Nebraska Local Technical Assistance Program
Nebraska Department of Transportation: Research Reports
Date of this Version
9-2024
Document Type
Article
Citation
Al Maabreh, A., Awawdeh, A., Maguire, M., Puckett, J., Kreiling, B. (2024). "High-Mast Tower Foundation – Phase II" NDOT Research Report SPR-FY23(013).
Abstract
Challenge: High-mast luminaire supports used in the US transportation sector are typically steel poles that are attached to a baseplate via a butt or socket weld. Many have failed primarily due to fatigue cracking at the weld toe. Significant research in the US improved the fatigue performance of these structures; however, failures still occur due to high-cycle cracking at the weld toes, or at other fatigue-prone details. Additionally, poles are failing in moderate winds that create galloping. Galloping is an aerodynamic phenomenon where the wind excitation frequency matches the pole’s natural frequency creating resonance. The top amplitudes are many times the pole tip diameter, e.g., 30 ft one way due to the inherently low damping. These movements and associated strains create low-amplitude fatigue (repeated yielding) typically causing the pole to fall.
Alternative design: An alternative to the traditional design is to directly embed the pole into a foundation shaft and backfill with concrete or gravel. This eliminates the fatigue-prone details associated with baseplates, welds, bolts, anchorages, and rebars. This process is routinely used in the electrical utility, telecom, and sports-lighting sectors. It can potentially remove the fatigue issue, provide a cheaper fabrication and construction method, and increase damping.
Research: This research was conducted in two phases: Phase I reviewed the literature, specifications, best practices, and construction methods. Phase II used this work to design, construct, and test four poles to near failure. Phase II work is the focus of this report. Four pole foundations were designed: two with concrete and two with gravel backfill. Two foundation depths of 12 and 16 ft were used, and the pole diameters were 36 in. The boring log was used to support LPILE (Ensoft Inc., 2022) and other analyses to check these designs and predict the performance. The AASHTO 700-year mean recurrence interval (MRI) wind load was applied to NDOT’s new standard 80-ft luminaire pole, and these reactions were used as the basis for this research.
Dynamic pluck tests were conducted in one direction followed by three static pulls near the 3000-year design load. Damping and hysteretic behavior were observed. Next, forced vibration was used to excite the pole over a range of frequencies, including the pole’s first three modes of vibration. The frequency response curves were developed. The load direction was then moved to the opposite direction, and all tests were repeated.
Observations: The groundline translations were within an inch in all cases. The hysteretic behavior was stable after some initial translations. As expected, the gravel foundations had more movement than the concrete, and the 12-ft foundation moved more than the 16-ft foundation. The downhole translations compared with the LPILE predictions. The 12-ft translation near the bottom indicates translation in the opposite direction of the pull and the rotation at that level was non-zero. The 16-ft foundation had a smaller translation near the bottom, and the rotation was smaller, indicating that the p-y behavior had become small. All these designs were reasonably aggressive, especially the 12-Concrete. The goal was to use a foundation small enough to obtain meaningful, and large enough data for analysis to support design.
Damping is essential to mitigate large-amplitude events. The wind speed whereby galloping will lock-in is a function of damping. This critical velocity is directly proportional to the damping ratio. Therefore, the inherent damping associated with direct embedment can drive the critical lock-in velocity to longer MRIs, significantly eliminating these troublesome events.
Summary: These tests demonstrated the application of direct embedment for high-mast towers used in the transportation sector. The tested poles behaved well even under extreme loads much larger than the 700-MRI design wind. The design requires a p-y analysis for the soil conditions to estimate the groundline translations. Acceptable (codified) translations have not yet been set and this research will help to guide those judgments. To set standards, NDOT could develop conservative shaft details for soil profiles typically located in this state. Although the designs in this research were shallow by design, increasing the depth to provide a more conservative shaft could provide yet more safety at a small marginal cost. Finally, these foundations are especially ductile, tough, and resilient as large displacements that might create voids can be readily backfilled.