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Bridges -- Design and construction, Strcutural dynamics


The most expensive and critical links in the nation’s transportation network are its bridges. Historical and contemporary bridge failures have highlighted our reliance on these structures. While the nation’s bridge management system is robust and well administered, the tools needed to evaluate individual bridges to determine their condition, whether for asset management or in response to a significant loading event such as the imminent Cascadia Subduction Zone earthquake in the Pacific Northwest, are currently highly specialized. The goal of this study was to develop a cost-effective, accurate, and easily deployed evaluation tool using widely available mobile technology, specifically iPods, to measure the dynamic structural response of a bridge subjected to harmonic forcing. Principles of structural mechanics, dynamics, and vibrations, as well as a significant body of literature, were leveraged to conceive a system that might complement existing visual inspection methods to support bridge condition evaluation and rating.

Any dynamic structural evaluation system like this requires consideration of the system accuracy, its potential users, and the time and effort required to use it. The Rapidly Deployable Structural Evaluation Toolkit for Global Observation (RDSETGO) consists of a relatively portable electromechanical shaker to supply a harmonic forcing to a structure and a network of iPods to measure acceleration response, all contained in portable plastic totes. The evolving deployment procedure requires estimation, identification, and confirmation of natural frequencies and mode shapes by peak picking and resonance testing methods. Modal damping ratios can also be determined using the half-bandwidth method to support development of a detailed modal model of a structure. Data collection is conducted in the frequency domain and has been performed manually in the work reported here. Post-processing consists of data entry and mode shape surface plotting.

Five bridges (two composite steel girder, two pre-stressed voided slab, and one composite prestressed concrete girder) with fundamental frequencies between 4 and 10 Hz were tested with various iterations of the system. Between three and six modal frequencies and mode shapes were measured for each structure. The first six modes and frequencies for the composite pre-stressed concrete girder bridge were within 3 percent of results measured by the Targeted Hits for Modal Parameter Estimation and Rating (THMPER™) system, which is deployed in support of the Federal Highway Administration’s Long-Term Bridge Performance Program.


This is a final report, NITC-RR-1022, from the NITC program of TREC at Portland State University, and can be found online at:

Dynamic_Evaluation_of_Bridge_Structures_with_Forced_Vibration.pdf (9179 kB)
Bridge Modal Analysis with Forced Vibration and Mobile Devices (Presentation)

NITC_Webinar_-_Dynamic_Evaluation_with_iPods.pdf (5448 kB)
Developing Practical Dynamic Evaluation Methods for Transportation Structures (Presentation)