Modal mass estimation for vibration serviceability assessment of footbridges
Recent high-profile failures of footbridges to carry pedestrians without excessive and uncomfortable vibration have shown the need for a better understanding of vibration serviceability of structures under pedestrian loads. Failure to predict excessive response may be due to misunderstanding the mechanism of pedestrian loading, but also due to inaccurate response calculations in code-based assessments during design. For a bridge expected to have a low natural frequency this assessment is a dynamic analysis using linear single degree of freedom models requiring realistic estimates of modal frequency, damping, shape and mass. If the design fails and a retrofit is necessary the same parameters are required, but are expected to be estimated to a higher accuracy by full-scale vibration measurement. Modal mass and modal damping are critical parameters for the assessment process yet are the most difficult to measure experimentally. This paper evaluates procedures for modal mass estimation via two case studies of problematic mass estimation through structural analysis end experiment. As well as traditional forced vibration testing, methods using calibrated footfall excitation are shown to be remarkably effective.