Fatigue Loading and Design Methodology for High-Mast Lighting Towers (2012)

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21.1 Problem Statement Although not often recognized as significant structures by the motoring public, ancillary highway structures, such as high-mast lighting towers (HMLTs), are important components of an efficient highway system. During extreme events, HMLTs greatly assist in ensuring safe travel for the public, which makes the robust performance of these structures over the long term critical for the smooth operation of traffic. Also, considering most HMLTs are located directly adjacent to high-volume highways, the impact a collapse presents to the safety of motorists is clear. Unfortunately, recent failures of these structures have raised questions as to the robustness and safety of the existing HMLT inventory. In the majority of cases, failure has been the result of load-induced fatigue. Fatigue cracking and total collapse of HMLTs has occurred. The cracking observed is generally found at the baseplate-to-column weld, handhole detail, and anchor rods, although cracking of the anchor rods was found to be less prevalent. Field observations and forensic studies of the failed HMLTs have indicated wind-induced vibration as a significant factor in the fatigue life of these structures. Natural wind can produce significant stress cycles due to dynamic loads from wind gusts. More complex aeroelastic phenomena such as vortex shedding also are known to produce fatigue damage. Since neither the magnitude nor the frequency of application of the loads that induce fatigue damage in HMLTs was clearly understood, NCHRP Project 10-74 was initiated to define a fatigue loading for the safe design of these structures. 1.2 Objectives The research program described herein was directed toward developing realistic and practical load models to be used for the fatigue design of HMLTs. Long-term field monitoring was performed to evaluate the magnitude and frequency of the loads experienced by these structures, as well as in-service response. Additionally, wind tunnel and dynamic field tests were combined to better understand the aerodynamic characteristics inherent to various HMLT sizes and shapes. Combining the magnitude and frequency of the loads with the associated aerodynamic properties for a given pole was then used to develop a realistic and rational load model for the fatigue design of HMLTs. This report presents results from field, laboratory, and analytical studies that have been used to develop rational criteria to be used during the fatigue design of HMLTs. Additionally, to ensure the fatigue loading is properly implemented into practice, a design methodology and specification for inclusion into the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals with associated commentary also was developed. C h a p t e r 1 Background

Background 3 In summary, the primary objectives of NCHRP Project 10-74 were to use data collected through field testing, laboratory wind tunnel testing, and analytical studies to 1. Develop loading and analysis criteria for use in the fatigue design of HMLTs. 2. Develop a design methodology and specifications with associated commentary for HMLTs. 3. Prepare recommended revisions to the existing AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals. The research team included Robert J. Connor from the School of Civil Engineering at Purdue University and Steven H. Collicott from the School of Aeronautics and Astronautics at Purdue University. Purdue University is the primary contractor with Dr. Connor as the prin- cipal investigator responsible for the project and Dr. Collicott as the co-principal investigator accountable for the wind tunnel testing portion of the study. Other members of the research team included Allen M. DeSchepper, research assistant at the School of Civil Engineering at Purdue University, Ryan J. Sherman, research engineer at the Bowen Laboratory for Large-Scale Civil Engineering Research at Purdue University, and Jaime A. Ocampo, research assistant at the School of Aeronautics and Astronautics at Purdue University. 1.3 Notes and Nomenclature Throughout this report, “AASHTO Signs” is used to refer to the Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals (2009). Where the bridge code is referenced, it is referred to as “AASHTO LRFD Bridge Design Specifications,” or simply “AASHTO Bridge Specifications,” or “AASHTO LRFD.” Furthermore, the writing in this report is generally consistent with the terms and nomenclature used in the AASHTO Standard Specifi- cations for Structural Supports for Highway Signs, Luminaires and Traffic Signals. For example, the term constant-amplitude fatigue limit (CAFL) is used instead of constant-amplitude fatigue threshold (CAFT), which is used in the AASHTO Bridge Specifications. In addition, “HMLT” is used to refer to the supporting structure and luminaire collectively. The tapered, tubular, steel supporting structure is generally called the pole.