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RE: seaint Digest for 2 Jan 2007

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Andrew - 

I'm curious about one other thing related to this failure investigation....
AISC has claimed for years that the inflection point should not be used as a
"brace point" for the LTB of a beam.  If those pictures demonstrate WHY you
don't want to use the inflection point for bracing, then count me as another
person who would love to see them.  To me, that's a much more effective
argument than anything they say in the AISC spec, commentary or seminar
series.  

For what it's worth, most of the engineers that I talk to (outside of heavy
hurricane or seismic regions) seem to scoff at me when I tell them that it
may not be a good idea to consider the inflection point as bracing.  They
come back with the same old lines that contractors use, "I been doing these
structures for 30 years and I always use the inflection point as bracing and
aint never had a problem".

Since AISC is getting more and more of a reputation as being ruled by ivory
tower academics, it'd be nice to be able to show these guys a real world
example of why you should or shouldn't make these sorts of assumptions.

Sincerely, 


Josh Plummer, SE
 
RISA Technologies
joshp(--nospam--at)risatech.com

--------------------------------------------------------------------------
From: "Andrew Kester, PE" <akester(--nospam--at)cfl.rr.com>
To: <seaint(--nospam--at)seaint.org>
Subject: lateral torsional buckling really happens!


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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