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Horizontal Vs Vertical Steel tube

  1. Jul 23, 2014 #1
    How does the orientation affect how much a steel tube can hold? or does it not really have an effect?
     
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  3. Jul 23, 2014 #2

    Simon Bridge

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    It's to do with the loading.
    The horizontal tube will crush more easily, and can bend in the middle (depending on how it is supported).
    The upright tube can still buckle in the middle - but only if it is not perfectly upright or already has a defect along it;s length.

    Experiment with drinking straws, or cardboard tubes - which way up do they hold the most weight?
    (You'll need more than one straw.)
     
  4. Jul 23, 2014 #3
    do the vertical tubes have a completely different weight capacity than the horizontal? or just different ways it can fail?
     
  5. Jul 23, 2014 #4

    Simon Bridge

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    They have different load-bearing capacities - just like any change in geometry does.
    Try the experiment suggested - get some cardboard tubes and lie them horizontally and see how much weight they support ... try again vertically.
     
  6. Jul 23, 2014 #5
    do you know how to determine the capacity of the vertical tube?
     
  7. Jul 23, 2014 #6

    Simon Bridge

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    You pile stuff on it until it breaks.
    Usually someone has already done it - you can look it up in tables supplied by the manufacturer.
    If you are buying it, then give your specs to the sales rep and they will select a range for you to choose from.

    You may want to look up "tensile strength" and "compressive strength".

    Note: a steel tube, end on, with a LOT of weight on it, will usually be driven into the ground.
    Consider: a nail is basically a steel cylinder. It will take a lot of pounding without bending provided it is hit end on. But hit it skew and it bends easily.
     
  8. Jul 25, 2014 #7
    How do you determine the Wall thickness for square steel tubing with a length of 5300mm that must be able to bare a work load of 10-12ton?
     
  9. Jul 25, 2014 #8
    i dont know a whole lot but i do know youre going to need more information than that. If you dont have to have certain width and height dimensons you could have litterally hundreds of possiblities. Is there certain width/height dimensions that the tube needs to be? or maybe a certain weight?

    I attached a document that i found very helpful and as you can see there is countless possibilites.
     

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    Last edited: Jul 25, 2014
  10. Jul 25, 2014 #9

    Baluncore

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    Firstly ,as height gets greater you will reach a point where column buckling occurs spontaneously.
    http://en.wikipedia.org/wiki/Buckling#Columns

    Secondly, what side loads such as wind are expected?
    Thirdly, how will you erect it without failure of the structure?
     
  11. Jul 25, 2014 #10
    We will be assuming no other outside forces like wind are acting on it.

    Here is a rough idea of what im talking about. I just wanted to find how much the vertical supports could roughly hold.
     

    Attached Files:

  12. Jul 25, 2014 #11

    Baluncore

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    Find the total cross sectional area of material in the leg wall.
    Find the minimum yield strength of the material and so compute the maximum safe compressive force.
    Divide by g to convert that force to a weight.
    Don't forget to subtract the weight of the leg and the frame above from that maximum.
     
  13. Jul 25, 2014 #12
    i did most of that,

    A=5.75in^2
    σ=30.36ksi
    I=31.75in^4
    L=5ft
    radius of gyration= 2.349in
    K=.5 (i just assmued fixed ends since its secured at the top and bolted to the floor)

    i then used this equation- Pcritical = (∏^2)(E)(I)/(KL)^2

    but my answers never made any sense so i wasnt sure what i was doing wrong.
     
  14. Jul 25, 2014 #13

    Baluncore

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    I am not at all surprised that you are not sure.
    If you insist on using archaic measurement systems or don't show your working, then I really can't help you.
    Use SI.
     
  15. Jul 25, 2014 #14

    SteamKing

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    The critical buckling load of the legs of your structure may not be the limiting feature of your frame.

    I refer you to this thread, where the buckling of a column similar to your frame was discussed:

    https://www.physicsforums.com/showthread.php?t=763056

    In addition to the strength of the legs, you also have to check the upper part of your frame if loads are applied on the horizontal members. In general, you want to examine the longest horizontal members in bending and shear, then check the shorter members, until everything is checked. Once you have identified the horizontal member or members whose loading is most critical, then you can check the loading in the legs to see if any problems arise there.

