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- To: seaint(--nospam--at)seaint.org
- Subject: diaphragm drift limitation
- From: SMO3(--nospam--at)aol.com
- Date: Mon, 8 Sep 2003 01:49:47 EDT
I sent this email a couple of days ago and then promptly left town. My seaint digest showed it to be a bunch of garbage, so I am trying again.
I am looking at a single story concrete tilt-up building that has a plywood sheathed panelized roof on steel trusses. UBC '97, seismic zone 4, no near source factors. The building's rough dimensions are 400' wide by 235' deep (aspect ratio of 1:1.7). The roof height is only 21' at the walls in the middle of the building. The problem, of course, is drift.
Calculating the roof diaphragm drift using the formula that combines the chord elongation component, sheathing shear deformation, nail slippage, and chord slippage (assumed 0" because chords are welded), I obtained DeltaS. Taking DeltaS multiplying by 0.7R (code section 1630.9.2) and multiplying by 1.4 to take it from allowable back to strength level, I get a drift of nearly 14". This is over 5.5% of the building height and 2.2 times that allowed by code (1630.10.2). Ouch. This is something that I have never allowed to happen before.
There is an exception in the code allowing this excessive drift, which reads "These drift limits may be exceeded when it is demonstrated that greater drift can be tolerated by both structural elements and nonstructural elements that could affect life safety."
The kicker here is that the building is designed by another engineer (a subcontractor, I am the new in-house guy) who thinks that the building drifting 14" or for that matter 18" or more (compared to the allowed of 6.3") to be not a big deal. He stakes out his position on the exception and on the worthless claim that he has done it numerous times. The problem is that he has never alerted any other discipline that the drift is excessive so they can detail their components (fire sprinklers and gas for instance). Furthermore, he has not educated the client. He views this drift to be not a problem structurally either. He assumes that the columns and walls are pinned at the base, and therefore could theoretically drift the top of these elements indefinitely without consequence because he argues that the vertical loads are carried axially through these vertical elements and the roof resists the horizontal component. This too, of course, is debatable.
He also believes that no additional damage will occur to the building due to the excessive drift.
What are some other opinions on this? How much drift is too much? If the drift does exceed code limits, what is a reasonable way to check the structural elements such as tilt-up wall and steel columns?
Mike O'Brien, SE
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