Assessment of structural integrity of wooden poles.

Ian A. Craigheada, Steve Thackeryb, Martin Redsta11', Matthew R. Thomasba Univ. of Strathclyde, Glasgow, Scotland Gi 1XJ

b BT Research Laboratories, Martlesham Heath, Ipswich, 1P57RE, UK.

ABSTRACTDespite recent advances in the development of new materials, wood continues to be used globally for the support ofoverhead cable networks used by telecommunications and electrical utility companies. As a natural material, wood issubject to decay and will eventually fail, causing disruption to services and danger to public and company personnel.Enternal decay, due to basidomycetes fttngi or attack by termites, can progress rapidly and is often difficult to detect bycasual inspection. The traditional method oftesting poles for decay involves hitting them with a hammer and listening to thesound that results. However, evidence suggests that a large number of poles are replaced unnecessarily and a significantnumber of poles continue to fail unexpectedly in service. Therefore, a more accurate method of assessing the structural:Lntegrlty ofwooden poles is required.

Over the last 25 years there have been a number of attempts at improving decay detection. Techniques such as ultrasound,drilling, X rays etc have been developed but have generally failed to improve upon the practicality and accuracy of thetraditional testing method.

The paper describes the use of signal processing techniques to analyse the acoustic response of the pole and therebydetermine the presence of decay. Development of a prototype meter is described and the results of initial tests on severalhundred poles are presented.

Keywords: Wood decay, wood poles, impact response, acoustic response, condition monitoring.

1.INTRODUCTION

Wooden poles are widely used by telecommunications and electricity utility companies for the support of overhead cables.They have been found to be a relatively cheap and reliable method for supporting an overhead cable network system. Polesare typically 150 - 400 mm in diameter and can range in height up to 10 m or more. Traditionally they are placed verticallyin holes, which are drilled into the ground to provide a cantilever type of support.

The tree most commonly used for poles in the UK is the Scots Pine (Pinus Sylvestris) but other species are to be found insignificant numbers. Wood is a natural, biological material and like all such materials is subject to decay. Decay in wood iscaused by certain fungi, which use the wood as a food source. The outer surface of a pole, although usually treated withcreosote, can be attacked by the fungus ascomytes. Such 'soft rots" are relatively easily detected by visual inspection andprobing and are generally slow to progress to ultimate failure of the pole. However, the internal parts of the pole can beattacked by basidiomycetes fungi, which enter the wood via checks in the pole, which occur as part of the drying outprocess. This decay can advance rapidly inside the pole with no external evidence visible to a cursory inspection.Subsequent failure of the pole can occur unexpectedly once the decay has weakened its structural strength sufficiently. Thedecay is found most often at or near the ground level where moisture conditions are most conducive to fungal growth.Unfortunately this is also the location ofgreatest bending moment on the pole, which magnifies the effect ofthe decay.

Replacement of a pole can cost as much as 1000 and they have an average life expectancy of45 years. The useful life of apole is affected by many factors, which are largely beyond the control of the utility operators. Variability in the quality ofthe wood, preservative treatment, site conditions, loading etc. can all lead to unexpected failure of the pole. This can resultin serious consequential losses in terms of public/personnel safety and system integrity. To avoid such failures, most utilitycompanies employ "pole testers" whose job it is to inspect poles periodically to decide if they are fit to continue supportingthe network until the next inspection is to be undertaken. The frequency of inspections is typically 5 -7 years. A number of

In Nondestructive Evaluation of Aging Materials and Composites IV, George Y. Baaklini, Carol A. Lebowitz,Eric S. Boltz, Editors, Proceedings of SPIE Vol. 3993 (2000) 0277-786X/OO/$1 5.00 131

utility companies employ a basic acoustic response testing method to determine the presence of internal decay. The testerstrikes the pole with a hammer and listens to the noise that is made. Subjective assessment is then made by the tester as tothe severity of any decay present. Surprisingly, this method is reasonably reliable but understandably testers tend to err onthe side of caution. It is generally recognised that a large number of poles are classified as 'defective" when the level ofdecay is insufficient to warrant their replacement. Such unnecessary replacements are costing utility companies ms peryear. Over the last 25 years considerable efforts have been made to improve upon existing testing methods and develop asystem, which is reliable, portable and inexpensive.

