State of The Practice and Art for Structural Health Monitoring of Bridge Substructures

CHAPTER 5. SUMMARY AND CONCLUSIONS

This project was originally intended to show the merits of SSHM via a review of the few well-documented cases where a concerted effort was in place to assess the long-term performance of foundations. While these efforts were underway, the I-35W bridge over the Mississippi River in Minneapolis, MN, collapsed in the middle of rush hour, killing 13 people and revealing to engineers the United States’ failing infrastructure. As a result, the project was redirected to aid MnDOT and FHWA in providing an effective yet economical means to monitor the new substructure during construction and for the future. This was possible largely due to the preparedness afforded to the research team as a result of the ongoing study. Therein, DAS units being tested on other sites could be redeployed immediately to obtain data for this fast-paced design-build bridge replacement project.

Two sites served as the primary proving grounds for the study: (1) the voided shaft test site in Clearwater, FL, and (2) the bridge replacement site in Minneapolis, MN. In both cases, data were obtained from below the ground surface from embedded instrumentation and used both to assess the health and performance of the elements and to review the capabilities of low-cost DAS. In that regard, hundreds of vendors provided DAS units of varied performance and economy, but this study chose to assess companies’ units to a large degree based on the cost. The ability to obtain data, upload remotely to a host server, and make spontaneous changes to the system configuration without a site visit were explored to the fullest. With very few exceptions, the systems performed well with an approximate cost of $160 per channel sampled for site 1 (Florida) and $170 per channel for site 2 (Minnesota). These prices included the loggers, cellular modems, enclosures, and power supply systems but did not include the cellular service contracts which were generally annual or biannual agreements. Embedded instrumentation varied and was generally more for site 2 based on the type of sensor.

A large amount of data was collected from site 2 and conceivably continues to be gathered (although presently unknown at the time of reporting). These data can be found in the attached appendix for completeness (archival purposes). Due to its electronic nature, it is readily usable for future analyses. Much of the analysis of these data is presented in chapter 4, but there are unanswered performance questions that remain. A full year of data collection is recommended to assess the substructure performance at the very minimum. This is presumably the course of action presently underway by MnDOT. However, multiple years and extreme weather events are likely to prevail that need to be caught by the DAS and used to alert transportation officials of possible changes in the substructural conditions.