From Old to New: Vibration Issues with Old Buildings and Their New Office Spaces

Abstract

Nationally, existing building owners constantly work to update existing building spaces to make them more appealing to potential lessees. These spaces are cleared of old heavy furniture and tall heavy partitions to make way for open plans with lightweight cubicle partitions and lightweight furniture. The change in office architecture along with the lighter paperless office lead to lower damping and reduced mass that often bring to light annoying floor vibrations that had never existed before. Structural engineers and architects are hired to assist in these updates, and often make the mistake of failing to review the adequacy of the floors for possible annoyances from walking induced vibrations. Once a problem has been identified, there is little guidance on the necessary standard of care of the renovation or the execution of a successful retrofit to mitigate the vibrations.Structural engineers need better guidance in the investigation and quantification of annoying floor vibrations of existing office space floors, as well as in estimating the likely success of possible retrofits. This paper explores these issues in a real-world office space with annoying walking induced vibrations. Field measurements quantified vibration characteristics and these were compared to established criteria to assist in determining the necessity of mitigation. Multiple mitigation measures were considered to reduce walking induced vibrations and these were explored through finite element modeling. Finally, the performance of a compact pendulum tuned mass damper (PTMD) to mitigate these vibrations was assessed through comparison of before and after field measurements. BACKGROUNDNew office renovations, as well as the general trend towards the paperless office, increasingly see a change from heavy wood furniture with full height hardwall partitions. These changes effectively reduce existing damping, which was sufficient to prevent occupants in the prior use from complaining of footfall vibrations. With the reduction of damping, some floors become exposed to resonant excitation from footfalls creating perceptible low frequency vibrations (below 10 Hz). The floor structures of concern are those susceptible to resonant conditions produced by footfall excitation, which occur mostly between 1.8 to 2.2 Hz. This excitation and its first three harmonics are typically of the greatest concern.