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Re: 1970's Panelized Roof - Question

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Dennis,
 
The problem - at least as I see it - is a little bit broader than a simple installation of the AC units. IIRC, "your" roof is not adequate for the code-prescribed loads without the new equipment. 
 
IMO, if the engineer has the knowledge of the inadequacy, than he/she has an obligation to take action to bring the roof to code regardless of the new equipment.  Such action seems particularly appropriate in the light of the known reduction of the allowable stresses.  When I faced that problem last year, Roger Turk helped me a lot by sharing one of his great (although scary) stories pertininent to the subject.  
 
In my case, we had to reinforce the entire roof framing system (all glulams and all purlins) to make it work.  I was lucky to have an incomplete set of original plans; the glulam material was unknown, but the purlins were clearly specified.  Still, they were about considerably overstressed under the code-minimum dead and live loads only, and with the current allowable stresses. 
 
With the wood safety factor of up to 6, some engineers may be cool about overstress (and the 20 PSF live load may never occur on the roof anyway).  My thinking, however, is that they did not introduce SUCH safety factor for nothing, and we should not take away from it.  These old roofs need to be brought to code - at least each time we have do something with them.
 
Steve Gordin, SE, PhD
Irvine CA        
   
  
----- Original Message -----
Sent: Tuesday, February 12, 2002 9:04 AM
Subject: Re: 1970's Panelized Roof - Question

Dennis,

As I see it you have four options.

1.) Add columns and foundations to reduce the GLB spans.

2.) Reinforce the GLBs.  Adding a MicroLam on one or both sides might be
a solution but there are other ways.

3.) Provide some structural framework (above or below the roof) to
redistribute the AC unit loading to something more acceptable.  I
believe this was suggested by Scott Maxwell.

4.) Move the AC units to another more acceptable location and distribute
the output as desired via sheet metal ducting.

I can't see any other acceptable possibilities.

Regards,

H. Daryl Richardson

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