What is Backstay Effect?

Yeah, that was me. One of my favorite topics actually:

Link
Link
Link

Basically, a shear wall terminating in a basement gets laterally restrained by the upper few floor diaphragms of the basement and turns into a propped cantilever rather than a pure cantilever. That can create a very high shear demand below main floor for buildings that go much higher above grade than they do below grade. Flip through the links. Once you see the right diagram, it will be pretty obvious.

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.