Architectural Engineer needs structural help

In the Architectural practice exam, there is a question I was hoping someone could help me with:

"In general, which of the following is the LEAST efficient frame in limiting drift from lateral loads?"

a. Moment resisting frame

b. X-braced frame

c. K-braced frame

d. Shear wall

Ans: A. Moment Resisting Frame

Can someone explain "efficiency" in the context of structural sytems? I understand stiffness increases as you go from moment, to brace, to shear wall, so is efficiency efficacy in this case? The moment frame is the least effective in limiting drift from lateral loads?

Thanks!

Dave

Philadelphia, PA

Dave, your instinct is correct. They're being a little cute with their terms, but what they're driving at is "pound for pound", all else being equal, the moment frame will give you the most drift because it's the least stiff inherently.

Keep in mind for other potential questions, though, that that lack of stiffness in general is what makes a moment frame most efficient at dissipating seismic energy, and therefore they are generally designed for much smaller forces than the other systems (member design in moment frames is often controlled by providing enough system stiffness to keep it within allowable drift limits).

Hope this helps, and good luck!