Keywords: structural dynamics, accelerometers, response spectrum, experimental modal analysis, curve-fitting.

The basic properties of a single degree of freedom (SDOF) oscillator model that describes the dynamic characteristics of a structural system can be encapsulated by three intrinsic parameters: equivalent stiffness, natural frequency and critical damping ratio. Traditional methods that have been used for evaluating these parameters in a teaching environment have been based upon conducting simple experiments, such as a "pluck test" on simple structural models in the laboratory, [1]. The teaching of more sophisticated techniques, such as the curve-fitting methods of experimental modal analysis (EMA) testing, has almost exclusively been facilitated using such simple laboratory models, as it has been prohibitively expensive until recently, both in terms of cost of equipment and in the logistics, of being able to mount tests in the field on real structures for this purpose.

By choosing newly developed low cost MEMS based tri-axial accelerometers that have recently become available and principally directed to the hobbyist market (for example in robotics) and simple easily accessible structural forms to test in the field (such as pedestrian bridges), it has become quite feasible to now run such field experiments for the purpose of teaching EMA techniques in structural system identification studies to classes with a manageable number of students.

A simplified SDOF curve-fitting procedure can then be applied to the acceleration response spectrum, obtained from the field measurements, based upon knowledge of the form of the input force spectrum or under the "white-noise input energy at resonance" assumption.

This paper describes the novel development and application of such a simplified EMA teaching approach that has been implemented by the author in his post-graduate class on structural dynamics at The University of Melbourne. Examples that have been implemented for this purpose include:

• Performing a "pluck test" on a vertical slender cantilever model and measuring the tri-axial acceleration response using a USB accelerometer.
• Performing basic experimental modal analysis on a pedestrian bridge undergoing vibration from pedestrian induced excitation from records of the adjacent vertical acceleration response, using a pair of USB accelerometers and a simplified approach.
• Determining the basic dynamic characteristics of a conifer undergoing excitation by high level winds during a 1 in 20 year storm, using measurements from a tri-axial accelerometer interfaced to a low-cost battery operated datalogging system developed by the author.

It is clear that the "hands on" involvement by graduate students in setting up the experiments concerned and their direct participation in the data acquisition has added another dimension to the teaching of structural dynamics to these graduate classes. The sense of "ownership" in the capture of individual data records by the students has heightened their interest and engagement in the classes concerned.

1

R.W. Clough, J. Penzien, "Dynamics of Structures", McGraw-Hill Book Co., 2nd Edition, 1993.

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