.89 inches. If you balanced the whole 5000 lb tank on just one leg (!!!) that would only give an axial stress of about 2.5 tsi which is zilch for steel. The limiting factor of the design is likely to be the joints in the stand (bolted? welded? whatever?) rather than the steelwork in between them.

    So your the main problem is making something stiff enough, and distributing the load evenly. The obvious way to get stiffness is to have some big triangles in the frame structure.

    If I was designing this I would try to get some information from the people who make tanks as to what the base of the tank needs to stand on. It could be a thin piece of glass that needs to be on a flat surface, or for safety it could be thick enough to distribute the weight itself, so it only needs to be in a "frame" round the edges. Also , look at some commercial designs and see how their structures work to carry the load.

    If your base (plywood) is very flexible, the load will be concentrated round the edges and cross members anyway. If it's very stiff, the load could also be concentrated at a few points if it's not flat enough compared with the base of the tank. You might be better off with a frame with an appropriate number of cross beams (back to what the tank makers recommend, again) and some soft material on top of the steel to cushion the glass against breaking.

    Another issue is getting the load from the legs of the frame into the floor. Depending what the floor is made of or covered with, there may be a limit on a concentrated load. You don't want a leg of the stand to punch a hole through one wooden floorboard, for example. You may need a "frame" at the bottom of the stand, so the load is guaranteed to be spread over the joists in a wood floor.

    Is the floor level or will you need to include some leveling devices in the legs?

    You might want to think about portability. For example two 48x48 frames standing next to each other would be easier to transport than one 48x96 structure.

    Sorry, no equations and ony one calc in the above - but thinking first and calculating later is usually a good idea when designing something.
     
    Last edited: Mar 13, 2007
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