Rafter unbraced length problem
Rafter unbraced length problem
(OP)
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.
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.






RE: Rafter unbraced length problem
"Acoording to my calculation with unbraced length =24', the rafter will definitely not be able to meet the required strength, ....."
RE: Rafter unbraced length problem
RE: Rafter unbraced length problem
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
RE: Rafter unbraced length problem
RE: Rafter unbraced length problem
RE: Rafter unbraced length problem
DaveAtkins
RE: Rafter unbraced length problem
RE: Rafter unbraced length problem
RE: Rafter unbraced length problem
RE: Rafter unbraced length problem
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.
RE: Rafter unbraced length problem
Your statement is fundamentally incorrect. Whether the bottom flange requires bracing is dependent on the member itself, not on the deck stiffness.
RE: Rafter unbraced length problem
Hokie is right to take issue with your approach Statiker, as you may be effectively restraining the CFS rafters agains LTB, but you will now have another failure mode governing, likely local buckling.
I posted the HERA research findings in an earlier thread, with a fairly detailed explanation. That said, the gist of the idea is that if your purlins are greater than half the depth of your rafter and connected to cleats passing across the top of your section with two or more bolts and full depth stiffeners are provided each side of the web, you have effectively restrained the section. I would advocate than anyone wanting to pursue removing fly-bracing by this method get a paper on the topic to ensure they're confident with it.
All of that said, you're almost certainly not going to get away with it in CFS; It's just too unforgiving.
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Rafter unbraced length problem
instabilities. global lateral tors. buckl. may be avoided by the deck itself and local buckling may be avoided by
maintaining the form of the cross-section for example by
local stiffeners. this is proven by experiments.
i think it is possible to analyse this correctly by solving the corresponding stability problem.
RE: Rafter unbraced length problem
RE: Rafter unbraced length problem
While you can appreciate the contractors comments and should treat him as an associate, the two of you are responsible for two different things.
He wants to get it built, he has no responsibility after it's up, you are responsible for a long time.
If you're truly uncertain, go with the bracing.
RE: Rafter unbraced length problem
I actually second Diarmud's comment, with a sprinkle of Ash060's pessimisum. Add the bracing, sleep at night.
AND a contractor with twenty years of experience (or a structure which has been field tested for that period of time) does not prove performance for as long as you must, and remain responsible for...
Good luck, Please send an update post with the end result,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Rafter unbraced length problem
Now I am going to tell the contractor that the existing roof might only meet the old code of 20 years ago, it does not meet current code requirement. So I will insist on putting these bracing member.
Thanks everybody again.