2. PREVIOUS TESTING METHODS

A wide range of technologies have been used in an attempt to produce a pole testing system that is more reliable than the"wheeltapper' approach. The main methods, which are described in the literature or have been developed commercially, areoutlined.

2.1 Ultrasonics

Ultrasound is a vibration which is at a frequency which is higher than the audible range (> 20 kHz). Usually pulses ofultrasound are directed into the pole in a radial direction. The time for the pulse to travel across the pole is measured. Thisprocedure is usually repeated at a number of locations around the circumference. The waves of ultrasound travel at speedsof approximately 2000 m/s in sound wood and slow down when passing through or around decayed sections. The time offlight gives an indication of the presence of decayed material. The method was pioneered in the 198Ots12 and developedcommercially3. It has been used successfully on building timbers but has had mixed success on utility poles. The cost ofthe instrument and the time required to survey a complete pole tend to discourage more widespread use.

2.2 Medical techniques

The location of decay within a pole can be likened to identifying fractures or abnormalities within the human body. It istherefore not surprising to find that techniques, long established in medical investigations, such as X rays, NMR etc. havebeen applied to locating decay in poles. Little published information could be found on the use of these techniques,presumably because developing a cost effective, safe and portable system would be difficult to implement.

2.3 Electrical impedance

As decay progresses in wood, the fungus produces ions, which change the electrical impedance properties of the wood.Initial trials of a tomographic sensor based on this approach have been described5 but the accuracy of the system wasinsufficient for it to be ofpracticai use. It is believed that further work on this system is ongoing.

2.4 CO2 detection

As well as the production of ions, the decay process results in the production of CO2. This can be detected by standardchemical analysis equipment and this approach has been studied and evaluated by a number of companies. However, areliable and cost effective system is yet to be developed.

2.5 Invasive technologies

Probes forced into the pole have been used in a number of guises over the years. It is believed that there is a directcorrelation between the wood's resistance to penetration and its strength. Italian researchers have provided some scientificbasis for this approach6'7 and used the method successfi.illy on timbers in historical buildings. The basis of the method is tomeasure the number of calibrated hammer blows per centimetre of penetration of a specially designed probe. An alternativemethod uses an instrumented drill8, which provides a graph of penetration resistance against drill progress. Both methodssuffer from the drawback that a number of tests will be needed to survey a complete pole and each penetration provides anew potential site for the initiation of decay.

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2.6 Proof loading.

Applying a test load to a pole, which is in excess of the design load, is probably the most direct means of assessing thefitness for purpose of a pole. This approach has been used successfully for testing lampposts9 and, in theory, could beadapted for wooden poles. However there are two major drawbacks :- 1) Many wooden poles are situated at some distancefrom roads so vehicles could not always be used to apply the necessary forces required, 2) failure of a pole in testing wouldprobably result in disruption to the network. For these reasons, proof loading has not been considered seriously for routinepole testing work.

2.7 Vibration

The decay of part of a pole will result in changes of the Young's Modulus and density in the region of the decay. Thesechanges have been considered likely to affect the vibrational characteristics of the pole and if these could be measured anestimate of pole condition could be obtained. Excitation of the pole is usually either by a single blow from an impacthammer or a repetitive, cyclical force. One of the earliest methods (The Resotest') relied on a square wave force input tothe pole approximately im from the ground and measurement of the response at intervals going down to ground level.Initial tests reported' seemed to be encouraging but little further information could be found (26 years later) so it wouldappear that the system failed to live up to its early promise.

There are a number of difficulties associated with applying vibrational techniques to poles. The bending natural frequenciesstart at approximately 2 Hz and occur every 10-20 Hz higher up the frequency range. The precise values vary depending ondiameter, number of wires that are supported, presence of stays, additional masses supported on the pole etc. Internal decaywill only affect the bending strength and hence bending frequencies slightly until the point of collapse so the likelihood ofdetecting any change due to decay is extremely small. Despite these disadvantages, the vibration approach has been usedsuccessfully on railway sleepers" where structural parameters between sleepers are much more regulated than for poles.The manufacturers of the sleeper tester have tried applying their method to poles but abandoned their research due to theproblems previously identified.

