Engineering Structures 80 (2014) 418–425

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Engineering Structures

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High accuracy measurement of deflections of an electricity transmissionline tower

http://dx.doi.org/10.1016/j.engstruct.2014.09.0070141-0296/� 2014 Elsevier Ltd. All rights reserved.

Abbreviations: RTS, Robotic Total Station; GPS, Global Positioning System.⇑ Corresponding author. Tel.: +30 2610997877.

E-mail address: stiros@upatras.gr (S. Stiros).

Fanis Moschas, Stathis Stiros ⇑Laboratory of Geodesy and Geodetic Applications, Department of Civil Engineering, University of Patras, Rion Patras 26500, Greece

a r t i c l e i n f o

Article history:Received 8 May 2014Revised 23 August 2014Accepted 2 September 2014

Keywords:Wind gust loadingCable-induced vibrationsVortex sheddingGeodetic monitoring

a b s t r a c t

The displacements of the top of an H-type, 30 m-high lattice tower of a 150 kV electricity transmissionline have been measured using a Robotic Total Station (RTS). Horizontal displacements approximatelyup to 30 mm and vertical up to 8 mm have been measured on a passive reflector set on the tower topduring days with moderate wind. Measurements are reliable and above the noise level which is deter-mined from measurements in a second reflector near the stable base of the tower. Displacements inthe cross-wind direction were found larger than along the wind. Such measurements, probably the firstto be made in a pylon, may be used to constrain models of their dynamics controlled by a very large num-ber of unknown parameters.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Measurement of deflections of various engineering structuressubject to various loads, especially dynamic loads, has been adream of structural engineers for centuries, at least since the firstmeasurements of the deflections of Eiffel Tower in Paris in the1880s [1]. Such measurements, however, became possible onlyduring the last few decades with the advent of new geodeticinstruments, GPS [2,3] and Robotic Total Stations (RTS, sometimesreferred to as Robotic Theodolites) [4–6], as well as instrumentssuch as tiltmeters [7], radar or microwave interferometers [8–10].The main advantage of these instruments is that they allowthe extraction of time series of deflections of measuring pointsrelative to a stable (global) coordinate system above the thresholdof a few millimeters to a few centimeters for relatively rapidoscillations (i.e. with a frequency lower than 5 Hz [11,12,6,13,14].A second advantage of these instruments is that they can recordquasi-static movements, including very long-period motions(<0.5 Hz; see [15]) which may be generated by wind loading[16], and which cannot be recorded by accelerometers.

The present paper is probably the first case of accurate mea-surements of dynamic deflections of the top of a lattice tower ofa 150 kV electric transmission line using an RTS set on the groundat a distance of up to a few tens of meters from the tower (Fig. 1).

These measurements are very unusual, and only rare reports ofmeasurements of the response of existing towers in terms of accel-erations exist (for example see [17,18]).This is mainly due to thefact that transmission lines are always on duty, and it is not easyto fix various types of electronic devices on them, while high elec-tric and magnetic fields make measurements by various electronicsensors rather impossible. For this reason the dynamic characteris-tics of electricity transmission towers and of transmission lines areeither predicted or estimated from models in wind-tunnel experi-ments [19–21].

The present study became possible because there was given theopportunity of a very rare service interval of the specific transmis-sion line (once per ten years) which made possible to fix on theupper part of the tower a small optical reflector. This was a typeof a prismatic (optical, passive) reflector which permits a very nar-row angle coded signal emitted by the RTS falling onto the reflectorto be passively reflected back to the RTS practically without loss ofenergy. The reflected signal is not sensitive to the electric and mag-netic field around the cables, while the RTS is equipped with a tar-get identification device and a servomechanism which permits toanalyze the received signal, track the movements of the reflectorand record its instantaneous coordinates in a pre-defined coordi-nate system (global system, independent of the study structure).

RTS has been successfully used in the past for monitoring var-ious engineering structures with different measurement rates,low rates for slow deforming structures such as buildings abovetunnels [22], and high rates for industrial chimneys [23] and evenlong- and short-span bridges excited by passing cars or

Fig. 1. An RTS roughly below the transmission line sighting to a reflector fixed onthe upper part of the tower, marked by an arrow. A second arrow points to a secondreflector (lower reflector, near the base of the tower), used to control themeasurement noise.

F. Moschas, S. Stiros / Engineering Structures 80 (2014) 418–425 419

pedestrians using RTS with sampling rates of the order of 5–7 Hz[6,14,24].

