double channel spacers

huangyhg

double channel spacers
guys/gals,
i have an existing 20ft. long c8x11.5 and i will be beefing it up with an additional c8x11.5 which will be bolted to the back of the existing, i want to leave a 戮鈥?gap in between the two channels, to do this i will be using 戮鈥?spacer plates, but my question is, along this twenty foot span i don鈥檛 know exactly how many plates need to be inserted so that these double channels will act as one continuous beam.
any information or direction to reference material on this subject would be appreciated.
check out our whitepaper library.
to find the maximum spacing for your spacer plates, you must first find the span at which the channels will ltb by themselves under the imposed load.  if your spacers are at that length or less, you won't have an issue with the channels buckling individually and can treat the channels as a unit.  
understand that cb will be close to 1 for this check (the moment could be distributed pretty evenly across the distance from spacer to spacer), but since you probably take cb to be 1 anyway, you shouldn't have a problem.
check the built-up member section of the aisc specification also.