hss design for shear

huangyhg

hss design for shear
i have a question regarding the shear strength calculations for hollow structural sections.
ref: aisc lrfd 3rd edition, 16.2-7, eq 5.2-2
in order to calculate the shear strength of an hss you must first calculate the critical stress, fcr.  one of the varialbes within that equation is "a", the distance from maximum to zero shear force.  is this variable refering to the maximum distance along the member or the cross sectional distance between the neutral axis and the extrem fiber?  
my calcs are resulting in rather large values for fcr compared to the limit of 0.6fy.  is there ever a case where the 0.6fy does not govern?
"a" is meaured along the   
thanks taro
if "a" is the distance along the member from max shear to zero shear than what is the value for "a" when you have a simply supported beam with a single point load?  the max shear and min shear theoretically occur at the same location.  are you suppose to use the maximum distance? i.e. l/2?
i too did some quick checks and can't find a situation where fcr would be below the 0.6fy limit for the shapes listed in the manual.  seems strange to me.