Thanks for the clarification aladdin76.
aladdin76 said:
I do not know what is meant by rotation. is it the rotation of the plastic hinge or the chord rotation of members (Drift/L).
You've got two separate items on the table here. In each case, FEMA-273 is attempting to limit curvature, and thus damage, in the members.
1) Curvature for a shear wall plastic hinge, often located at the base of the wall, is indexed to the amount of rotation occurring over the estimated length of the plastic hinge. That's figure 6-2.
2) Curvature for a coupling beam is indexed to the chord rotation estimated in each of the coupled shear walls that serve as the chords for the composite wall, taken at the elevation of the coupling beam. This is figure 6-3.
The key to understanding this, in my opinion, is to realize that curvature is the parameter of interest for members dominated by inelastic flexural behavior. Rotation and chord rotation are just how we get to curvature.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.