DL,
Timber frames are not inherently stiff structures. They flex. Yes, even frames with a lot of knee braces flex when loaded by wind. They are strong but not stiff.
Modern building regulations require stiff buildings that do not flex. It is incredibly difficult (you can substitute "expensive" for difficult) to build an all wood-joined timber frame that meets code requirements for lateral stability. So to build code compliant timber frames buildings, most often we rely on the envelope to provide lateral stability.
This is typically not a problem when the frame is enclosed in any of the typical manners -- sips or stick framed walls. Those systems can be made to meet the requirements quite easily. It becomes an issue with alternate enclosure systems and especially with open air structures.
