Chris: R = 1.5 certainly feels like a conservative value. I'm confident that an unsheathed timber frame can dissipate a good amount of energy under reversible loads, so strength isn't the real issue. Serviceability under such loads would then become the problem.
The obvious consequence of R = 1.5 is that seismic loads become quite large when elastic or nearly elastic response is assumed. In general, I'd be surprised if a conventional timber frame that relies on knee braces for lateral load resistance could be designed to resist the required seismic loads and satisfy the corresponding drift limits. This suggests that an alternative framing concept (such as story-high bracing as used in German Fachwerk) might be in order. -- Dick
