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?

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Request for Question Clarification by redhoss-ga on 24 Jul 2006 14:52 PDT

I think that I can probably help you. Do you know what the live
load/snow load is for your area. Also, what type of roof do you have
or do you know the dead load.

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Clarification of Question by mcphersn-ga on 24 Jul 2006 17:37 PDT

Hi,
     Not much of a snow load here in the arid zone (0), but a friend
of mine has roughly calculated the total roof load, which includes
about 3000 square feet not supported by the glulam, at 80,000 lbs. 
Not sure how realistic this is, but it's what I have...

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

Good job.