I have two 2x10?s 18 feet long supported in the middle by one
2x4. Windows are now in these openings. I?d like to remove the center 2x4 and replace the windows with a 16?x 7? garage door. What size (thickness) flitch plate would be needed to strengthen the 2x10?s to support the open span? (Keeping within the 9¼? height.) Single story, 4/12 roof, rafters are perpendicular to header. No live load, just ceiling joist. Snow load for western NY. Next wall parallel to header is 22?feet away.
The calculations to determine the individual tributary loads and the resultant header load are pretty straightforward, the flitch plate calculations a little more involved. I don't think you are going to find anyone on a newsgroup (even a PE) who will be willing to give you an absolute answer.
And a more complete deescription would be needed-any roof overhangs, roof style, stick or truss framed roof, etc. To many thngs not mentioned.
If I'm not mistaken, a ceiling live load has to be applied to any space with a headroom greater than 42 inches. I would imagine snow load is at least 50 PSF in western NY. The local authority would tell you that value.
This is the same info given to the Glulam (paralam) company. They spec?d 3, 1 1/2?x 9 1/2? GluLam beam. I would just like to use steel to bring the 2x10?s to the same spec.
OK, at first glance I'd say they calculated the load capacity of the existing configuration and used that to spec a clear span. Maybe I'll run some numbers to check that....
Well, if two 9' 2x10s have S = 3x9.25^2/6 = 42.8 in^3 and M = 1000S = 42800 in-lb = W9x12/8, W = 3169 pounds, it looks like the 2x4 post (it might support 5250 pounds) isn't the limiting factor. The uniform load on an 18' beam would be 2x3169/18 = 352 pounds per foot.
The 1981 NAHB ap note says an 18' 2x10 Hem Fir beam with a 1/2"x9" plate can support 215 lb/ft of uniform load, so a 0.5x352/215 = 0.82" plate (or 2 246 pound 7/16" plates) might support 352 lb/ft. They suggest
1/2" bolts every 20" near the top and every 40" near the bottom and something like 2 2x6 jack posts with 3/4" plywood plates at each end.
When you calculate the bending stress (S=67.7) and deflection on the glulams specified, (E=1.8E06) with a 352 pounds/foot load, Fb is about 2000 psi (2400 psi allowed) but the deflection is about 0.9 inches, assuming a 16 foot span (garage door width).
If you use the 1/360th of span as the limit, and 16 foot unsupported span you get about 210 pounds/foot.
It does meet 1/180 of the span. Still, even 352 pounds per foot is low if the header is supporting a roof. If the opposite wall is 22 feet away, that's only a 32 psf total load.
Thanks Rick, I was looking at the other side of the building. It has an 18? opening with the same room dimensions. Of course same roof load. There are three 2x12?s supporting the same ceiling attic space. What would be the psf on that span? It has been there for 17 years. I?m leaning toward the 3 LVL?s (glulam) because I can?t seem to get a straight answer on a flitch plate. Seems costly 4.45 per beam, per foot.
Hey Rick, all this time I was comparing between a LVL and adding a flitch plate. Wouldn?t three 2x12?s support everything and cost one third the price of LVL? Where is the LVL advantage?
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