huangyhg

uneven slope lenght roof main beam size
i鈥檓 planning to build a 22鈥檟 27鈥?(exterior dim) garage with a cathedral celling at the second floor (usable attic for additional storage and possible studio) with uneven roof slope lenght but having the same slope angle; 4/12. on the second floor the two walls running along 27鈥?won鈥檛 be of equal height, one will be 8鈥?and the other will be about 4鈥?height. the celling will be made of i beams of 16鈥?x 18鈥?for the long span and 16鈥?x 9鈥?for the short span, (all install at 24鈥?c/c) (16鈥?beams are needed for an insulation of r 40). the main central beam (which is not in the center of the 22鈥?but at about 2/3 of the lenght of the 22鈥?side) will be supporting all the ceiling i beams. if i want that main beam to be able to run along the 27鈥?lenght of the garage (- the thickness of the walls=26鈥? and being the only support of the whole roof ... what size does it have to be and what material shall i consider best; wood or metal ? should i consider bracing some i beam together in a 鈥楢鈥?shape structure in order to give more rigidity laterally ?
note: weight factor for snow is 60 psf over here. wind factor is not really a consideration so weight should be calculated for maximum charge. the dim. surfaces of the roof are 9鈥?x 27鈥?and 18鈥?x 27鈥?both having a slope of 4/12 and will be recovered by standard painted corrugated metal sheets.
best regards to all of you
when you say ceiling is made of i beams, you mean engineered wood products, such as tji's, right? not actually steel w sections?
a 27' span for a wood beam supporting what sounds like a lot of load probably will not work. maybe a flitch beam, a w14 section.
an average wood 12x20 in tall is required to limit long term deflection to under 1 in. this is maybe too intrusive in the attic so maybe you would want.
a w14x30 braced atop every 2 ft seems strong enough and has about 1.25 in deflection or l/260 under my 0.84 klf total service level load, that can work for a roof.
this seems like an ideal application for wood trusses at 2 feet on center. that would eliminate the massive center beam, as the trusses could easily span between the bearing walls. the downside is that you would end up with a higher structure (3-4 feet higher at the ridge)than you would with your scheme.
not that i'm trying to sell trusses....
if you use trusses, i dont think you'll get the deesired cathedral ceiling.

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