It can be seen that considerable research effort'" has been put in worldwide over the last 30 years in an effort to improvethe reliability of wooden poles used in supporting cable networks. Because of the limited success of previous, alternativestudies it was decided to look more closely at the standard acoustic method which has been relied upon for decades and seeifit could be improved by using modem technology.

3. THE IMPACT RESPONSE METHOD

Striking any object or structure with a hammer causes it to vibrate and this vibration may cause the surrounding air tovibrate thus producing the sound ofthe impact that is heard. It was considered that it was this sound that pole testers use toidentify decay in poles. The structure, which is struck, usually vibrates at one or more of its natural frequencies. Usually,only frequencies which lie in the range 0 I 500 Hz can be excited by a hammer blow'2. Verbal description by pole testerssuggested that the characteristic pitch or frequency ofthe impact fell when a decayed pole was stmck compared to a "good"pole. This implies that one or more ofthe natural frequencies reduce due to the presence of decay.

To investigate this supposition, the acoustic signal and vibration response were recorded from a number of telegraph poles,which were specifically used for training pole testers at one ofB.T's Training Centres. This work, reported in 13 confirmedthat lower frequencies were observed within the acoustic spectrum when decayed poles were impacted by a hammer. Thework resulted in an algorithm. which was implemented on a lap-top computer. The acoustic signal was picked up with aBruel & Kjaer l/2inch microphone and the signal digitised and fed into the computer using a Pico-Log AID board. Theequipment arrangement can be seen in Figure 1.

The laptop-based instrument was used to assess the condition of a number of poles and the predictions compared to theassessment of experienced pole testers. The output from the pc was a display of the acoustic spectrum, a frequency valuebased on the algorithm and a rating from 0 10 indicating the "severity" of the decay. Figure 2 shows a typicalmeasurement screen from the system.

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Figure 1 Laptop based prototype instrument.

Figure 2 Output display from laptop based instrument

The results from the system tended to agree with the assessment provided by traditional methods but there were a fewsignificant discrepancies. These were due to dominant frequency peaks, which were sometimes evident throughout thespectrum and were random in nature. These peaks, when present, tended to result in the algorithm "mis-diagnosing" thepole condition. The shortcomings of the lap-top system were considered to be due to using the algorithm on a single impactsound whereas the original algorithm was based on an averaged spectrum taken from 16 impacts. it was considered that theaveraging process tended to remove the dominance of "random" peaks and result in a reduced mis-diagnosis rate.

To overcome the problems it was decided to collect sounds from a much larger selection of poles and investigate a numberof different algorithms to identify the best for use with a single impact.

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4. DATA COLLECTION & ANALYSIS

A number of operational poles were identified at various locations throughout the U.K. Sound recordings were taken ofhammer impacts applied to various positions on the pole. The positions included were generally within 2m of the groundand measurements were taken at positions where decay was suspected and also on sound sections of the pole. The presenceof decay was confirmed afterwards by drilling the pole. The data was digitised at a rate of 1 1025 samples per secondenabling frequencies of up to 5. 5 kHz to be assessed. The various impact sounds were stored in the form of .WAV files for:ftirther processing. In total, 570impacts were recorded.

.A software analysis system was developed, based around the FFT process, to enable various algorithms to be applied to thedata and evaluated. Figure 3 shows typical output from the system. Factors such as the weighting, band pass filtering,number of data samples, smoothing etc. could be changed and their effect readily evaluated. In order to assess the accuracyofthe algorithms proposed, 100 test samples were identified. 50 were from poles, which were known to have decay present

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Audio Sampling Rate; 11025 samples per second.Samples Presented to FFT: 1024 samples from sample 70 to 1093.Fourier Weighting Window: Cos2.Frequency Weighting: None.Founer Low Frequency Cutoff: Disabled.Fourier High Frequency Cutoff: Disabled.Fourier Smoothing: Enabled. 3 point averaging.Fourier Vertical Scale: Relative (i.e. values adjusted so highest peak is always at 1.0).

Key Frequencies: ow cutoff 377Hz, balance 528Hz, high cutoff = 538Hz, bandwidth = 161Hz.Figure3 Typical results from the sound analysis

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and 50 were from poles that were considered to be in "good" condition. The algorithms were each applied to the test suite ofimpact sounds and their performance was evaluated. The output of each algorithm was a characteristic frequency and it wasfound that "good" poles tended to produce a higher characteristic frequency than poles that had decay in the vicinity of theimpact site.

