Authored by Ghyslaine McClure*
Abstract
This paper presents the dynamic characteristics of 20 steel telecommunications monopoles as extracted from ambient vibration measurements made directly on the base and on the structure itself at low height. Velocities are measured during a minimum of 30 minutes at each site with highly sensitive micrometers, synchronously at the different positions. The records are analysed using a frequency domain decomposition algorithm available in commercial software. Statistical analysis of the extracted results indicate that the fundamental natural frequencies could be identified with high precision (standard deviation less than 4%) while the corresponding total equivalent damping was less precise, with standard deviations between 6 and 28%. The low-cost, non-intrusive AVM tests are deemed appropriate to evaluate the vulnerability of telecommunications monopoles to wind-induced instabilities.
Characteristics of HSLA Steel
This experimental study is motivated by the need for telecommunications monopoles designers to assess the vulnerability of tall and smooth structures to wind-induced instabilities such as vortex shedding and flutter. Recent editions of North American telecommunication structures design codes, such as CSA S37-18 [1] in Canada and ANSI/TIA-222-H [2] in the US have provided guidelines to address fatigue in poles (monopoles and tripoles, in particular) that preclude the knowledge of their modal damping in the fundamental mode. Also, the evaluation of the critical wind velocity for vortex shedding requires the knowledge of the fundamental or first few lower natural frequencies of the structures. These dynamic properties are difficult to predict by calculations and in situ ambient vibration measurements (AVM) provide the ideal way to determine them while taking into account the effects of the foundation, antennas and all attached appurtenances.
Tested Structures and Protocol
With the collaboration of the owners and their engineering consultant, it was possible to conduct AVM tests at 20 cellular telecommunications monopoles sites located in Québec, Canada, between 10 May and 12 July 2019. Site No. 16 is shown as an example in Figure 1A. The structures vary in height from 21,3 m to 50,9 m. Most of them are made of smooth circular tubes while a few are multi-sided. Although their detailed geometric/mass characteristics cannot be disclosed, their dimensions varied from approximately 600-1800 mm width and 6-12 mm thickness at the base and 400-1300 mm width and 5-13 mm thickness at the top. Three types of joints are used: bolted, welded and section overlaps. The sensors are Tromino 3G ENGY micrometers [3] that measure the velocity of the structure to an accuracy of 10-6 m/s (μm/s). Sites Nos. 1 to 11 have used two sensors, one placed on the concrete base and another attached to the structure using a kevlar sling system. Sites Nos. 12 to 20 have used a second sensor on the structure using a magnet support system, as shown in Figure 1B. In all most cases two 30-minute long records were collected in three orthogonal directions; only one test was performed at the last tested sites as the preliminary analysis of the results indicated good coherence and accuracy. The measurements of the sensors were all synchronized using a wireless antenna. The wind speed at the site was measured using an EXTECH Mini Thermo-Anemometer 45158, at a height of approximately 2 m above ground. In some cases, the base of the monopole was sheltered from wind or the wind speed was too low to be measured (Figure 1A,1B).
Operational Modal Analysis
Once the in-situ data acquisition is completed, the recorded 30-min velocity time histories are transferred from the individual TROMINO sensors to a personal computer where they can be analysed by the ARTeMIS Modal Standard Software [4]. This software allows the identification of the modal frequencies of the structure using the Enhanced Frequency Domain Decomposition (EFDD) Method as well as the approximate global damping corresponding to each identified mode. An example of ARTeMIS data extraction window is shown in Figure 2 for a single record at Site No. 16 (Figure 2).
The EFDD method uses a peak-picking algorithm in the frequency domain to select resonant peaks. The peaks selected are not single values yet, but a band of the width of the peak. This band is identified with the modal assurance criterion (MAC), which poses a boundary of acceptance of the peaks. The MAC value varies between zero, representing a non-consistent mode shape, and one, an entirely consistent one. [5] A default MAC value of 0.9 was used for this study, which yielded consistent results. This MAC value ensures that the automatically identified peak is one of real resonance. A power spectral density function is then defined around the identified peak and used to obtain the frequency, through a trip back to the time domain, to obtain the zero crossing times. The damping ratio estimates are obtained through logarithmic decrement of the correlation time functions corresponding to each natural frequency identified. [6] The resulting fundamental frequencies and equivalent damping ratios are the two indicators that are the focus of this study. The EFDD method was used a number of times in different software, showing its high reliability for frequency extraction. [7] However, damping ratio estimates have larger uncertainty, in the order of 20%, and better estimates are found for longer measurement samples. [8] The uncertainty in the damping estimation method is mainly attributed to the nonproportionality of damping observed in real structures, as opposed to the classical logarithmic decrement assumption associated with the ideal linear viscous damping model.
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