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

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

70 4.1 Conclusions The findings presented in this report are the basis for the proposed revisions to the fatigue design provisions of the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. The results of the experimental investigation into wind loading, dynamic response, and fatigue analysis of HMLTs are summarized in the following conclusions: 1. The 0.5 percent damping ratio given in Section 11.7.2 of the current edition of AASHTO Signs (2009) is applicable for Mode I vibration of HMLTs within a reasonable statistical confidence; however, it will yield unconservative results if used to evaluate higher modes. It is known that vortex-induced vibration of HMLTs occurs in the second and third modes. Any effort to compute loads associated with vortex shedding should adjust the damping ratios accordingly. (It is noted that in the proposed specifications, such information is no longer required.) Recommended damping ratios are based on the 80 percent confidence limit in Table 102: 0.75 percent, 0.3 percent, and 0.1 percent for Modes I, II, and III, respectively. 2. Increases in the mean wind speed typically result in an increase in the number of damaging load cycles. The fatigue-limit-state pressure range also tends to increase since the magnitude, or scale, of the stress-range distribution changes with increasing cycles. However, the shape of the distribution remains essentially the same, and the resulting effective pressure range for fatigue design (including buffeting, vortex shedding, and associated dynamic response) does not change. 3. Mitigation of vortex-induced vibration mainly affects the accumulation of load cycles. For the double-wrap rope strake tested, results show a significant decrease in the number of stress cycles accumulated on a per day basis, while the corresponding constant-amplitude effective fatigue stress-range and fatigue-limit-state stress-range are essentially unaffected. 4. Infinite life design is appropriate for HMLTs. The number of lifetime loading cycles exceeds the limiting number of cycles at the constant-amplitude fatigue limit for the most common HMLT fatigue detail categories (e.g., Category E) in poles designed to the earlier versions of AASHTO Signs. 5. Static pressure-range values were developed and recommended for fatigue design of HMLTs, and account for both geographic variation in yearly mean wind velocity and variation in experimental data. The proposed static pressure-range values are 5.8, 6.5, and 7.2 psf corresponding to yearly mean wind velocities of 9, 11, and 13 mph, respectively. 6. Static pressure-range values were developed and are recommended for fatigue evaluation of HMLTs. The pressure range used to determine if an HMLT is capable of attaining infinite life is the fatigue-limit-state pressure range of 5.8 psf. The pressure range used to determine the finite life for a given HMLT can be reasonably represented by a constant-amplitude effective pressure range of 1.3 psf. Both values are based on the average yearly mean wind velocity of C h a p t e r 4 Conclusions and Recommendations

Conclusions and recommendations 71 9 mph in the United States. Data showed that greater measured mean wind speeds do not significantly influence the corresponding constant-amplitude effective pressure range. 7. Stress range cycle frequencies are recommended for fatigue evaluation of HMLTs. The cycle frequency values are 9,500, 15,000, and 23,000 cycles per day corresponding to yearly mean wind velocities of 8, 10, and 12 mph, respectively. A cycle frequency of 7,000 cycles per day is recommended for HMLTs mitigated against vortex shedding. 8. The coherence of the “lock-in” phenomenon is largely due to the configuration of the pole. Although a face-upwind configuration may or may not lock in, the vertex upwind configuration undoubtedly does for every type of cross section. This means that poles with a vertex toward the prevailing wind are more prone to lock in and exhibit the phenomenon due to the location where the wind separates. Of interest, the phenomenon also seemed to be more prevalent on the 12-sided model. Lock-in happened not only for a much tighter data frequency spread but also for a greater range of diameters. This seems counterintuitive since the 16-sided model is closer to a cylinder and the researcher expected it would exhibit a more uniform response. So far, lock-in has been confirmed up to 18 inches on a 5-foot model or about 25 percent of the wind tunnel model’s diameter. To make sure this isn’t just the whole model moving, there are also tests where only a bandwidth of 9 span-wise inches are locked in. Further study may prove this to be more than 18 inches as the static mount was not tested before the completion of this report. 4.2 Suggested Research Based on the results of this study, the following topics are suggested for future research: 1. Wind tunnel testing of HMLT luminaires—Conduct wind tunnel tests of luminaire assemblies commonly used by industry to determine reasonable values of effective projected area (EPA) and/or drag coefficients to be used for design. Within the design community, little is known about the aerodynamic properties of luminaires, particularly the interaction of individual lighting elements with their adjacent supporting system. 2. Investigate the effectiveness of commonly used mitigation devices—Mitigation can be an effective means for reducing vortex-induced vibration and increasing the fatigue life of HMLTs. Many types of devices exist that may either disrupt the formation of vortices or provide additional damping. However, at present their effectiveness and impact on the design of new poles is not well understood. 3. Investigate the phenomena responsible for large-amplitude oscillations—Evidence exists that shows that HMLTs can experience large dynamic oscillations at moderate wind speeds that may significantly reduce fatigue life. These events appear to be very rare, exceeding the 1:10,000 probability, but may be responsible for low-cycle fatigue behavior leading to collapse or significant cracking.