Determination of pedestrian loads in the presence of multi-modal lateral bridge vibrations

Simultaneous excitation of multiple lateral modes due to pedestrian loading has been observed on the London Millennium Footbridge and Clifton Suspension Bridge in the UK. However, all previous experimental studies aiming to determine the pedestrian loading on laterally oscillating ground have focused on a single mode only. This shortcoming has arisen as a result of the lack of capability of previous experimental setups to deliver multi-mode vibrations. As a consequence, in all existing modelling approaches, the results from tests in a single mode are assumed to be representative of pedestrian behaviour universally. Therefore, uncertainty remains as to whether the predictions of dynamic structural behaviour from these models can be trusted. To address this problem, tests have been conducted on a newly-developed instrumented treadmill mounted on top of a hydraulically actuated base, allowing for treadmill motion representing simultaneous motion in multiple bridge modes. This paper presents the results of these tests and relates them to the performance of the inverted pendulum pedestrian model previously shown to be capable of generating destabilising forces on the structure in multiple modes simultaneously.