When the World Trade Center towers were being planned, the builders discovered that nobody really knew how much sway occupants of tall buildings could tolerate.

When the World Trade Center towers were being planned, the builders discovered that nobody really knew how much sway occupants of tall buildings could tolerate. Wind tunnel tests showed that the (relatively) light buildings would sway more than expected and the builders feared that any publicity about this would scuttle the project. They began conducting two secret research programs. One investigated methods of dissipating energy to reduce sway. The other determined how much a room could move before occupants noticed it. This second investigation was conducted under the pretense of "free vision exams." Participants were led into a room that was (unbeknownst to them) mounted on rails and moved by hydraulic rams. The amount of simulated sway was increased until somebody spoke up. The engineers eventually determined that the sway could be brought within acceptable (though still detectable) levels by installing viscoelastic dampers between the floor joists and the building's perimeter columns. The whole story is told in the 2003 book City in the Sky, which is a fascinating read.