The aims of the present study are:

(a) to report for the first time directly measured displacementsof the top of a lattice tower under wind loading by moderatewind for the studied area,

(b) to study the measured displacements in combination withthe measured wind characteristics in the area and to com-pare the results with existing analytical studies,

(c) to extract possible periodic characteristics (oscillation fre-quencies) of the tower response,

(d) to evaluate the performance of the Robotic Total Station forthe measurement of the 3-D deflections of a transmissionline tower.

2. Characteristics of the tower, of the transmission line and ofthe terrain

The measured lattice tower is part of a 150 kV transmission-linecrossing the Campus of the University of Patras, Greece, in a north-eastern direction (azimuth approximately 50�). The tower isapproximately 27 m high, and is founded on a 10 m high hill. Thetower top is located at an absolute elevation of 134 m above sealevel. The studied tower is a truss structure made of L-shapedrod elements connected together with bolted joints. Five transmis-sion-line cables are connected on it. Three transmission cables areconnected via �2 m long insulators hanging from the tower, whiletwo lighting protection cables are connected on the top of thetower (Fig. 1). This type of tower is usually referred to as H-type.

The location of the tower is characterized by smooth reliefand moderate wind. Since the tower is located in the Universityof Patras Campus it is characterized by a terrain category III

(Area with regular cover of vegetation or buildings or with isolatedobstacles with separations of maximum 20 obstacle heights, suchas villages, suburban terrain, permanent forest, see paragraph 4.2in [25]) corresponding to a roughness length factor z0 equal to0.3 m according to [25] (Eurocode 1 Part 1–4; Wind actions onstructures).

3. Methodology

The intention of the study was to measure the instantaneouschanges of the position of a reflector (and hence the oscillations ofa selected point of the upper part of the tower) during periods ofstrong wind in three axes, along cable, across cable and verticalusing an RTS set on stable ground and the passive reflector installedat the top of the tower. The RTS was setup on a tripod, on a pointrather protected from the wind and from oscillations introducingnoise to the pylon measurements. In addition, the instrument usedin the present study is equipped with an automatic built-in controlsystem aiming to interrupt the measurements if the instrument issubject to oscillations or tilting above a certain limit. During the fieldmeasurements the same instrument was used, installed on the sameposition, while during each session of measurements the winddirection remained practically constant (Table 1).

Field and experimental evidence [26,6,14] indicates that RTScan record oscillations with amplitude above a few mm and withfrequency below 4 Hz, but its level of performance deterioratesin the case of atmospheric turbulence [6,14]. In order to confirmthat results are reliable, a second reflector was fixed at a lowerpoint of the tower not expected to oscillate during measurements(for details see paragraph 4). Since no significant oscillations of thesecond prism at the bottom the tower were expected, these mea-surements provide an estimate of the measurement noise (seeSection 4 for details). Because the RTS can track only one point atthe time, the overall strategy was to measure alternatively sessionsof about 30 min long on the upper and lower reflectors, permittingto control the noise in measurements, as well as possible drifts ofthe instrument. This procedure permits to identify statistically sig-nificant (i.e. above the noise level) displacements at the top of thetower. Spectral analysis of the measured deflections in each axis isexpected to permit to extract the dominant frequencies of deflec-tions and constrain certain dynamic characteristics of the pylonin the framework of the transmission line. A last problem is thatthe high sampling rate of RTS is not stable, and this forces to adopta special spectral analysis methodology. Details on the spectralanalysis procedure are given in Section 5.3.

4. Field measurements

A few hours interval of interruption of the operation of thetransmission line due to maintenance and development works,gave the opportunity to install close to the top of the study towera high quality AGA super-type reflector (see Fig. 1 and paragraph1). Measurements were made using a Leica 1201 Robotic Total Sta-tion (RTS), each time temporarily installed approximately 60 mfrom the tower. The RTS permits to obtain 3-D instantaneous coor-dinates of the reflector system with a nominal sampling rate of10 Hz. Since the instrument experiences sampling rate instabilities,the real mean sampling rate was around 6–7 Hz and measuredcoordinates are not equally spaced in time (for details see [6,26]).The measured coordinates reflect oscillations of the specific partof the tower because of wind excitation, plus some measurementnoise. The reflector coordinates are obtained in a Cartesian Systemwith two horizontal axes, one parallel and one perpendicular to theline of sight of the RTS, and one vertical axis. During the measure-ments analyzed in the present study, the RTS was installed right

Table 1Details and meteorological information for the two surveys of the studied tower.

