Comparing braced frames to moment frames, is like comparing triangles to squares...
The square can easily deform and requires rigid joints if used as a lateral system to resist wind/seismic...
The triangle can only deform if the member itself shrinks or extends axially (along length) - which is difficult to do significantly (PL/AE). The square with rigid joints does not need to deform axially, it simple needs to bend and it will translate. So moment frames are about 20 times more flexible than braced frames (+/-10). To make a moment frame into braced frame, simply add a diagonal...
