huangyhg

rafter unbraced length problem
we got a difficult issue with our current design vs. an existing roof. following is the senario:
i designed a flat roof structure for a workshop in a 40psf ground snow area, lower portion in attached pdf file shows part of the roof design plan view. in my design i would add one bracing member at middle of the 10jwe12 cold formed steel rafter to reduce the unbraced length of the rafter to half(12') based on required flexural strength calculation. but the contractor says it is un-necessary to add this bracing member and he showed me an existing workshop roof which is near exactly the same rafter span except it does not has the bracing member.
acoording to my calculation with unbraced length =48" will definitely not be able to meet the required strength, unless the designer of that existing roof considers the top felt and a thin steel roof sheet have the bracing function to the 10jwe12 cold formed steel rafter. but to me, these soft and flexural material can not be relied on as an bracing member. on the other hand, this existing roof has already stand there for over 20 years without any problem according to the contractor.
what are you guys thoughts and suggestions?
thanks a lot.
sorry, the third paragraph should be corrected as:
"acoording to my calculation with unbraced length =24', the rafter will definitely not be able to meet the required strength, ....."
you can only trust a contractor as far as you can throw him
in light gage steel building design, the rafters are frequently laterally stabilized using 28 or 29 gage steel panels. i am leary of their structural strength, especially if you walk on them wrong, but the system does seem to work.
what is the gage and depth of the steel sheeting? if the decking is 1.5" deep, 26 gage material, it should be ok, depending on the attachment to the joists. screw the felt!
mike mccann
mmc engineering
don't forget wind uplift on the rafters. if you rely only on top flange decking/roofing, your unbraced length may be limited for gravity loading but would be 24 feet under uplift.

the bracing or bridging
when i design cold formed steel roof framing, i put in bridging @ 4'-0" oc. it does not need to be full depth bridging; it can be channel bridging running through the openings in the rafters. and you can assume the rafters are fully braced by the roof deck, if the deck is steel deck, plywood or osb. if you have a standing seam roof, then i agree, the rafters would need the kind of bridging you are proposing.
daveatkins
checked the existing roof without bracing
again, the top flange is braced, but not the bottom. don't ignore uplift.
i very rarely see no bridging on this type of thing. i second every else.
this problem of lateral torsional buckling may be solved
by proofing that the rotational stiffness of the deck is
higher than the required rotational stiffness to
avoid lateral torional buckling.
the result will be that you need no bracing.

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