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Old 06-27-2006, 11:21 AM   #1
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Beam Sizing


Before I finalize things, I'll get input from an engineer, but I'm curious what folks see regularly in the field. I'll be replacing approximately 20' of load bearing wall with a wood beam. There is an attic and roof above. None of the attic has been converted to living space, but we would like to install a beam that would permit that someday. Ceiling height in the living space is 10', so we can accomodate a beam of almost any size. Glulam is always an option, but I am curious mainly about regular dimensional lumber, particularly doug fir.

From experience, would you think a 6" x 12" would work? Is something larger than that typically used for a 20' span? I know that a steel W beam is always an option (a 12" W beam currently supports a 20' span in the basement, with the living level, attic and roof above), but I'm trying to get a sense of how feasible the wood beam option will likely be.

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Old 06-27-2006, 10:23 PM   #2
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Beam Sizing


20 Feet is pushing it. With select structural Doug fir you could get 20' 7-1/2" with four 2x12 if they are only supporting 7'6" of joist length (ie one end of a 15' room). However, you might find it hard to get such wood, and if you use #1 and #2 you can only span 16' while supporting 7'6" of joist. You could perhaps have a larger solid timber cut, which would require grading and an engineer to sign off on it as inspectors (around here at least) don't 'do' timbers. These numbers are from British Columbia, but are probably similar to wherever you are assuming that gravity is the same there. Good luck.

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Old 06-28-2006, 06:32 AM   #3
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Beam Sizing


The way I would do that is sandwich 3/16 inch sheared plates between the 2X12's...at least 2. I think there are better ways to do that, and considering you want to have a future potential of adding say, 150# live load plus more dead load..I bet you will go with 4X16 glue lam as a minimum...and maybe a steel H beam, boxed (my choice).
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Old 06-28-2006, 12:27 PM   #4
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Beam Sizing


Thanks guys, it sounds like 20' may be a bit much for a solid lumber beam of any size. BTW, the joist span on each side of the proposed beam is 12'.

Is there an online source for information on the span capabilities of a
4" x 16" glulam beam??
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Old 06-28-2006, 12:54 PM   #5
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Find out what company makes the glue lam's you are interested in and contact directly...Lowes commercial desk or HD will have the tables for Boise Cascade beams.
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Old 07-17-2006, 04:14 PM   #6
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Beam Sizing


how is the roof framed? is it supported by the bearing wall?

it really depends on how much of the roof and attic load (attic = typically 20psf) that the beam will be carrying. the beam will only be carrying half of the supported span (on each side). so 6' on each side of the beam (12' total supported span), along it's entire length

it's also difficult to calculate unless we know your location (roof snow loading).

20' does seem like a pretty significant span, tho. have you considered LVL?

if you can sketch it up, i can give you an LVL calc sheet to give you an idea. shoot me a PM if you'd like.

Last edited by kevin hassell; 07-17-2006 at 04:20 PM.
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Old 07-19-2006, 01:37 PM   #7
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Beam Sizing


Maximum span for a center bearing girder for a 24' wide structure is only 10' 10" when using 4 2x12s. (International Residential Code)

Only way you can span this is distance of 20' is with an engineered girder.

If this center wall also supports a center bearing attic wall, the maximum span is reduced to 7' 8".
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Old 07-24-2006, 01:23 PM   #8
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I'm trying to get some idea of the practicality of a 24' span for the central girder supporting the roof of a single story house. The span of the rafters extending from the central ridge girder is about 21', which I'm thinking of sizing at 4x12 (rafters). Can a glulam beam be manufactured that could conceivably support this load? I'm now at the stage where I need to make some decisions about the design, before I pay for an engineer...
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Old 07-24-2006, 02:57 PM   #9
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McPhersn, the span (horizontal) of the rafters from ridge to wall is 21'? Giving you a room 42'x24'. Or the distance measured down the slope is 21'? What is the pitch?

Anything can be done for enough $$. Is this an exposed (cathedral) ceiling, is that why you are using 4x12 rafters? If not cathedral you don't need the 4" measurement, it helps very little with the span.
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Old 07-24-2006, 03:21 PM   #10
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Right. The span of the rafters from ridge to wall is less than 14'. It is a cathedral ceiling, with 1 inch rough cut planking on the rough cut 4X12 rafters. I'm new to this forum, obviously...
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Old 07-24-2006, 03:25 PM   #11
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And the pitch is 2.5 in 12. (I'm in the boonies of Arizona)
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Old 07-24-2006, 11:34 PM   #12
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OK, Arizona, so I guess we're not too concerned with snow load?

It's just a matter of talking to an engineer and finding out which option to go with. Is the interior of the building an open plan, or can you put intermediate supports in? To answer your question: It can be done.
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Old 08-04-2006, 12:39 PM   #13
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Beam Sizing


I have a question regarding the use of an engineered beem. I am adding a sunroom to my home that is 28' x 14' and I want to know the correct method for supporting/attaching the beam to an existing load bearing wall. I want to create a cathedral design with a ridge beam and 2 x 10 rafters. The rafters will span the wide aspect of the room.
The beam will be almost centered above an existing sliding glass door.
What design is need to place the beam approxiamately 3 1/2 feet above the top plate of the load bearing wall. Thanks to all responses.


To follow up on the question of beam needed, the other writer was correct, you need to know the span and weight factors for your desired application and the pro desk can give you the deminsions.
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Old 08-05-2006, 12:40 AM   #14
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Quote:
Originally Posted by Riddler
Thanks guys, it sounds like 20' may be a bit much for a solid lumber beam of any size. BTW, the joist span on each side of the proposed beam is 12'.

Is there an online source for information on the span capabilities of a
4" x 16" glulam beam??
Riddler, you will need glu-lam-beam 3-1/8 x 13-1/2 or a
5-1/8 x 12" for a 20 foot beam with 12 foot joist.
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Old 08-05-2006, 12:46 AM   #15
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Beam Sizing


Quote:
Originally Posted by Thomas
I have a question regarding the use of an engineered beem. I am adding a sunroom to my home that is 28' x 14' and I want to know the correct method for supporting/attaching the beam to an existing load bearing wall. I want to create a cathedral design with a ridge beam and 2 x 10 rafters. The rafters will span the wide aspect of the room.
The beam will be almost centered above an existing sliding glass door.
What design is need to place the beam approxiamately 3 1/2 feet above the top plate of the load bearing wall. Thanks to all responses.


To follow up on the question of beam needed, the other writer was correct, you need to know the span and weight factors for your desired application and the pro desk can give you the deminsions.
Answer received today.
Place a post on the top plate after reinforcing the header by removing one of the 2x12 and replacing with two 3/4" plywood and one 1/2"plywood 12" glue and fasten to original remaining 2 x12 single thereby forming a new laminated stronger header.

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