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lateral torsional buckling really happens!

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I just got back from looking at a tornado damaged PEMB in Daytona Beach, FL. It happened on Christmas Day, go figure, it is Florida... We also practically had to run the AC, Scott in Michigan would like to know.
 
It tore off the wood framed and metal deck mansard canopy on the east end, but did not damage most of the rest of the walls and canopies. I went on the roof carefully while I waited for the owner to arrive and unlock the door. Parts of the roof were buckled upward and other downwards, in excess of 2-3 feet. It was really amazing, I have never seen anything like it. Not a piece of deck was missing though some had been torn in half due to the partial collapse. There were also some projectile holes in the deck.
 
Once inside, several of the 3 foot deep beams as part of the moment frame had failed in several places in what to me seems bottom flange compression failure. They were not on the ground but they were deflecting and were severely distorted. Many of the purlins were now horizontal as they had failed in lateral buckling I would assume.  I did see some bottom flange beam bracing along some of the beams. One of the worse areas of deformation was near where a beam failed at the column connection, and these were the deep tapered beams that are deeper at the columns then got less deep towards the center. In another part of the building the drop ceiling was still intact. 30 yards away a single story shingle roof was completely intact. Two blocks away an apartment complex had several buildings unscathed, but in the center of the complex it looked like bomb was dropped on a couple of the buildings. Tornados are definitely random, much more so then hurricanes, so it seems in my forensic experience.
 
I design canopies and roof beams in high uplift areas and always have to remember to check them in uplift due to the lack of compression flange bracing, and will usually just upsize them a little to handle that. Of course you can put bottom flange bracing but that is usually more work and expensive then a slightly bigger beam.But in my head I always think there is no way that would ever be the failure mode, the deck would get ripped off or the light gage framing before the load would ever be extreme enough to cause that. I have been wrong.
 
Andrew
 
Andrew Kester, PE
ADK Structural Engineering, PLLC
Lake Mary, FL
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