Performance of Pedestrian-Load Models through Experimental Studies on Lightweight Aluminum Bridges

Aluminum structures provide a high strength-to-weight ratio, are corrosion resistant, and are esthetically pleasing. Hence, their use in bridge construction has recently begun to increase, especially for pedestrian bridges. Because of their relative light weight as compared with other structures, they often exhibit fundamental vertical frequencies outside the range of normal pedestrian walking frequencies. Despite this, they tend to be lively structures, easily excited by pedestrians, resulting in large-amplitude accelerations. An experimental program was undertaken by the authors to study the vibration characteristics and performance of three full-scale aluminum pedestrian bridges, two in the laboratory and one in the field. Although the lowest fundamental frequencies for all three bridges fall outside the normal range of pedestrian walking frequencies, there is the possibility of higher harmonics of walking loads near resonance or in resonance with the fundamental vertical mode. The aluminum bridges studied were instrumented and subjected to a range of pedestrian-load tests to understand the performance of existing periodic models in predicting the dynamic response. The performance of these models (assuming pedestrian forces to be a summation of harmonics) has thus far only been assessed for cases in which the fundamental frequency is resonant with the first harmonic of walking, often considered the most severe case for design. This is the first time, to the knowledge of the authors, that these models have been studied for the case of bridges that are nonresonant with the fundamental mode. This article notes important observations regarding the performance of these models in predicting the serviceability of bridges made out of lightweight materials, such as aluminum, for cases in which the higher harmonics of pedestrian walking are either nonresonant or near resonance with the fundamental vertical flexure mode.