Event Date Max wind speed (m/s) Wind direction Temperature (�C) Measurements

A 10th March 2010 �14 E 11.7 Reflector only on tower topB/C 30th January 2012 �15 NNE 7.5 Reflector on tower top and base

Fig. 2. Schematic representation of the location of the measured tower relatively tothe meteorological station from which wind speed data were obtained. Heights ofthe meteorological station and tower base and top are indicated in absolute scale(from mean sea level) and relatively to the ground level (scale on the left side of thefigure).

420 F. Moschas, S. Stiros / Engineering Structures 80 (2014) 418–425

below the transmission line (Fig. 1) with the line of sight parallel tothe transmission cables. As a result the reflector coordinates wereobtained in an axis parallel to the transmission cables (along cabledirection), an axis perpendicular to the transmission cables and avertical axis.

The typical accuracy of the RTS used is 3 mm ± 2 ppm for dis-tance measurements and 1’’ for both horizontal and vertical anglemeasurements, as specified by the instrument manufacturer. Theoverall accuracy of coordinate changes is however defined inexperiments in which the movement of the target (prism) isknown independently, as the zone of apparent displacements ofa non-moving point (see Section 5.1) [26,27] This last approachwas adopted in the second survey analyzed below.

A large number of measurements during different environmen-tal conditions were carried out between 2006 and 2013, followingpractically the same methodology. The events characterized by thestrongest wind, analyzed in the present paper, occurred in 2010and 2012. During surveys carried out after 2011 a second reflectorwas fixed at a level of 2 m above the base of the tower, in a pointfor which no wind-induced deflections were expected. The reflec-tor at the bottom point was stable, hence measured apparent dis-placements reflect only noise. The amplitude of this noise isexpected to be higher than the noise at the top of the tower dueto atmospheric turbulence along a ray-path close to the ground.As a result measurements from the bottom reflector provide anestimate of the accuracy of the noise level at the top, thoughoverestimated.

Measurements of the tower displacements were made in inter-vals characterized by wind and even of no wind, during varioustimes in the day. Clearly, tempests are characterized by atmo-spheric turbulence (instability and inhomogeneity of the atmo-sphere along the RTS ray-path) which may affect the quality ofoptical measurements; for this reason the focus of the presentstudy was placed on periods with moderate wind and a clearatmosphere.

The present study is focusing on two representative surveys,each about 30 min long, on 10 March 2010 (event A) and on 30 Jan-uary 2012 (event B). During the 30th January 2012 survey a secondcontrol event (event C) was analyzed. During event C the reflectorwas installed near the base of the tower and the measurementsgave the opportunity to assess the noise levels of the RTS measure-ments as explained in Section 3. Information about the wind speedand direction was obtained from the archives of a meteorologicalstation installed at the roof of the Civil Engineering Department,University of Patras building (http://www.hydrocrites.upatras.gr/WindSpeed.aspx). The station is located 560 m away from thetower at an elevation of approximately 82 m (see Fig. 2) and itssampling rate is 1 sample per minute. As a result, wind measure-ments provide an estimation of the wind characteristics at an ele-vation approximately 50 m slightly lower than those expected atthe top of the tower in a small horizontal distance from it. Becauseof the relatively low sampling rate, certain wind gusts may nothave been recorded. During the measurements of the tower oscil-lations a wind speed of 14–15 m/s was recorded at the meteorolog-ical station. Wind characteristics during the time of eachoscillation event are summarized in Table 1.

A rough estimation of the wind speed at the top of the tower(elevation difference of 70 m from the ground) can be made using

the wind speed measurement from the neighboring meteorologicalstation and Eq. (1) [28] supposing a power-law distribution of thewind speed profile.

mmr¼

ln zz0

� �

ln zrz0

� � ð1Þ

where:vr the reference wind speed and the zr the reference height(wind speed and height at the meteorological station),v and z are the wind speed and relative height at the tower top,z0 the roughness length factor defined in paragraph 2.

Using Eq. (1) and replacing the wind speed recorded at themeteorological station, the height of the meteorological stationand the height of the tower top, a wind speed of approximately19 m/s is calculated for the tower top. This is probably a lowerbound estimate because of effect of the relief at the base of thetower (the tower is based on an approximately 46 m-high hill)[29,25], but this estimate seems not to differ significantly fromthe real wind speed value.

