What size i-beam to support a flat roof 32feet by 40feet the ibeam will span the 32foot distance?
32 by 40 foot flat roof with the ibeam spanning the 32 foot distance which will only be supported by each end no center posts.
The roof construction currently is 2 x12 by 20 foot spruce joists on 12" centers tied to a center beam made of 3 layers 2x12 nailed togather and supported by wooden posts. I would like to remove the wooden upright posts and install a ibeam under the wood beam to open the area. I am located in Canada and snow load is a concern.
Answer:
"I am located in Canada and snow load is a concern"
The snow load is a critical piece of information, it would be best to have this engineered by someone familiar with the local conditions and codes.
To give you a really rough idea, assuming a total load of 100psf which is just a random guess, I have no idea how much snow you get. . By the way, you used feet, Is that common in Canada? anyway, I will continue with feet and pounds. Using that assumption, the load on the beam will be 100 x 20 (half of the 40' span is supported by the end walls) x 32 or 64,000lbs, According to the AISC 9th edition (a bit old) ASD manual, that load could be carried by, for example, a W21x62 or W16x77 (second number is weight per foot), weighing about 2000 and 2500 lbs respectivly, I'd guess about $0.50 (US) per pound is a resonble ballpark guess for steel prices, so you are looking at somewhere over $1000, for the beam, so spending a couple hundred for an enginner (if it is even that much) is not such a large percentage of the project.
It depends on the depth of the beam you're willing to use, and my steel book is at work. The beam would get progressively lighter as you go from 12 inches to 14, 16, 18, 24 inches deep. So you have to know how deep you can get away with. 150 pounds per foot is just freakin' ridiculous. I'll try to remember to bring the book home from work tomorrow and check if you got a real answer.
Estimating the roof weight at 12 psf and taking the snow load at 65 psf, you'll be supporting around 50,000 pounds. The lightest I found were W18x55 (1.4" deflection) and W24x55 (.94" deflection). I agree with the advice to get an engineer, he'll probably want the bottom flange braced to the joists. He can also look at your roof and estimate (rather than guess) its weight. Good luck with your project.
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