fb back to back channels

huangyhg

fb back to back channels
hello all,
i have an existing sturcture which i have modeled in staad that contains a double channel beam (c7x9.8 w/.75" spacing, 15 feet long supported at each end, 36ksi steel).  i ran the analysis and the staad output is saying that this beam will be overstressed due to the comobined loading eq. h1-3 aisc 9th ed.  this conclusion is not too unreasonable to me, since i am trying to put quite a bit of load on this beam but just out of curiosity i looked into the numbers staad used in it's calcs and in the fb/fb part of the combined stress equation they used fb=14.65, which is about 0.4*fy, why is the reduction factor for this so much lower than the usual 0.6*fy?
i assume this may be due to my laterally unsupported length of my compression flange being to great (15ft), but this is where i am starting to get a bit confused. (chapter f aisc 9th ed.)
check out our whitepaper library.
it is because your unbraced length is greater than lc given by f1-2.  i used the equations in f1.3, eqns f1-6 through f1-8 and got the same answer staad gave you for fb.
thanks uc,
i mistakenly used ft. as my units in f1-2 as opposed to in. which of course led me to believe my lb did not exceed lc.  re-calculated (with in.) and i too am now getting answers that are correlating with staad when using f1-8.  guess silly mistakes like these are why i'm still an eit, couple more years to go..
thaks again for your help!!
anyone can make silly mistakes.  the difference is that when you do the same kind of calculations quite a bit, you can recognize the silly mistake quicker and redo it.