5. Analysis and results

Measurements of each survey were recorded in a local Cartesiansystem with one axis aligned with the direction of the transmissionline (along cable direction), an axis aligned with the directiontransverse to the transmission line (cross cable direction) and avertical axis (see Section 4, paragraph 1). In the following sectionsthe maximum deflections of the tower top under strong wind andthe frequency content of the displacement signal are extractedfrom the RTS measurements. Furthermore the noise level of themeasurements is also assessed.

Fig. 3. Time series of apparent deflections of the reflector at the base of the towerduring event C (control event). Reflector is not moving, and apparent deflectionsindicate measurement noise, their max amplitude indicates the maximum width ofthe zone of noise, marked by dashed lines. Time series corresponding to the towerbase are relatively small, thus they are plotted in a smaller area compared to timeseries corresponding to the moving tower top (Figs 4 and 5).

Fig. 4. Time series of deflections of the reflector at the top of the pylon forapproximately half an hour of observations during event A. Measurements in gray(around seconds 470, 1260 and 1640) indicate high amplitude displacementsmarked by a gray ellipse in Fig. 6. A gap in the measurements betweenapproximately seconds 500 and 600 is due to power loss of the RTS. Spectralanalysis was carried out on the time-series on the right of the vertical line (aftersecond 620) in order to identify the dominant frequencies of the tower deflections.

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5.1. Evaluation of measurement accuracy

The accuracy of instantaneous RTS measurements in conditionsnot very different from those reported in this article has beendefined from zones ±2 mm wide in from apparent displacementsof stable (non-moving) reflectors, but this value may somewhatfluctuate depending on the environmental conditions during mea-surements [6,14].

In the present study, the measurement accuracy was in additionassessed from the recordings collected from the stable reflectorinstalled at the bottom of the tower. Apparent displacements atthe lower reflector during event C (control event), are shown inFig. 3, and were collected during an interval of approximately20 min just before the measurements to the reflector at the topof the tower (event B), under similar environmental conditions.

The minimum and maximum apparent displacement measuredfor the bottom reflector (see Table 2 for displacement values), forma zone of 6 mm, 4 mm and 8 mm for the cross cable, along cableand vertical axes, respectively. These zones are indicated in thegraphs of Figs. 3 and 5. The noise zones defined above, overesti-mate measurement noise because of near-ground atmosphere per-turbations close to the lower reflector and provide an upper boundof the uncertainty of measured displacements (see above).

5.2. Displacements of the tower top

The displacement time series of the tower top during events Aand B are presented in Figs. 4 and 5, respectively. In these graphs,the zones of noise derived from the corresponding apparent dis-placements of the lower reflector, clearly exaggerating noise (seeabove) are also shown. Computed horizontal apparent deflectionsof the upper reflector at the top of the tower during events A andB are much larger than the estimate of the noise, hence reflect realdeflections. The latter reach approximately 30 mm in the along andcross – cable axes, and up to 8 mm in the vertical axis.

A combined plot of the instantaneous coordinates of the upperreflector in the horizontal axes (x–y plot) is shown in Fig. 6 forevents A and B. In the same figure the instantaneous coordinatesof the lower reflector during event C, indicating measurementnoise, are also shown.

5.3. Displacement spectra

Spectral analysis of the displacement time-series of the towerbase (shown in Fig. 3) and top (shown in Figs. 4 and 5) was carriedout in order to identify the noise frequency content and the signif-icant frequencies of the displacement signal respectively. The dis-placement spectra were computed using the Normperiod code[30], an implementation of the Lomb Periodogram which is basedon the Least Squares (LSQ) method. The basic reasons for adoptingthis technique is:

first, the instability of the sampling rate of RTS at its highestrecording frequency and gaps in measurements (Fig. 4). Usualspectral techniques (FFT etc.) require continuous time series

Table 2Peak displacements of the tower top recorded during event A and peak displacements of the

Direction Peak displacements (mm)

Event A (tower top) Event B (tower top)

max min max

Cross cable 13.4 �29.6 10.8Along cable 13.6 �22.4 9.1Vertical 7.4 �8.6 4.2

with constant sampling rate, and if these requirements arenot satisfied, before their spectral analysis time series arereconstructed using interpolations (and this leads to additionalnoise), or are divided into segments (and this leads to loss of thelonger-period information.second, the Lomb Periodogram permits the calculation of thelevel of statistical significance of the computed spectral peaks

tower top and base, recorded during event B.

