Office floor vibrations: modal parameter identification and vibration monitoring

Several people working on the 2nd floor of an office building complained about disturbing floor vibrations. The building consists of five floors. In the contrary to the other floors, the 2nd floor slab has no floor-to-ceiling secondary walls neither beneath nor on top of the slab. The modal parameters of four floors were identified using ambient vibration testing (AVT)-technology and a 5-kg-medical ball as a vibration generator. The fundamental natural frequencies of the floors were: 2nd: 7.4 Hz, 1st, 3rd and 4th: 11.5...12 Hz. To monitor the vibration intensity and to identify the source of the vibrations, a triaxial velocity sensor was subsequently mounted in a critical point of the 2nd floor slab. The vibrations were monitored for two months using a newly developed internet-accelerograph. This allowed on-line checking of the vibrations and downloading of the data on an external server on a daily basis. Processing of these data yielded that no other source of the vibrations could be identified than people walking on the floor. Rating of the measured vibrations according to ISO 2631 yielded that during working hours, the vibration level was up to 3.6 times higher than "satisfactory". At the moment, discussions are ongoing to find an optimum solution for the problem. 1 THE PROBLEM AND THE APPROACH TO FIND A SOLUTION Several people working on the 2nd floor of an office building complained about disturbing floor vibrations. The building consists of four similar floors. To identify the problem and to achieve a reasonable basis to evaluate a solution, two kinds of tests were performed: a) System identification of the floors (determination of natural frequencies, mode shapes and damping coefficients), b) Monitoring of the 2nd floor vibrations for two months.