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fire resistance ratings

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Greetings,
 
I have a question about determining fire resistance rating (1997 UBC) of a masonry composite wall.  I have an ungrouted nominal 6 inch CMU interior wall, a 3 3/8 grouted and reinforced collar joint, and an exterior nominal 3 inch brick veneer.  I have calculated the fire resistance of each wythe and the collar joint .... no problem.  Is it permissible to use equation 13-3 of Standard 7-7 for combining the fire resistance of each wythe?  I get drastically differing results if I simply add the fire resistance ratings algebraically or use the exponentials as suggested in formula 13-3 (or 7-1 for multi-wythe concrete walls).
 
Thanks in advance,
Dave