    Because it appears you are using open structural tubing in some of the horizontal members of your frame, a more detailed investigation of the tubing structure may be required, depending on the max. loading of the frame.

    Once you have the loading of the structure determined, then the weld design can commence.

    Open structures generally do not attract much in the way of wind loads, unless you are designing the frame to withstand a tornado or some other type of cyclonic wind.

    If you expect some significant side loading, say from seismic activity, or if the frame is to be mounted on a moving platform, then additional investigation will have to be carried out, to check the strength of the frame under combined vertical and lateral loading.

    Don't worry about the units for your calculation so much, as long as you use consistent ones. Converting units back and forth just provides another source for errors to creep into a calculation.
     
  16. Jul 28, 2014 #15
    I determined the allowable load for each of the beams so i know how much they can hold individually assuming the load was evenly distributed on their top face. I also determined that each of the four vertical beams can hold 131,000lbs before failure( assuming the load is applied directly straight down and no other forces are acting on it) i think that is correct but not 100% positive. I feel it makes sense but im not sure.

    I would they like to know even if the 131,000 is wrong how would i go about determining the max load for the entire structure? would i just say its roughly 131,000*4 (for each leg) or is there more to it than that?
     
  17. Jul 28, 2014 #16

    SteamKing

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    Great! What are the allowable loads you determined for these members? From your sketch, it doesn't look like the horizontal members are all the same cross section. Can you provide details?

    The max. load in the vertical supports may not be the limiting feature of this frame.

    There's probably more to it than just the max. allowable load in each leg, but I can't say because I don't have all the details of your structure and your calculations. :smile:
     
  18. Jul 28, 2014 #17
    The following are the max allowable UNIFORM loads on the horizontal beams:
    *assuming a weight is applied evenly across the top surface to each beam*

    HSS 6x6x1/4in Square Steel Tubing

    for the four equal length 8ft beams (purple if the color shows in the picture):
    i got an answer of 24kips or 24,000lbs for each.

    for the baby blue beam 12ft that is perpendicular to the first four:
    i got an answer of 16Kips or 16,000lbs
    i felt this made sense bc as a beam gets longer it would be able to hold slighlty less.

    Lastly the front beam (dark green). This beam is actually an 12ft I-beam so i had a harder time figuring out the uniform load but i was able to determine a max point load at the mid point of 8.6kips so i know it can at the very least hold that much.

    i even went on to determine the weight of each beam which is about 19.02lb/ft for the square tubing


    Also it might be hard to tell in the picture but the four purple beams are notched and fit inside the I-beam where they were then welded.
     
  19. Jul 28, 2014 #18

    SteamKing

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    Because of the layout of the horizontal members, you can't assume that all of the loads are evenly distributed. For example, the two purple beams in the middle of the frame, which connect the two 12-ft beams, impose point loadings on the longer beams at the points of connection. Even though the loads on the purple beams may be evenly distributed, the same cannot be said for the longer beams. You have to analyze the longer beams with a mix of point loadings and distributed loads.

    In other words, the longer beams must support the loads directly imposed on them as well as the load imposed on the shorter beams.

    BTW, which section did you select for the I-beam?
     
    Last edited: Jul 28, 2014
  20. Jul 28, 2014 #19
    i realize the point load part, and in my hand drawn sketchs of this set-up i made the two middle purple beam have a point load on both end beams. The one thing i was not sure about is how large that point load is? lets say i assume the two middle beams can hold a max of 24kips (uniform load) then does that mean i can split the difference between the front and back beam? for example each middle purple beam would apply a point load of 12 kips on the front and back horizontal beam.

    also i used a W 6x20 A36 I-beam
     
  21. Jul 28, 2014 #20

    SteamKing

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    Yes, if you create a free body diagram of one of the middle beams with a distributed load, you'll see that the end reactions of this beam are each half of the total distributed load. These reactions must then be applied to the long beams so that they can be analyzed in turn.
     
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