Chapter 1. Introduction
This research project, "Dynamic Bridge Substructure Evaluation and Monitoring," was sponsored by the Federal Highway Administration (FHWA) under contract number DTFH61-96- C-00030. The research was funded to investigate the possibility that by measuring and modeling the dynamic response characteristics of a bridge substructure, one could determine the condition and safety of the substructure and identify its foundation type (shallow or deep). Determination of bridge foundation conditions with this approach may be applied to quantify losses in foundation stiffness caused by earthquake, scour, and impact events. Identification of bridge foundation type may be employed to estimate bridge stability and vulnerability under dead and live load ratings, particularly for unknown bridge foundations.
Chapter 2 introduces the research project scope and presents the results of a literature review of related topics. Chapter 3 describes the bridges in Texas that were selected for modal vibration tests during the research. Three bridges and four substructures were selected to provide footing (shallow), pile cap on piles (deep), and piles (deep) foundation types to investigate the type of foundation. Chapter 4 presents the modal vibration testing program, looking at instrumentation and field test procedures used to measure the vibration responses of the bridges. Of particular interest is that selected piles of two bridge bents were able to be tested in a sound, undamaged state initially, and then as selected piles were first excavated and ultimately broken, to simulate scour and earthquake damage events, respectively. Chapter 5 details modal vibration data processing and analyses performed on the four tested bridge substructures for the three bridges. Example time, frequency spectra, and modal transfer function (TF) data are presented for the bridge substructures and sound, excavated, and broken pile states. The TFs were found to show increased flexibility (lower stiffness) for substructures as they were excavated and ultimately broken. However, comparisons of modal TF data for two similar bridges and substructures with shallow and deep (footing and pile cap on piles) foundations did not show promise for only experimental data indicating shallow versus deep bridge foundations. The possibility of determining substructure damage or foundation type, or both, using structural modeling and structural parameter estimation techniques to obtain so-called super-spring elements was extensively researched as discussed in chapter 6. Unfortunately, the outcome of the structural parameter estimation analyses was that the damage states could not be predicted reliably, nor could the foundation type be determined.
As a consequence of this inability of theoretical approaches to identify either damage or substructure foundation type while experimental data showed the effects of the damage, the completion of the research was delayed while a new analysis technique for identifying instantaneous, brief changes in frequency associated with nonlinear responses of structures to damage was investigated. This technique is known as the Hilbert-Huang Transform (HHT), and the results of these analyses of the modal vibration test data are presented in chapter 7 for the two bent substructures that were tested in sound (undamaged), excavated, and broken pile states. The HHT method was found to have considerable promise for identifying expected but brief changes in frequency associated with nonlinear responses that are masked by the dominant linear responses of otherwise sound structures in traditional modal vibration testing and analysis. Chapter 8 presents a preliminary discussion of the potential for integrating modal vibration testing and HHT analyses into bridge management systems in the future. Chapter 9 presents the conclusions of the research and recommendations for future research on the HHT method.