Vibration Serviceability Performance of an Externally Prestressed Concrete Floor during Daily Use and under Controlled Human Activities

Abstract

The vibration serviceability of an as-built long-span concrete floor was assessed using field measurements taken during the structure’s daily use and under controlled human activities including a group of people walking and jumping at specific frequencies. The floor serves as the waiting hall of a railway station and has a unique external prestressing system. The measured natural frequencies and recorded accelerations were compared with threshold values specified in design guidelines. The fundamental frequency of the floor was estimated to be approximately 2.2 Hz, which does not satisfy the requirements of current Chinese design codes for concrete structures and PCI design guidelines. The recorded vibrations were much lower than the vibration amplitude thresholds specified in several design codes for similar types of structures. This positive result is consistent with interview results from users stating that there have been no complaints from passengers or station staff relating to the floor’s vibration performance since the station was open to the public three years ago. Unless the external prestressing system is changed, numerical analysis demonstrates that it is too expensive and in some cases technically impossible to increase the natural frequency of a floor to 3 Hz to satisfy the recommended threshold value. The authors conclude that the frequency threshold value given in the current Chinese code and PCI design guideline is too strict for this floor. An approach that sets a vibration amplitude threshold seems more appropriate for assessing the vibration serviceability of this kind of long-span concrete floors. A multiplication factor of 30 to the ISO vibration baseline curve is recommended as an appropriate assessment criterion for floors serving as waiting halls in railway stations. Vibrations of the floor under different volumes of users were recorded and analyzed to capture the influence of crowd configurations on the vibration. Observations revealed that the vibration induced did not show much dependence on crowd configurations. Besides, floor vibrations under various group sizes were investigated. Even though they were found to be dependent on group size, the floor vibrations did not change linearly with group size as a result of imperfect synchronization.