Risk-Based Inspection and Strength Evaluation of Suspension Bridge Main Cable Systems (2023)

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137 C H A P T E R 8 Conclusions, Workshop and Suggested Research Conclusions The test of any successful project is whether it delivered on its objectives. The two main objectives of this research were to: 1. Develop Guidelines for RBI and strength evaluation of suspension bridge main cable systems using probabilistic approaches. 2. Plan and conduct a workshop for suspension cable bridge owners to demonstrate the use of proposed guidelines. Due to circumstances outside the control of the research team, the workshop that was originally scheduled for Phase III of this project had to be postponed until the end of the project, due to the COVID-19 pandemic. The workshop was held on June 29, 2022 at the Keck Center of the National Academies in Washington, DC. The following items highlight the specific tasks that were included in the original Amplified Work Plan along with the resulting outcome: 1. “Our Research Approach is to look at two alternatives for meeting the stated objectives of this research project. The first alternative will be to review the methodology stated in NCHRP 534 and determine what needs to be done to address the perceived shortcomings and see what needs to be done to make it even better. The second alternative will be to evaluate a new method such as the Random Field theory outlined under Task 4.B.4.b of this proposal.” a. Based on the questionnaire responses and input from the research team, it was determined that many of the comments on NCHRP 534 focused on the application of the cable strength methodology, specifically on grouping of specimens and issues related to cracked wires. Therefore, it was determined that the research would focus on making improvements to the cable strength calculations (based on the Random Field theory), while retaining the field inspection procedures that the engineering industry was already accustomed to. 2. “The development of specific guidelines and research into possible monitoring and testing methodology to assess helical strand cables is a valuable endeavor that will be considered as part of this project. The RT will include a description of the general approach and some case studies in the Guidelines.” a. A description of the inspection methodology for helical strand cables has been included in the Guidelines under Article 2.2.5.2 based on experience from actual cable investigations performed by the research team. Due to the fact that it is not possible to extract wire samples from helical strand cables, the for cable strength

138 evaluation proposed in the Guidelines (based on the Random Field Method) is not applicable. 3. “A database of wire test data will be assembled and studied to obtain a more complete understanding of wire properties and degradation rates. Certain assumptions inherent to NCHRP Report 534, such as the percentage of cracked wires (which affects the number of sample wires), will be re-evaluated on the basis of a more statistically relevant number of structures.” a. One of the significant changes between the “weakest link” methodology in NCHRP Report 534 and the Random Field methodology in the proposed guidelines is the fact that the Random Field Method accounts for the variation in strength along the length of the wire within the panel. As a result, it is critical to know both the corrosion stage and the ultimate strength of each wire specimen in order to utilize this approach. Unfortunately, the existing database of wire test data provides only typical data from stress-strain tests; the corrosion stage rating of individual specimens was not required as part of NCHRP 534 and is therefore not available. As a result, most of the previous test data was not relevant for this study. Fortunately, there was one bridge, which had been analyzed using both NCHRP and the BTC method, for which this corrosion data was available. In addition, the research team performed two additional cable investigations during the course of this project (including a follow- up investigation of the first bridge) which provided two additional data sets for this study. 4. “Every bridge cable(s) is unique as to the cable length, diameter, number of cables and wires, their condition, maintenance and inspection history, propensity for corrosion and level of deterioration, types of corrosion, propensity for cracked wires, number of broken wires, etc. The inspection plan must take these and other considerations into account and develop a recommended inspection program that is specific to the bridge and its cable(s).” a. Given the variety of conditions mentioned above, combined with limited information about conditions inside the wrapped cable, it is necessary to bring forward experiential information (aka engineering judgement) to develop an inspection plan rather than relying solely on probabilistic methods. The procedures and recommendations contained in NCHRP 534 were compiled from experienced engineers and therefore were carried forward into the proposed Guidelines. 5. “The current practice of wire sampling should be revisited and the use of statistical methods to improve the current wire sampling protocol should be considered.” a. The number of wire samples was revisited and specific recommendations on the number of wire specimens (short sections cut from the wire samples that are used for tensile testing) has been provided, along with the statistical basis for the recommended number. 6. “Information from different owners on methods used for wire cutting and wire repair for the interior wires will be collected to come up with a recommendation on how to take samples from deep inside the cable.” a. No recommendations were obtained from the questionnaire. It was determined that, in general, it is not practical to extract samples from deep within the cable unless the wires are already broken (and it is not recommended to take broken wires as sample wires as they are known to contain pre-existing cracks and can artificially skew the wire tensile test results). 7. “Include inspection methodologies for anchorage strands. The strength calculation for the anchorage splay has specific issues that are not covered by NCHRP NCHRP Report 534.”

