Hi Stuart,
See Ed Levin's and Tom Nehil's Frame Engineering articles in the Guild's journal, Timber Framing, for good summary articles on this topic, particularly issue #87, March 2008. Ed's articles include calculating the joinery design capacity once you know the load.

The only practical way to calculate lateral loads on frames and the resulting stress on joinery that I know of is through Finite Element Analysis (FEA) and that is done by a qualified engineer with computer simulation.