rafter ties and reduced rafter spans irc 9code0

huangyhg

rafter ties and reduced rafter spans irc (code)
in the irc tables  such as r802.5.1(2) there is a reduction of  rafter spans based on the height of the rafter ties above the plate. i.e. a 2x12 spf at 24" o/c can span 21'-0"
and if the rafter ties are at mid height factored by 0.58 = 12.18'.  my calculations show this to rafter to be overstressed.  has anyone else found this to be true? or
and at 2/3 the overstress is even worse.  
in addition  table (1)(ceiling not attached) has a span of 18' and table (2)(ceilig atached) has 21' and it seems to  me that the spans in table (2) should be less than table (1)
in regards to rafter ties not collar ties  the dissertation in paragraph 802.3.1 appears to allow these ties at 48" o/c but that makes the overstress in the above  cases even worse.
has anyone had this come up?
i'm surprised that noone has commented on this thread