area of a compression flange regarding daf

huangyhg

area of a compression flange regarding d/af
i have a common combination section trolley beam.  its an s-beam with a channel riveted to the top flange.  the unbraced length is large so i need to calculate fb.  i can use the radius of gyration with equation f1-7 from the manual of steel construction asd 9th edition to get fb, however the value is only 13.3ksi.  does anyone know how to calculate the area of the compression flange for this combination section so that i can use equation f1-8 from asd?
is it simply the area of the s-beam flange plus the cross section of the channel?  
the aisc limits the use of equation f1.8 to sections that have a compression flange that is approximately rectangular.  i have always interpreted this to mean that a section with a cap channel does not meet this criteria.  therefore, one would use the appropriate equation with l/rt.  this seems to be correct as the aisc does not list values of d/af in the section properties table for combined sections.  they do, however list values of rt.
would it not be possible to at least use the flange of the s-beam and the web of the channel and consider that rectangular?  since they are riveted together, the could be considered as one right?
i am not sure.  i do know that the aisc places the restrictions on equation f1.8 because it is a"boiled-down" version of a general stability formula.  the aisc has formulated it based on the most typical shapes (w and s).
you may want to look into a stability textbook.  i know that there is one out there that was edited by galambos, et al.  i do not have the reference in front of me.  perhaps you can lay your hands on one.
weron4u,
if you assume that this built up section behaves as a single   
i have been using the aisc formula for years when designing cranes girders and crane runways.
the area of the compression flange consists of the area of the cap channel plus the area of the flange (ref. gaylord and gaylord, second edition, page 6-23).
a second reference is the aisc design guide number 7, in appendix a, table 2, their tabulated d/af values; they did consider the area of the cap channel.
i have seen other references that use less than 100% of the cap channel area. it is my opinion that this is conservative.
i hope this helps.