Date of Graduation

5-2014

Document Type

Thesis

Degree Name

Bachelor of Science

Degree Level

Undergraduate

Department

Civil Engineering

Advisor/Mentor

Grimmelsman, Kirk A.

Committee Member/Reader

Heymsfield, Ernest

Committee Member/Second Reader

Hale, W. Micah

Abstract

The aging and deteriorating state of bridges in the US, along with the many limitations of the visual inspection data that is used for assessing and evaluating their condition, have provided motivation for research on experimental methods to quantitatively describe and evaluate their in-situ performance and condition. Ambient vibration testing is one such global characterization approach that has been widely explored due to its low cost and ease of implementation for in-service bridges. The testing is used to identify the modal properties of the structure, typically the natural frequencies, mode shapes, and damping ratios. Although ambient vibration testing has been used for many structural identification and health monitoring applications with bridges, the measurements are subject to uncertainty from a number of different sources that can limit the reliability and effectiveness for many practical objectives. One possible source of uncertainty that is particularly challenging to quantify and evaluate relates to the actual nature of the uncontrolled and unmeasured dynamic excitation of the bridge that leads to its measured vibration responses. The uncontrolled dynamic excitation in an ambient vibration test comes from natural environmental inputs and operating traffic loads, and is assumed to be spatially distributed on the structure and to have broadband, uncorrelated Gaussian white noise characteristics. Presently, variations to this assumed character have only been evaluated analytically or indirectly from the measurement results. Both of these approaches are subject to limitations that only permit qualitative assessments of the excitation related uncertainty. This paper describes a study that was designed to experimentally evaluate the characteristics of the ambient dynamic excitation on the identified modal parameters for a full-scale truss bridge in a direct manner using controlled excitation from a spatially distributed network of dynamic exciters attached to the bridge. This novel and low-cost dynamic excitation system was developed by Dr. Grimmelsman and enabled the research team to apply controlled dynamic excitation to the bridge that was consistent with the characteristics normally assumed for ambient vibration testing and for known variations to these characteristics. The modal parameters identified from these controlled excitation cases were compared with those identified from uncontrolled ambient dynamic excitation of the bridge. The results showed that the effective bandwidth of the uncontrolled ambient excitation was relatively narrow, and that most consistent and reliable identification could be obtained when spatially distributed, broad band white noise excitation was supplied to the bridge using the dynamic excitation system. The dynamic excitation system was also observed to lead to bridge vibrations that were substantially larger than those induced by ambient natural sources demonstrating that it could be an effective tool for characterizing and evaluating excitation related uncertainty in ambient vibration testing for other short to medium span length bridges.

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