139 a. Inspection methods for anchorage strands have been included in Article 2.2.5.3. Strength calculations for anchorage strands have not been included however. 8. “The case of suspension bridges with four cables needs to be addressed.” a. A sentence was added under Article 2.2.4.1 to state that “When there are more than two cables on the bridge, the same inspection pattern described above should be applied to each pair of cables.” 9. “The [dehumidification] data gathered [from the Akashi Bridge], as well as any other examples found, will be integrated into the RBI process.” a. Dehumidification was included in the RBI process developed for determining the required frequency of main cable inspections. Unfortunately, the owners of the Akashi Bridge did not provide information about the long-term performance of their dehumidification system. This would have provided valuable data as it was one of the first uses of this technology and has been in place for the longest period of time. 10. “As an alternative to NCHRP NCHRP Report 534, another probabilistic methodology will be investigated to determine the required frequency of main cable inspections involving cable openings. As an alternative, a more complex reliability-based approach will be proposed to determine the first location where the main cable will be opened.” a. A risk-based methodology was developed for determining the required frequency of main cable inspections (as well as other components of the main cable system). 11. “The planned strategy [for the total number of wedges] will be worked out in the detailed study.” a. After a review of typical inspection practices gathered from the survey respondents, it was determined that using the standard procedure outlined in NCHRP 534, with eight wedge lines for the full length of the panel, provided acceptable results and no changes were needed. 12. “Results of such intermediate inspections will be taken into account in the proposed methodology.” a. The results of intermediate inspections are accounted for in the risk-based approach for determining inspection frequency. 13. “The RT will also consider appropriate ways to include findings from NDE/SHM technologies into the planning of future inspections.” a. The data obtained from NDE/SHM technologies (e.g. acoustic monitoring) are accounted for in the risk-based approach for determining inspection frequency. 14. “Our study will include evaluation of the splayed cable strands in the anchorages; a subject that is not covered in NCHRP NCHRP Report 534. Our team has performed in-depth inspection of strands on numerous suspension bridges and have adapted the NCHRP methodology for main cables to assess the strength of the strands of four cables.” a. Inspection methods for anchorage strands have been included in Article 2.2.5.3. Strength calculations for anchorage strands have not been included however. 15. “The Research Team will look at ways that information coming from nondestructive evaluation (NDE) techniques can be input into the proposed methodology for cable strength estimation as well as into the inspection program for both parallel wire cables and main cables comprised of helical cables.” a. A review of current commercially available NDE technologies reveals that they are not sufficiently developed to supplant the current practice of visual cable inspection through unwrapping and wedging. The data obtained from NDE/SHM technologies (e.g. acoustic monitoring) are accounted for in the risk-based approach for determining inspection frequency.

