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RE: lateral torsional buckling

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That reminds me of another illustrative example, a letter in IStructE journal a few years ago, used a steel tape measure to explain the different modes of failure. That’s a really slender cantilever beam that definitely twists and fails.




Steven CONRAD Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust


Roy Harrison & Associates

Consulting Engineers (Structural)

PO Box 104

Para Hills

SA 5096

South Australia

tel: 8395 2177

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From: Andrew Kester, PE [mailto:akester(--nospam--at)]
Sent: Thursday, 4 January 2007 01:42
To: seaint(--nospam--at)
Subject: re: lateral torsional buckling



Maybe I incorrectly used the term "lateral-torsional buckling", but I always understood that this was the proper term for when an unbraced beam that fails due to compression flange buckling, and as a result twists such as being in direct torsion. I am not much of a textbook mechanics guy, but I double checked in a steel textbook I have and what I saw in the field is what the book describes as "lateral buckling and the resulting torsion".


So this particular beam was braced on the bottom flange intermittently (evidently not enough for this particular uplift load),  and it was a deeper and more slender type beam because it was one of those PEMB tapered beams. These are more vulnerable to this kind of failure. Once in uplift, the bottom flange goes into compression and starts trying to kick out. The textbook did say the tension flange tends to keep that flange straight and restrain the compression flange from buckling. But this can be overcome when the buckling stress of the bottom flange becomes large enough to overcome the tensile restraint, which I believe to be the case in this building. "When the compression flange does begin to buckle, twisting or torsion will occur, and thesmaller the torsional strength of the beam the more rapid the failure."


I am not trying to be argumentative because I think we are pretty much saying the same thing. I just want to make sure I am using the term correctly.


I have found two everyday items that can illustrate this physically. A slender but wide piece of trim wood, the more slender the better. Simply hold it between your hands like a deep beam and have someone press straight down in the center and the beam will deflect until it laterally buckles and twists. You can do the same with a piece of metal flashing or drip edge, though you may push it beyond plastic range which means your piece of flashing will be useless. The wood will bounce right back as long as you don't push it to failure....




ADK Structural Engineering, PLLC
Lake Mary, FL