One of the open-plan office floors in the very prestigious new Mailbox office development had been designed with a very low profile floor to provide more space within the building. Unfortunately the design had a natural frequency of 3.6Hz and had very low damping.
Consequently, as staff walked across the floor, the vibration amplitude would rapidly build-up and after a few steps, the whole floor would be oscillating with quite high amplitude. This caused PC monitors to sway and some staff to complain of incipient motion sickness.
Based on a detailed analysis of the structure and response, a minimal set of disguised stiffening beams were designed and fitted (to avoid impinging on the open feel of the office). These increased the vertical stiffness by 70% and introduced more damping, solving the problem.