Modal parameter changes of lowly damped civil structures due to people-structure interaction

The interaction between civil structures and people has been a widely studied topic in the last years and the comprehension of the whole system dynamics is considered a valuable tool to the purpose of structural health monitoring. This paper is intended to give evidence of people effects on the modal behaviour of very lowly damped civil structures, the considered is case is a steel stair walkway, placed at the ground floor of a new skyscraper in a Spanish city. The modal analysis of such a walkway has been carried out in different conditions, by forcing the structure by means of a hydraulic actuator and exploiting ambient vibrations. This allowed to achieve full knowledge of the stair dynamic behaviour alone. Furthermore a comparison between the two identification methods has been made possible. Then tests with 22 and 44 people walking on the stair have been performed to assess the structure vibration levels under operational condition and therefore consider all serviceability issues according to the international standards. In the meantime vibration data were used to compute the modal parameters of the complete system (stair + people). The results show that there is a significant damping ratio increase for the first modes at low frequency, once increasing the number of people. This limits the overall vibration levels, although the excitement is increased with the number of people. The damping ratio increase due to people is about 400 % for the first eigenmode when 44 people are walking together.