Methods of Monitoring and Evaluating Structural Performance

The ASCE/SEI Committee on Methods of Monitoring and Evaluating Structural Performance developed a 1-day track on “Bridge Testing and Health Monitoring” for Structures Congress 2005 that was held in New York. This track consisted of four technical sessions, each on a different monitoring and evaluation approach. The sessions covered critical areas for bridge testing and health monitoring. These four sessions were on (1) Nondestructive Load Testing and Strength Evaluation of Bridges; (2) In-Service Evaluation of Bridges; (3) Field Monitoring for Fatigue Evaluation of Existing Bridges; and (4) Corrosion in Bridges. A subset of the above papers was selected for this special issue, and authors were invited to expand their presentations at the Congress into journal papers. These papers have gone through a rigorous peer review process based on ASCE publication standards, and only the accepted papers are included in this issue of the Journal of Bridge Engineering.

John DeWolf, Sreenivas Alampalli, and Masoud Sanayei coordinated the 1-day track at the Structures Congress 2005 as well as this special issue of the Journal of Bridge Engineering. Members of the Committee on Methods of Monitoring and Evaluating Structural Performance played a major role in the review of the papers in this publication. The goal has been to provide information for use by both bridge engineers and researchers who are conducting field bridge-monitoring studies. The papers developed from these sessions should serve those who want to learn more on testing, monitoring, and evaluating bridges. These papers also offer guidelines on the different bridge health monitoring approaches.

This issue of the Journal of Bridge Engineering contains 10 papers on Bridge Testing and Health Monitoring. “System for In-Service Strain Monitoring of Ordinary Bridges” is authored by Daniel A. Howell and Harry W. Shenton III. This paper presents an In-Service Bridge Monitoring System (ISBMS) to provide near real-time web-based monitoring of live load strains in a bridge. The small, battery-operated monitoring system can be rapidly deployed and can be programmed and interrogated via a user-friendly web interface. The ISBMS has been designed to be portable and used on an “as-needed” basis as a diagnostic tool or for health monitoring of ordinary bridges. The system has been tested in the laboratory and in the field on a heavily traveled steel-girder bridge. The data obtained from the ISBMS can be used for load rating, fatigue assessment, monitoring of stresses due to overload vehicles, and long-term health monitoring of bridges.

“Monitoring of Steel Railway Floor Beams Pre-Stressed by Steel Plates” is authored by Masahiro Sakano, Hironori Namiki, Syuji Yajima, Yasuhiro Koide, Hitoshi Furuta and Dan M. Frangopol. Steel cables and tendons are commonly used in reinforcing steel beams as well as concrete beams. However, the structural details for the cable anchors in steel beams tend to be complicated, and the effect on reducing live-load stresses is not always significant because of the relatively small stiffness of cables and tendons. Using high-strength steel plates instead of cables and tendons, the structural detail of the anchor area becomes simpler. In this study, the steel plate prestressing method is applied to beam specimens and intermediate floor beams of a steel railway through girder bridge. The behavior of the reinforcing steel plates and reinforced steel beams is monitored during prestressing and live loading in order to assess the effects of prestressing and reinforcement.

“Damage Localization and Finite-Element Model Updating Using Multi-Response NTD Data” is authored by Masoud Sanayei, Erin Santini Bell, Chitra N. Javdekar, Jennifer L. Edelmann, and Eugene Slavsky. This paper presents an approach for finite-element model updating, based on data collected from a nondestructive test. The approach allows for the simultaneous use of both static and modal NDT data. A new damage index is developed for damage localization of structures. This method depicts changes in physical properties of each structural element compared to its initial state. Deficient or potentially damaged structural elements are then selected as the unknown parameters to be updated by parameter estimation. The method is used to evaluate data from the test of a laboratory grid model of a bridge deck. Multiresponse parameter estimation has been utilized to update the stiffness of bearing pads and both the stiffness and mass of the connections by using static and dynamic NDT data.

“High-Cycle Fatigue of Diagonally-Cracked RC Bridge Girders: Field Tests” is authored by Christopher Higgins, William C. Farrow, Brian S. Nicholas, and Tanarat Potisuk. This paper presents the results of a study to investigate the influence of diagonal cracks on the service life of reinforced concrete deck-girder bridges. Diagonal cracks have been identified in many of these bridges, which have been exposed to millions of load cycles during their service life. Four bridges with diagonal cracks were instrumented to collect information on both the strains and crack displacements when the bridges were subject to ambient traffic conditions and controlled test trucks.

