Institute for Transportation

About the research

Currently, the Minnesota Department of Transportation (MnDOT) is experiencing loose nut problems. To address this issue, MnDOT has funded a research project titled, “Re-tightening the Large Anchor Bolts of Support Structures for Signs and Luminaires,” which ended in June 2018. In this project, the researchers developed guidelines with tables and charts for tightening and re-tightening anchor bolts. MnDOT will implement these guidelines. The objective of this proposed project is to verify the effectiveness of the new tightening methods, and if necessary, make adjustments or modifications to the proposed specifications. This project will also address several issues that were found during the Phase I project.

The researchers will complete the following tasks for the Phase II project:

1. Interview MnDOT construction/maintenance crews and contractors to get their feedback on the new specifications.
2. Visit sites where bolts are installed with new and old specifications and inspect their performances.
3. Continue Phase I research to study the effect of snug tight, grip length, and other details.
4. Investigate the overstressed condition under snug tight stage for smaller anchor bolts from ¾”φ to 1¼”φ, mostly used for supports for small signs, light poles, and traffic signals. Develop the specifications specifically for snug tight procedures and verifications in stress level for the smaller anchor bolts without damaging or yielding them.
5. Continue Phase I research to provide a practical method to tighten bolts with a yield strength of 105 ksi. Generate a guideline or specification specifically for MnDOT Spec 3385, Type C anchor bolt tightening procedures.
6. Continue the fatigue test from Phase I to study the effect of different stress ranges.
7. Continue to monitor the sign structure at Snelling and TH 36. The researchers started the monitoring in August 2017 and have collected valuable data. Continuous monitoring will provide long-term data that will benefit not only this project, but also others related to wind load.
8. Finalize the specifications for tightening procedures and the bolt verification form to allow on-site verification of proper installation of bolts by MnDOT construction/maintenance crews and contractors.
9. Funded by an NCHRP IDEA project, the researchers at Iowa State University have developed a capacitance-based smart installation and monitoring system. The overhead sign structure at Snelling Ave. and T.H.36, which is currently equipped with monitoring system for Phase I research project, can be used to implement this new technology for future applications.