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Expert: - 1/17/2010


Question

ceiling structure
I'm working on a structural problem in my 1909 house and trying to decide how to proceed. When the house was built a 14' wide opening in the wall was made for a floor to ceiling bay window. The floor under the bay is cantilevered out 18" over the edge of the foundation. The ceiling joists (2x6 on 16" centers) run perpendicular to the wall and are nailed to a single 2x6 "header" that spans the entire 14' opening and rests on each end on top of a double 2x4 top plate (king plate) that sits atop 2x4 studs that go down to the foundation wall eventually (we are talking standard, floor by floor framing here not balloon framing). The heavy plaster ceiling has sagged, especially over the bay opening as one might expect. I'd like to add a glulam beam that would span the opening and hold up the old header as well as catch all the joist ends from below. I'd need to shorten the studs on either side of the opening, (adding some extra cripples) and jack the glulam into place on top of them but under the top plates as I don't want to lose the connection between the top plate and the bay window walls that connect to them.
The first question is how to calculate the load on the glulam and size it accordingly. My gut tells me a 3 1/2 x 12" glulam would be plenty but it not only holds up the weight of the roof but also the attic floor. The attic floor joists also span 14' from the (load bearing) wall opposite the opening. If I went with a 40lb per square foot load for the attic floor and another 20lb per foot for the roof I'm up to around 6000 lbs at the center of that opening (plus the weight of the structure itself.) The current header sags about 1.5" in the center and I'd be happy with a 1/4" of deflection with the new beam.
Is that enough info? I can post sketches or photos which would undoubtedly make things clearer.

Thanks,
David

Answer
Hi David, 60-65 pounds a square foot is a pretty standard calculation for a load such as this.  I think you can safely use this figure when ordering your LVL (laminated beam) or glulam..I'm not sure if you are referring to an LVL in your term glu-lam..A glu-lam beam is generally a beam made up (in your case 3 1/2" wide) of 2x4s laid flat and glued together stacked on top of each other.  An LVL is actually made up of laminating horizontal strips of plywood together.  LVLs are 1 3/4" wide so in your case you would need two of them nailed and glued together..Whether or not a 4"x12" glu-lam or a double 1 3/4 x 12" LVL will handle this span (it should) can be answered by the supplier of the material.  I am not an engineer so all I can say is that I have used the above materials in similar applications with success.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
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    CommentHi Bruce, Thanks for the quick answer, you've given me the confidence to continue with this job which is what I needed. I figure what's there has lasted 100 years so anything extra will be better than nothing. What we can say is that this old timber is pretty amazingly strong. Have a great day off I hope. David


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Bruce E. Johnson

Expertise

I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.

Experience

Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.

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