Event C (tower base – measurement noise)

min max min

�27.2 2.6 �3.5�16.9 2.6 �1.5�7.8 3.9 �4.1

Fig. 5. Time series of deflections of the reflector at the top of the pylon during eventB. Dashed lines indicate the zone of noise derived from the maximum amplitude ofapparent displacements of the stable, lower reflector shown in Fig. 3 (control eventC). In gray (at around seconds 450 and 1260), deflections marked by an ellipse inFig. 6.

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third, its performance has been tested using experimental andfield data [14,26,30,31].

In particular, direct comparison of natural frequencies of struc-tures derived by non-equidistant measurements from RTS and byequidistant measurements from accelerometers and GPS indicateddifferences smaller than 0.05 Hz (see [14])

Spectra of the 3-D apparent displacements are shown in Fig. 7.For the computation of the spectra, the whole time series, about600 s long were used in the case of Event B and C, while for eventA the time series after the large data gap (around 600 s from theevent start) where used. The noise spectra, derived from event C,contain energy in the low frequency band (<0.04 Hz) while no sig-nificant frequencies can be found above 0.3 Hz. The spectra charac-terized by decreasing energy from lower to higher frequenciesresemble the typical spectrum of wind speed [1,16,32]. A detailedview of the low frequency part of the spectra is shown in Fig. 8. Agroup of frequency peaks concentrated around 0.26–0.27 Hz canbe clearly identified in the along-cable direction spectra for eventsA and B corresponding to the displacements of the tower top. Thesepeaks probably indicate that one of the eigenfrequencies of thetower, probably the fundamental one, falls close to 0.27 Hz. Fre-quency peaks lower than 0.1 Hz can also be identified but theydo not correspond with each other in the spectra of different

Fig. 6. X–Y plot of instantaneous coordinate changes of the reflector at the top of the towpoints, mostly indicates multiple recordings. Arrows indicate the mean direction of thelevel of noise (uncertainty). The strong majority of dots in the ellipses correspond to thedeflections in the cross wind direction (arrows) is evident.

surveys. These peaks could be attributed to frequencies inducedby wind loading or by the oscillation of the transmission linecables. The spectra of the apparent displacements of the towertop indicate that the apparent displacements are contaminatedby low-frequency noise (below 0.1 Hz) with high energy. Classicways to counteract noise could be the use of low-pass or band-passfiltering (for example Chebyshev or Butterworth filters [33,34])which however require equidistant measurements [35]. Anotherpossibility for removing low-frequency noise could be the use ofMoving Average filters which have been used in the past in thecase of non-equidistant RTS measurements [14,27]. The problemwith this type of filter is that it does not have a clear responsein the frequency domain [35] thus it could remove useful lowfrequencies (for example 0.27 Hz along with low-frequency noise).

In the present study the value of the tower natural frequencieswas identified by multiple detection of the same or similar fre-quency peak in the spectra of several vibration events.

6. Discussion

The installation of a reflector at the top of the tower permittedto record its deflections during two different events of pylon exci-tation by wind with maximum speed of 14–15 m/s. Because of therelatively low sampling rate of the anemometer and its differencein the elevation with the pylon (about 20 m), the real maximumvalues of the wind may be somewhat underestimated.

RTS permitted to record peak horizontal displacements of 29.6and 27.9 mm at the top of the tower. Because the accuracy ofRTS is well-known [6,14,26] and recordings of a second reflectornear the base of the tower were available and defined the measure-ment noise level, these results are reliable. What they show is thatthe deflections of the tower were broadly similar when it wasexcited at different days by relatively strong wind of somewhatsimilar speed and in a direction subnormal to the transmissionline.

A tower with similar geometrical characteristics has been stud-ied using analytical FEM models by Battista et al. [20]. Analyticallycalculated deflections in the horizontal axis reached amplitudes of0.6–0.7 m under wind loading with a velocity of 45 m/s. The firstthree natural frequencies of the tower were 0.158, 0.172 and0.2 Hz. On the other hand the deflections measured in the presentstudy for a similar lattice tower reached smaller amplitudes(approximately 0.03 m). This difference can be attributed to sev-eral reasons which do not permit direct comparison with theFEM predicted deflections in [20]:

er during an interval of approximately 30 min for events A, B and C. Plotting in gridwind. Measurements corresponding with event C provide and upper-bound of thedeflection peaks marked in Figs. 4 and 5 with gray color. A tendency for increased

Fig. 7. Spectra of apparent displacements of Fig. 3 (whole time-series), 4 (part between seconds 620 and 1890) and 5 (whole time-series). A dashed line indicates the 99%level of statistical significance. The highest energy is concentrated at the low frequency band while no significant frequency peaks are identified above 0.3 Hz.