It was then necessary to identify a threshold or cut-off frequency that would discriminate between "good" poles and thosewith decay present. Unless the system was lOO% accurate there would be the likelihood that some decayed poles would bemisdiagnosed as being "good" and some "good" poles being incorrectly condemned as being decayed.

In order to quantify the accuracy, two criteria were identified:-

4.1 Minimum error setting.

A cut-off frequency was selected so that the number of good poles which were condemned was equal to the number ofdecayed poles which were mis-diagnosed to be "good". The number of cases mis-diagnosed should approach zero as thealgorithm accuracy improves.

4.2 Maximum caution setting.

A second, higher cut-off frequency was identified to ensure that all decayed poles were identified. The number of "good"poles condemned then became a reflection of the accuracy of the system. A perfect system would have both cut-offfrequencies the same, resulting in zero mis-diagnoses and lOO% decayed pole detection. The performance ofthe algorithms

Pole Decay Algorithm PerformanceI - Mm. Error 2 - Max. Caution.

a Series I Series 2Figure 4 Algorithm performance. Better algorithms have a high Mm. Error value and a low Max. Caution value.

tried was found to be disappointing and considered unlikely to lead to a reliable instrument. Many of the initial algorithmsproduced more mis-diagnoses than correct assessments. Further inspection of the recorded sounds in the time domain andfrequency domain resulted in the proposal of additional algorithms, which produced better results. The performance of thesealgorithms on the test suite of pole impacts can be seen in Figure 4. The most successful algorithm was "am" with a successrate of 84% in terms of minimum error and 26% of "good" poles condemned on the maximum caution evaluation.

Figure 5 shows the distribution of frequency parameters for algorithm "am". The minimum error threshold frequency is at950 and the maximum caution threshold at 1070.

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5.HARDWARE DYELOPMENT.

The accuracy of the algorithm was considered to be sufficient to warrant design and production of 5prototype meters basedupon the software developed. The meter will be a battery powered, handheld system similar to that shown in Figure 6. Themicrophone will take in the sound from a hammer/pole impact and the signal will trigger the digitisation process. The soundwill be recorded for approximately 90 msec and this signal will then be the basis for the proposed algorithm. Calculation ofthe characteristic frequency for the sound and comparison with the threshold frequency will enable display of the frequencyand determination of a "decay parameter" probably on a scale of 0 10. A reading of 10 should indicate a "very good" poleor one where there is a high degree of probability that no decay is present whereas a reading of 0 or 1 would indicate severedecay with some certainty. Values in between these extremes are likely to provide a less certain diagnosis but it is expectedthat it will be more reliable and consistent than the present subjective,aural techniques. At the present time, the prototypesare being manufactured and it is expected that field trials will commence in the near ftiture.

137

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Figure 6 Schematic diagram of pole decay meter.

6.DJSCUSSION

The data collection was undertaken by BT personnel and the recording system worked very well The impact sounds werereadily converted to WAy files and subsequent data handling was straightforward The signal analysis software wasextremely user friendly and allowed efficient evaluation of the algorithms under consideration. The system underwentconstant improvement during the research as minor shortcomings were identified and remedied. At the present time, thesystem has been automated to such an extent that the 100 test sounds can be analysed in a matter of a few minutes

The initial results from the 570 pole sounds were based on the original algorithmU, which relied on smoothing the averagedspectrum and identifying a lower threshold frequency The results, on the whole, tended to follow earlier results but therewere a significant number of anomalies, which were usually due to individual peaks appearing at higher (>600 Hz) or lower(K 150 Hz) frequencies causing radical changes to the threshold value obtained It was considered that this was due toconsidering a single impact in the current tests whereas the original work was based on a spectrum averaged from 16impacts on the same section It was felt that the averaging process probably reduced the influence of these peaks, whichwere of a somewhat random nature, in a practical implementation of the algorithm, it is unlikely that a tester would want toimpact the pole a number of times to enable averaging to be undertaken A meter based on a single Impact would thereforebe preferable

In an attempt to eradicate these anomalies associated with single impacts, a number of different algorithms were tried. Thealgorithms were based upon a range of different frequency characteristics of the signal. However, their success ratecontinued to be disappointing.

