bottom flange bracings

huangyhg

bottom flange bracings?
on a gabled steel moment frame, my architect requested me to take out all bottom flange bracings making it fully unbraced. top flange is effectively braced by purlins. with wind uplift controlling, i'm always using the full width of the frame (sloping) as my compression flange unbraced length. i've seen some designers and not realizing that node pt. at ridge is not a braced point unless provided with other positive means. any one has a different approach to this condition?  

can you rephrase the question and post a sketch of your situation?
don't you have a ridge beam (perpendicular to gable frame)?
there's not a ridge beam so the full rafter span is unbraced at bottom flange. it is 100' span symm. single gable at 3:12 slope, i'm using about 103' unbraced length of compression flange for wind uplift. node pt. at ridge should not be assumed as braced pt.
you are correct.  it is unbraced for its full length.
daveatkins
so, was your question lb = 103' or 206'?
forget above the ridge beam, it is immaterial for this case. i agree 103' is correct, because the true unbraced length of concern is shorter than that.
thanks guys. it's just that other designers can sometimes overlook this without specifying the correct lb in their model,i.e., using lb=51.5' instead of 103'
i assume you told your architect you can't comply.
ba
the other guys are correct that the full span length must be used if the bottom flange is totally unbraced, which probably makes your design very unwieldy.
there are proposed design methods which use the strength of the purlins, bolted to cleats with at least two bolts, and combined with web stiffeners, to brace the bottom flange indirectly.  there have been some discussions on this site about using stiffeners as braces, but i don't have a specific reference.
australian steel institute (asi - formerly known as australian institute of steel construction - aisc) publishes a book called "limit state design of portal frame buildings" by woolcock, kitipornchai & bradford. i have the first edition (1991), but i know it has been updated since then.
a couple of quotes from the first edition:
from 5.3.2.3.2 "without fly bracing":
"... theoretical and experimental studies  have confirmed that translational restraint alone acting at the level of the tension flange, such as that provided by purlins, is virtually ineffective [to increase the lateral buckling capacity] ... it is possible to design the purlin-rafter connection for some rotational capacity by providing two or four friction bolts to the cleat ..."
from 5.8 "fly braces":
"in this case, the bottom flange should be braced by using a wider purlin cleat and four high strength bolts, and a web stiffener on one or both sides to prevent distortion ... there is some evidence that the stiffeners are unnecessary. however, until testing confirms this, it is recommended that at least one side of the web be stiffened."
hope this helps!
i would recommend looking into using torsional bracing so that your shape does not get needlessly large.  see appendix 6 of the aisc 360-05 (american institute of steel construction) specification.  
"2. torsional bracing - it is permitted to provide either nodal or continuous torsional bracing along the beam length.  it is permitted to attach the bracing at any cross-sectional location and it need not be attached near the compression flange.  the connection between a torsional brace and the beam shall be able to support the required moment given below..."