engineering School of Engineering and Applied Science Videos

Good Vibrations

Two engineering professors, about thirty five students and the university mascot Willie Pride shook up one of the university’s pedestrian bridges – literally – to illustrate the basic concepts of structural dynamics.

For one hour, Assistant Professor of Civil Engineering Manuel Miranda, Assistant Professor of Mechanical Engineering Alex Pesch and their students ran and jumped across the West Pedestrian Bridge and then measured the vibrations after each activity.

“Engineering is definitely challenging and there are a lot of concepts that need to be understood,” Dr. Miranda said. “The best way to understand them is by doing experiments and performing hands-on activities that motivate students to understand why it’s important to learn the material we study in class.”

The experiment was part of the Fred DeMatteis School of Engineering and Applied Science’s celebration of National Engineers Week, which was February 19 -25.

To accurately record data, Professor Miranda and Pesch’s students attached several accelerometers to the structure of the bridge, at its mid span. Two accelerometers were also connected to a signal conditioner and oscilloscope in order to visualize the vibration waveforms.

Mechanical engineering student, Susanna Planck-Kuney, ’18 helped organize the event and said, “It’s supercool because everything has a natural frequency—cars, people, motors.  It’s a very cool phenomenon because if you vibrate something at its natural frequency it gets really excited.”

The experiment considered two test frequencies: 2 Hz and 3.75 Hz.  To simulate the lower frequency, participants were asked to jump up and down with both legs at 120 beats per minute to the rhythm of a metronome.   To create the second test frequency of 3.75Hz, which the engineering students predicted to be close to the natural frequency of the bridge, Professor Miranda had participants jog quickly in place at 225 beats per minute. Other activities included everyone jumping in unison, as well as running and walking across the bridge at varying speeds to measure the response of the bridge at different frequencies.

The results of the experiments indicated that the bridge’s natural frequency is actually closer to 3.55 Hz which according to Professor Miranda is a pretty interesting finding.  He said, “Our computer predictions were within 6% of the measurements which is actually pretty good because it means that the values we calculated based on our computer model are very close to the measured values.”  While the bridge may be a little bit more flexible or softer than predicted, he assured us that it is still 100% structurally sound.

Professor Miranda plans to conduct the bridge vibration experiment annually with his students to track whether the natural frequency of the bridge changes over time due to ageing of the structure.

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