A number of alternative algorithms were identified after considering the time histories and spectra of the 100 test samplesmore closely. The accuracy of these algorithms improved with the better ones achieving typical values of 80% for theminimum error setting and 25% for the maximum caution setting. The best algorithm was identified and it's performance isshown in Figure 5. Initially a value of 84% was obtained for the minimum error value and 26% of good poles werecondemned in the maximum caution setting. However, it can be seen (Figure 5) that there is a cluster of circular dotstowards the lower left corner of the graph, which are separated, from the other circular dots for the poles without decay. Bymoving the cut-off frequency from 950 to 1050 almost all decayed poles are identified (98%) with no increase in thenumber ofgood poles condemned.

Furthermore, the cluster of circular dots which gave a relatively low frequency are all from tests conducted at Harrogate onAmerican Southern Pine poles or European redwood poles rather than Scots Pine. In addition it was raining during testing ofthese poles and the wood surface was reported to be wet. It would appear that either the wood type and/or the wetness of thewood has resulted in a lower frequency for these good poles resulting in the erroneous diagnosis of decay present. It is feltthat both of these factors could be taken into account, after further research, resulting in a much-improved performance ofthe meter. If this cluster of circular dots are discarded, the algorithm gives a virtually, clean separation of circular (good)and diamond (diamond) points. The success rate for minimum error would become 98% and the maximum caution criterionwould result in 3% ofgood poles being condemned.

It could be argued that the decayed samples are from poles with significant decay present (at least sufficient decay to bedetected by current methods) and that it is unlikely that the algorithm will perform as well on poles with small levels ofdecay present (the borderline cases). This is probably true but it should be remembered that the test result diagnoses arebased on a single impact. A tester currently will usually impact the pole a number oftimes to get a general impression of thedecay. This could be done with the proposed meter and figures averaged to improve diagnosis reliability. It should also beremembered that if there is only slight decay present it is likely that the pole is not in imminent danger of collapse and thechances are that it will survive until the next inspection, so reduced reliability in these cases may be acceptable.

The most successftil algorithm was therefore considered to be most appropriate for implementation in the prototype meters.Because of the unresolved matter with the cluster of poor results it was decided to make the cut off frequency adjustable toallow further investigation and refinement to be undertaken. The output from the meter was chosen to be a scale from 0 10. Scores of 4 and under should indicate decay and scores of 5and above a good pole. The further the reading is from themid range the more reliable the diagnosis is likely to be.

The meters are being designed to be easy to use so that there is little scope for operator error. It is envisaged that the metercould be manufactured relatively cheaply (several hundred pounds) so that each tester could carry one around in theirtoolbag. As the meter relies on an acoustic signal, then any ambient, background noise may affect the accuracy of theanalysis. It is considered that the meter should be capable of reliable operation at any location where current, aural testmethods are used.

It is anticipated that 5 prototype units will soon be available for initial field trials to be undertaken. The meters will be handheld, batteiy powered devices for ease of portability. The analysis should only take 10-1 5seconds and then a further impactcan be captured. Averaging around a circumferential line on the pole will be easy to undertake to improve reliabilitycompared to a diagnosis after a single impact. Use of these meters will provide valuable information on the performance andreliability of the system and enable further research to improve the algorithm by taking into account factors such as woodtype, pole diameter, moisture content etc if applicable.

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7. CONCLUSIONS

Over 500 pole/hammer impact sounds have been recorded and analysed in the time and frequency domains. Approximately40 algorithms have been tested on this database to find the best at identifying differences in the sound between decayed andnon-decayed poles. The best algorithm resulted in correctly diagnosing 84% of decayed poles in a representative samplewhilst condemning 26% ofgood poles. 18% ofthe "good" poles were tested when the wood was wet and ifthese results arediscounted the algorithm resulted in an accuracy of98% with only 3% of"good" poles incorrectly diagnosed as decayed.

Prototype meters are currently being manufactured, based on the identified algorithm and ftirther testing and field trials areplanned.

ACKNOWLEDGEMENTS

The authors would like to express their gratitude to BT Research Labs for the support provided to enable the work to beundertaken and agreement to publication of this work. Thanks are also expressed to BT Training Centre for allowing accessto their facilities.

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