The skinny towers put residents where they want to be — near a window with knockout views. Yet like reeds in a marsh, these skyscrapers are prone to bend when the wind blows hard. Ideally, that bending is imperceptible. So today's engineers battle not only the force of gravity but the the whirling pockets of wind, known as vortices, that can cause towers to shake or vibrate. Failure to do so would make the motion of the building — or "acceleration," as engineers call it — uncomfortably noticeable.