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Square steel tubing for a beam...did I do this right?

  1. Jul 30, 2015 #1
    I need to come up with a beam (two of them, actually) that will hold a large steel box suspended over a hole in which it will ultimately be buried. The box is 9 feet long by 5'6" wide and weighs 1600 pounds. The box will not rest across the length of the beam, it has to rest on two supports on either side. The hole will be just over 6 feet wide, so my beams will rest on the edge of the hole, and the box on the supports on the beam. A rough diagram:

    ==|=====||=================||====|== Beam
    Hole Support Support Hole
    edge edge

    The hole will be 73 inches wide, the bar 85 inches, and the support rails 18 inches inside the edge of the hole. I added 25% to the weight of the box for safety and I think 2-inch square tubing with 1/8 inch walls will work - I found formulas to calculate the stress on the beam in this scenario and then some more to calculate the stuff the formulas need: section modulus came out to 0.5517, weight on the support 500 lbs (2000 / 4 supports), so a bar that long with the stress 18" from the end should be feeling 16,311 lbs/sq. inch. (right? I assumed the part of the bar outside the hole would not factor into the calculation...)

    The challenge I had was finding out the yield strength (I hope that's correct - I'm looking for how much stress it can take without being ruined) of that material...came up with nothing beyond a vague post that 36000 psi is as far as you want to go for 'steel'. So, it looks like my 2" tube will work, but I'm not an engineer and am skeptical of both my calculations and the information I found. So I'm asking professionals...what do you think?
  2. jcsd
  3. Jul 30, 2015 #2


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  4. Jul 30, 2015 #3


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    I think your setup is thus:

    Code (Text):

         ------------ Box ----------------         Box = 2000# (1600# + 25%)
        18"  ||                     ||   18"
    ==|======||=====================||======|==    Beam (1 of 2) L = 85" each
    Hole   Support                Support  Hole
    edge                                   edge
      |<-------------- 73" ---------------->|
    You should check with a supplier to obtain the correct material properties for the steel tubing sizes you select. Regular structural steel is ASTM A36, which has a yield stress of 36,000 psi, but steel tubing is often fabricated from higher-strength steels, like ASTM A500 or A513, which is why you should check before you buy.
  5. Jul 31, 2015 #4
    OK - I saw formulas for that...but why? Once I learn how much the bar will bend, what will I do with that information? Educate me, please?
  6. Jul 31, 2015 #5


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    Sorry for delay in replying .

    There are several reasons to consider deflection of beams :

    (1) Simple practicality . If beam deflects a lot then it isn't going to do the job of supporting your load .

    (2) Beams which deflect a lot are usually an indication of them being inadequate for strength . Some beam sections - like I beams - can also become unstable and buckle when they have large deflections .

    (3) Beams which deflect a lot are often very 'springy' and if suporting a ponderous load the whole lot might start to oscillate .

    Generally deflections of a safe design of beam are very small compared to length of beam .

    Not very scientific but always ask yourself whether you would be happy to stand underneath any steelwork carrying a heavy load !
    Last edited: Jul 31, 2015
  7. Aug 3, 2015 #6
    That seems reasonable, but how do I know if my beam is bending 'a lot' (Or 'too much' if we're getting down to brass tacks)? I assume there's some amount that's acceptable, but how can I figure out the beam's deflection is acceptable?
  8. Aug 4, 2015 #7


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    Part scientific and part common sense - if it bends enough to see then it's too much .

    For your arrangement say 0.1% of beam length or about 2 mm at mid point of beam .
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