Fig. 8. Spectra presented in Fig. 7 but zoomed in the frequency band from 0 to 0.5 Hz. Significant frequency peaks can be identified below 0.1 Hz for all axes while a significantfrequency peak at 0.26–0.27 Hz is identified for the along cable direction.

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(a) insulators/dumpers at the contact between cables and thelattice tower significantly reduce the tower topdisplacements,

(b) the difference in the wind directions and the wind speed(�19 m/s during the measurements and 45 m/s in the ana-lytical study),

(c) the fact that the studied tower was more rigid compared tothe tower of the present study as the fundamental naturalfrequency was identified around 0.27 Hz indicating a morerigid structural system which is expected to present smallerdisplacements.

Significant displacements with values close to 7 ± 4 mm havebeen recorded in the vertical axis of the tower, in agreement withnumerical studies [20]. Such vertical deflections can be a result ofvertical oscillations of cables or/and to wind blowing with a certainattack angle relative to the horizontal [17], perhaps as a result ofwind turbulence at the top of the pylon [36]. Vertical displacementpeaks generally correlate with peaks in the horizontal axes asrevealed by Figs. 4 and 5.

A point of discussion is whether the values of identified naturalfrequencies could be affected by environmental conditions andespecially temperature. Temperatures at the time of field measure-ments differ by approximately 4 �C from day to day (see Table 2).Past studies on bridges have shown that above 0 �C changes inambient temperature can only slightly modify natural frequenciesof a structure, less than 0.01 Hz for a 12 �C increase [37,38]. Fur-thermore, the studied tower is entirely made of steel, and hencenot subject to changes in its natural frequencies imposed by differ-ential response of various materials to changes in the ambienttemperature.

An important outcome of this study is that it documents deflec-tions larger in the cross-wind than the along-wind direction(Fig. 6). Several possible explanations should be investigated. First,larger cross-wind deflections may indicate vortex shedding effects.It is however not easy to accept this explanation due to the rela-tively small surface of the structural members of the tower.Another possibility is that they reflect a response to forces gener-ated by oscillating cables: slight along-wind oscillations of thetower, mainly because of gusts (derived from sub-periodic maximain along-cable deflections in Figs. 4 and 5), excite cables which sub-sequently oscillate in the vertical sense and impose along-cablehorizontal forces on the tower (see Battista et al. [20]); an effectcommon in structures interacting with cables such as guyed mastsor cable stayed bridges [39,40].

A final remark is that the displacement pattern at the top of thepylon is characterized by a combination of large quasi-static dis-placements, usually referred to as ‘‘background response’’ [16],and high frequency motions due to wind loading. Transmissionlines and guyed masts are also expected to induce large-periodoscillations [19,41], with periods up to several seconds and evenlarger [15]. Such quasi-static or aperiodic motions could not becaptured with instruments like accelerometers [17]. Still, eventsA and B, with duration of about 30 min each, do not provide anyevidence of such movements (Figs. 4 and 5).

7. Conclusions

A rare opportunity to install a reflector at the top of an H-typelattice tower permitted to record the 3-D deflections of the topof the tower during events relatively strong for the area (windvelocity 14–15 m/s) with an accuracy of a few millimeters.Observed deflections were small, up to 30 mm, but they showeddeflections larger in the cross-wind direction than along-wind,and they permit to put some realistic constraints in the modelingof such towers and of transmission lines, i.e. to a complex struc-

tural system controlled by numerous unknown parameters (atmo-spheric and soil conditions, etc.)

The presented methodology is low-cost and effective and thus itcould be introduced in the monitoring of the structural health ofelectricity transmission networks. Still, it will not be effective inthe case of atmospheric conditions obstructing optical measure-ments such as snowfalls or heavy rain falls. Especially in the caseof icing episodes, usually dangerous for transmission lines (at leastone major event in the last 15 years in Greece; [42] with cleanatmosphere, measurements are possible if the reflector on thepylon is cleaned from ice using special devices.

Acknowledgements

We thank the Public Power Corporation of Greece and espe-cially Mr. Kritikos and Mr. L. Georgiou of the Transmission Sectionof Western Greece who embraced our idea and installed the reflec-tor on top of the tower. Panayotis Yannopoulos is thanked for pro-viding the meteorological data. Comments of three anonymousreviewers are highly appreciated.

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