Fb Back to Back Channels

[rulljs]

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.)

 

[UcfSE]

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.

 

[rulljs]

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!!

 

[JStephen]

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.