Ridge girder for proposed house
Asked by: mcphersn-ga
List Price: $20.00
24 Jul 2006 12:05 PDT
Expires: 23 Aug 2006 12:05 PDT
Question ID: 749083
I need to know if it's possible to construct a glulam beam that will support a 24 foot span in the central ridge of a house. The rafters extending from this beam would be 4X12 on 32 inch centers. The rafters themselves will span a maximum of 14 feet at right angles to the girder. If it's possible to manufacture such a beam, what would be its approximate dimensions?
Re: Ridge girder for proposed house
Answered By: redhoss-ga on 26 Jul 2006 06:09 PDT
Hello mcphersn, I didn't know much about Glulam beams until your question but I do now. There is some real good info on Glulam products here: http://www.aitc-glulam.org/TechnicalInfo/ All the info needed to answer your question is there. First we need to determine the load on your beam. The number your friend calculated is probably very accurate. So, the dead load would be: Roof weight in pounds per square foot = 80,000# / [3000 + (24 x 28)] = 80,000 / 3672 = 21.8 psf Since you say you live in a light snow area we probably can use 20 psf for the live load. Total load = 21.8 + 20 = 41.8 psf The beam supports 14 feet of the roof load so the load on the beam is: w = 41.8 x 14 = 167.2 lb per ft We don't know the specifics for a beam you might use so I will use the span table for a Douglas fir-Larch beam. For a 24 foot span we need a beam that will support 167.2 lb per ft. It looks like your best choice would be either: 3 1/8 x 12 @ 9.1 plf (lb per lineal foot) 5 1/8 x 9 @ 11.2 plf These tables are calculated based on a maximum deflection of l/180. Even though I am sure that the American Institute of Timber Construction knows exactly what they are doing, I normaly use l/240 or l/360 for roof beams. If we were to use l/360 it would double the value we are looking for in the table. This would mean that we are looking for 334.4. Your choices would then be: 3 1/8 x 15 @ 11.4 plf 5 1/8 x 13 1/2 @ 16.8 plf The answer to your question would be that there are Glulam beams that will work for you. I am certain that the company that you buy your beam from will have great expertise in selecting the best beam for your needs. Please ask for a clarification if there is any of this that you don't understand. Redhoss
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