March 14, 2013 — MONTREAL, CANADA – The MAADI Group engineering firm, Montréal, Québec, has had their 145-foot-long Brossard, Quebec aluminum pedestrian bridge chosen for a footbridge vibration study by Professors Scott Walbridge and Sriram Narasimhan and their team at the University of Waterloo’s Department of Civil and Environmental Engineering in Waterloo, Ontario, Canada. The aluminum pedestrian bridge, which spans Daigneault Creek, was chosen for this field study on vibration effects because it is the longest footbridge of its kind in Canada, it is located near a major center and the bridge offers good access to its underside across its entire length.
The vibration research study was conducted by Professors Walbridge and Narasimhan, with lab technician Richard Morrison and graduate students Ann Sychterz, Pampa Dey and Ayan Sadhu in fall, 2012. The complete vibration study, “Results from Modal Testing of the Daigneault Creek Bridge,” is being reviewed by the CSCE General Congress in Montréal for presentation and publication this summer.
The vibration study’s results may greatly influence the future of pedestrian bridge design. Professor Walbridge states, “This was a great opportunity to record vibration measurements on a structure that has the distinction of being the longest of its kind in Canada. A key motivation for our work is that we hope this study will lead to the economical construction of aluminum pedestrian bridges in the future with even longer spans. Little exists in current scientific and technical literature, in terms of focused study on the vibration characteristics of natural frequencies and damping ratios for aluminum pedestrian bridges. We believe our work in this study will make a significant contribution to the state of knowledge in this field.”
In the Brossard bridge vibration study, the footbridge was instrumented with twelve wired accelerometers installed on mounting brackets fastened to the bridge’s bottom chords. Custom aluminum clamps were used to mount the vibration sensors, and signals recorded through a portable computer that tested and measured the vibrations. Loading patterns were assessed during field tests for vibrations caused by walking, running and jumping at various locations along the pedestrian bridge.
Professor Walbridge is a member of the Technical Committee for the Canadian Highway Bridge Design Code, Chapter 17, Aluminum Structures. He conducts research in the area of fatigue assessment and strengthening of metal structures. Professor Narasimhan has conducted extensive analytical and experimental research on vibration behavior of structures retrofitted with passive and active damping systems, most recently focused on understanding the behavior of linear and non-linear structures and control devices, and the processing of measurement data from vibrating structures to assess their condition.
“MAADI Group is pleased to have our pedestrian aluminum bridge utilized in this important vibration study,” said MAADI CEO Alexandre de la Chevrotiere, “Data from these tests will help engineers to design long-span footbridges that are stable and secure, as well as strong and long-lasting. Aluminum pedestrian bridges are proving to be a safe and viable option for infrastructure projects. As engineers, we strive to improve bridge design from many technical standpoints, including the structure’s vibration damping qualities.”
The vibration tests confirmed that the MAADI Group’s engineering design tasks were done correctly. The study shows that the bridge’s fundamental frequencies do not fall within the range of normal walking frequencies (1.6-2.4 Hz). The four modes of natural frequencies of the test subject MAADI Group bridge correspond to 3.4 Hz (vertical), 5.9 Hz (vertical-torsional), 14.7 Hz (torsional), and 17.1 Hz (lateral mode of the truss sides).
As stated in AASHTO (Guide Specifications for Design of Pedestrian Bridges): “The potential for significant response due to dynamic action of walking or running has been recognized by several analyses of problem bridges and is provided for in other design codes, such as the Ontario Bridge Code. Research into this phenomenon has resulted in the conclusion that, in addition to stiffness, damping and mass are key considerations in the dynamic response of a pedestrian bridge to ensure acceptable design.”