140 16. “In addition to all these NDE technologies, the research team will look at the positive effects of cable dehumidification.” a. Dehumidification was included in the RBI process developed for determining the required frequency of main cable inspections. 17. “The inspection forms contained in NCHRP NCHRP Report 534 as well as those currently in use by stakeholders will be evaluated to ensure they are compatible with the RBI requirements to be developed in this project.” a. After a review of typical inspection practices gathered from the survey respondents, it was determined that using the standard inspection forms included in NCHRP 534, were compatible with the RBI requirements and no changes were needed. 18. “Develop illustrative examples that can encompass a more realistic cross-section of conditions that may be found in future inspections. These examples will be used to test the newly developed methodology and compare its results with the results using NCHRP NCHRP Report 534 methodology. The detailed illustrative examples will demonstrate the full range of applicability of the proposed methodology.” a. Example calculations for both the RBI approach and the main cable strength evaluation are included in Appendix A and B of the Guidelines, respectively. Implementation Plan A ballot item will be developed, most likely a joint item between T-14 and T-18, for adoption of the guidelines as an official AASHTO document. Given the timing of the project, the ballot item will be progressed after the completion date of the NCHRP project. We highly recommend that once this project is finished, that each of those on the 70 bridge owners on the contact list for the questionnaire be officially notified of the availability of the new Guidelines. NCHRP-Sponsored Workshop An in-person workshop was held on June 29, 2022 at the Keck Center of the National Academies in Washington, DC. The 18 (18) invited participants included bridge owner representatives, AASHTO and FHWA representatives, NCHRP panel Chair and review members and research team members. A complete list of attendees and a workshop agenda can be found in Appendix B of this report. Workshop attendees received materials and instruction on the content of the proposed Guidelines through a series of interactive sessions. Throughout the workshop duration, attendees provided feedback to the research team on the developed Guidelines. This input was summarized and presented to the project panel at a meeting held August 1, 2022. Key points raised were: Helical Strands. There is a need for additional guidance on strength estimation of helical strand cables. However, due to limited data, no change to the number of corrosion stages for helical cables versus parallel wire cables should be made. Due to insufficient data, no changes to the calculation of strength from what is shown in the guidelines is recommended. The I-74 bridge project (decommissioning of two helical strand cable suspension bridges) might provide an opportunity to obtain valuable insight. Dehumidification system. Provide language in Guidelines for inspection, the effect of plant operation, and how to minimize increase in humidity during cable inspection. Request for modification of the attribute tables for RBI. These changes were included in the Guidelines. Historical Inspection Data. Modification of attribute tables to be able to use historical inspection data to adjust number of panels for inspection. However, no change in extrapolation to non-inspected panels, and no change in how to use historical inspection data for asset management was deemed

141 necessary. Add language in Guidelines to limit safety factor to critical value from all previous inspections and that historical inspection data is not a substitute for inspection of remaining panels. Fractographic Examination. Include it in Guidelines as a mean to evaluate failure mode, include text in Guidelines (including helical strands). Indicate in guidelines that it may still be relevant even if not used for strength calculations. RBI. There is need to include language in the Guidelines to clarify that a risk assessment panel (and specific tables) are needed for each bridge, as the Guidelines are just the starting point. Request to modify attribute tables to account for dehumidification. Fatigue. Provide clarification langue in the Guidelines regarding fatigue issues in cables. Also discuss sustained dead load effect on cracking. NCHRP Report 534. Provide clarification language on the validity of NCHRP Report 534 and the incremental nature and improvement of the proposed Guidelines coming from NCHRP Project 12-115. Additional topics include providing guidance on the demand side for calculation of FOS. The research team, with input from the NCHRP project panel, made changes to the Guidelines accordingly. Future Research As with any research project, there is inevitably more work that could be done to refine and build upon the information in this report. The following items are known areas where the research team feels that future research would be beneficial: 1. Due to limited available wire test data, the proposed Random Field Method has only been benchmarked using data from two bridges. A larger sample size is needed to truly test the effectiveness of the method. The probable error and confidence level of the results using this method could be better defined through more field experience. Further statistical work could be done to compare probable error between the Random Field the NCHRP Report 534 methodology. 2. The Random Field Method only considers the ultimate strength of the wires and does not explicitly take into consideration the ultimate strain data. Evidence suggests that there is a correlation between broken wires and sample wires with low ultimate strain. Perhaps due to strain compatibility, these are the wires that break first? This is not captured in the proposed method. 3. The list of attributes for main cable suspension systems for use in the RBI methodology could be more fully developed. 4. Example calculations could be provided for bridges with helical strand main cables. In addition, calculations for anchorage areas or other unique configurations (e.g. unsupported backstays, etc.) could also be provided. 5. Determine if there is a correlation between the percentage of Stage 4 wires in a panel and the number of cracked wires in that panel. Stage 4 wires and the number of broken wires in that panel. Cracked wires and the number of broken wires in that panel. 6. Further evaluate the methodology for the estimation of the number of broken wires. 7. Further evaluate the methodology for determining redevelopment length of broken wires. 8. Perform research on means to more objectively and reliably classify wire corrosion stage in the field, i.e. using cameras or other NDT methods rather than visual methods only. 9. Further investigate the use of an LRFD reliability index for suspension bridge main cables.

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