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Stress computation

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  • Stress computation

    Does anyone have a source for a formula to calculate the torsional stress in square structural tubing?
    I just need one more tool,just one!

  • #2
    Machinerys Handbook has a section on calculating beam stresses. The maximum stress is at the outer edges, so the formulas are the same. If you do not understand the formulas, I would not recommend calculating the stress unless this part is not critical, you will not have a feel whether you did it correctly or not. What is your application? What would happen if this part fails?



    • #3
      Mike its the main load carring member in a structure its already in use and working perfectly but now we have to put some numbers on it to satisfy building codes,The stresses involved are a combination of both bending and torsion and because its a hollow square section there aren't to many formulas out there to fit what we are looking for, the A.I.S.C book doesn't have them either.We think that surely somebody has run into this problem before as hollow rectangular sections are becomming more popular everyday.
      I just need one more tool,just one!


      • #4
        sounds like something you should hire a civil engineer for, not a good idea to take this into your own hands. That kind of beam is common in mechanical engineering, but the formulas for combined stresses get complicated. You might find an approximation in a strength of materials textbook, but it probably will not satisfy code officials, they like to have a PE stamp it.



        • #5
          The people who put out that material also have books on the properties of their sections, and the manner of computing stresses etc.

          That said, a letter from a PE, with his "crunch" in the corner will make nearly any building inspector go away happy. (assuming there is no obvious violation).


          • #6
            I work for a PE we had to go thruogh this before on a similar project long way around the barn very slow had to develop our own equation we where hoping to find some one who knew of a good ref.for these formulas but after an extensive search none is avalible,oh well I guess its back to pages and pages of permits,and explainations,and delays. And as for the people who put out these sections having info on their properties the standard for the industry is "gee nobody has ever asked for that before" 45 out of 47 supliers.

            [This message has been edited by wierdscience (edited 02-25-2003).]
            I just need one more tool,just one!


            • #7

              I have a book from an aluminum Co with the info, and another from a steel co with the same info. Both are older.

              New liability-reducing policy?

              More and more companies are deleting propoerties info and basically saying "WE just make it, YOU use it, YOU figure it out".

              And if it is made in China, there ARE no standards.


              • #8
                You might try Roark's Formulas for Stress and Strain. It's the "bible" for stress engineers.


                • #9
                  i doubt you'll find "a formula" to satifsy what you're looking for as most of them are restricting enough when you read the application conditions to make them useless unless you're working under ideal conditions in a lab and you know the composition 100%.
                  (not to mention, age, condition, etc)

                  best way to do this is numerically so if you've got some specific numbers you want to post i can set the old meatgrinder here on it and let you know what it comes up with.

                  numeric solutions these days, especially with the current power of desktop computers, are almost always the way to go.

                  unless you're doing design work in which case numeric trial and error becomes tedious.

                  tell us more.



                  • #10
                    i knew something about this question bothered me...

                    here goes: (square) tubing should not be used where (significant) torsion will occur. hollow tubing is best used in tension and, when suitably sized, in bending.

                    if this is your case, the torsion might not be significant compared to the bending moment AND, if the section is large enough compared to the loading distance, you may only need to verify that section/face and not the entire beam/tube.

                    such as a loaded t-weld on one face of square tubing.. you may only need to verify that one face/patch wont shear off.

                    finally, maximum stress will occur at the point on the surface of the tube CLOSEST to the axis.. ie, the faces, not the corners.
                    they buckle there.

                    thought i'd get that off my chest. thanks



                    • #11
                      Just to follow up on the Roark suggestion...

                      I looked it up and there is a formula for a rectangular tube in torsion. But it says that there will be higher stress in the inside corner unless there is a generous radii. So KnuckleH is right about the special condition making the formula useless.


                      • #12
                        Now were on the right track the torsion in this structure is not a significant force yet it is there the only reason the question comes up is because the load is slightly offset from the center line of the tube, if it were centered it would be a simple bending solution.Even thuogh the torsion is minor the powers that be want it expressed as a maximun stress limit.
                        I just need one more tool,just one!


                        • #13
                          If the "powers that be" value the answer, they should be willing to pay accordingly. Tell them to hire a licensed engineer. With the details you've given, the calculation sounds easy for a PE. Around here (NoCal) it should cost a few hundred at most.


                          • #14
                            Already been done has is and will work perfectly in about a thousand units sold and installed world wide,only problem is some inspectors in some code departments want more than is practically needed i.e. certain cities(a few out of many)in places like Cal.(no-offense)want that little extra something that makes them special.The total maximum load for this tube section in absolute certitude throughout the universe is 6,000#on a six by six by .250"wall tube 68"long!Almost everyone agrees that a tank could drive across this section and not deflect it but some people still persist.Maybe its because the want that extra layer of protection over thier but.
                            I just need one more tool,just one!


                            • #15
                              Must be L.A.........

                              They had and may still have, their own electrical testing and certification lab. Nominally it operated to UL, but in actual fact, they operated to UL as interpreted by English Second Language persons of middle-eastern origin.

                              Smart enough people, but some of the interpretations of the standards were "interesting", due to *less than perfect* knowlege of english!

                              Luckily, they finally agreed that ETL could do what UL does, and the ETL mark was equivalent, so they quit bothering us.

                              I have no idea what they might get into with respect to building code type issues. The mind boggles.

                              BTW, we have a lot of 6" x .25 tube holding up mezzanine (as columns) in our factory, and a bit of larger stuff too. Seems to be working....................

                              [This message has been edited by Oso (edited 02-27-2003).]