“Overview of Vibrational Structural Health Monitoring with Representative Case Studies” is authored by Kai Hsieh, Marvin W. Halling, and Paul J. Barr. This paper explores how techniques based on measured bridge vibrations can be used for damage detection. The basis of the approach is that structural damage can result in permanent changes in the distribution of structural stiffness. The paper uses the direct relationship of stiffness, mass, and damping of a multi-degree-of-freedom system compared with the natural frequencies, mode shapes, and modal damping values as a basis for structural health monitoring. Included are the results of live-load testing on three different bridges.

“In-Service Evaluation of Cable-Stayed Bridges: Overview of Available Methods and Findings” is authored by Armin B. Mehrabi. This paper is based on a review of previous investigations conducted during the last decade for condition assessment of cable-stayed bridges. Results of these investigations have been used to develop a unified approach for in-service evaluation and problem solving in these aesthetic structures. The unified approach is based on modifications to traditional inspection methods. The result is that nondestructive techniques may be used for health monitoring of critical elements of cable-stayed bridges.

“Estimating Fatigue Life of Bridge Components Using Measured Strains” is authored by Sreenivas Alampalli and Ryan Lund. This paper presents the methodology, with an actual case study, to estimate the remaining fatigue life of bridge components using the measured strain data under actual vehicular traffic. They used the procedures outlined in the AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel Bridges to evaluate the bridge. Infrastructure owners often need the estimated remaining fatigue life of bridges to make appropriate, cost-effective decisions regarding repair, rehabilitation, and replacement of existing bridges. The paper provides a step-by-step procedure for conducting such tests and subsequent analysis, along with the limitations associated with this methodology.

“Assessment of Bridge Remaining Fatigue Life through Field Strain Measurement” is authored by Y. Edward Zhou. This paper outlines a step-by-step procedure for evaluating the remaining fatigue life of existing riveted bridges. It further confirms that field measurements often show that existing bridges have infinite remaining fatigue, even when the analytical methods indicate otherwise. Current AASHTO specifications for fatigue evaluation of existing bridges are reviewed, and case studies of three major highway bridges are discussed in the paper. This paper also concludes that the 1990 AASHTO fatigue guide specifications may produce conservative results when compared to the 2003 AASHTO LRFR guide manual.

“Identifying Effective and Ineffective Retrofits for Distortion Fatigue Cracking in Steel Bridges using Field Instrumentation” is authored by Robert J. Connor and John W. Fisher. This paper discusses retrofitting fatigue sensitive bridge details associated with out-of-plane distortion or other unanticipated secondary stresses, which are attributed to the majority of fatigue cracking in bridges built before the 1980s. Due to the lack of predictive models to evaluate distortion-related cracking behavior and assess the effectiveness of retrofits adopted, evaluation of these details through in-service field testing is often needed and recommended. This paper provides guidance on field instrumenting, with a case study that uses field testing to demonstrate the ineffectiveness of initial retrofit strategies.

“Development and Implementation of a Continuous Strain Monitoring System on a Multi-Girder Composite Steel Bridge” is authored by Santanu Chakraborty and John T. DeWolf. This paper describes the implementation and evaluation of a long-term strain monitoring system on a 3-span, multi-steel girder, composite bridge located on the interstate system. The remote monitoring system has been used to augment current bridge inspection and management strategies. This paper shows how the remote strain monitoring system has been used to gain a better understanding of the actual behavior of a bridge through measurement of direct bridge parameters, including the location of the neutral axes and the load distributions. These are then compared to the design parameters.

We believe these papers will contribute to the state-of-the-art for nondestructive bridge testing, evaluations, and health monitoring. The Editors thank the paper reviewers for their diligent comments. These have improved the quality and have ensured that the papers are useful to the bridge engineering community. The Editors also thank the ASCE staff and Bruce Peterson, Editor of the Journal of Bridge Engineering, for giving us the opportunity to develop this special issue and providing required assistance. Coordinating efforts provided by Linda Banks of the New York State Department of Transportation during the review process are greatly appreciated.