Joe:

Ed Levin is THE person to talk to on this subject. See his article in Timber Framing # 25 on engineering the Speed River bridge in Guelph, Ontario (a former Guild project) as well as various posts in this BB including:

http://www.tfguild.org/ubbcgibin/ultimatebb.cgi?ubb=get_topic;f=5;t=000005

For more info, use the search box on the first page of the web site on the term "speed river".

Brian