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Journal of Wind Engineering and Industrial Aerodynamics 96 2008 1103 1110 Wind induced transmission tower foundation loads A fi eld study design code comparison E Savorya G A R Parkeb P Disneyb N Toyb aDepartment of Mechanical Wind loading Design code 1 Introduction The wind induced foundation loads on a standard height National Grid Company Blaw Knox L6 transmission line tower Lomas 1993 shown in Fig 1 situated in an exposed location at Winterbourne Abbas in Dorset in the South West of England were monitored from March 1995 to November 1999 These loads were measured by strain ARTICLE IN PRESS 0167 6105 see front matter r 2007 Elsevier Ltd All rights reserved doi 10 1016 j jweia 2007 06 033 Corresponding author Tel88256 faxE mail address esavory eng uwo ca E Savory gauge arrays attached to all four of the tower legs immediately above the base giving the strains associated with member tension or compression only The strain data together with the wind speed and direction both measured at the top of the tower were simultaneously logged typically every 10min providing a comprehensive data set for analysis over the four and a half years monitoring period An interim paper on the research programme containing data from the fi rst half of the monitoring period together with some details of the instrumentation and data processing has been published previously Savory et al 1998 The present paper summarises the main fi ndings from the complete data set As part of the interim data analysis Savory et al 1998 the measured leg strains obtained for a limited number of cases of wind speed and direction were compared with the leg strains computed using the usual British Standards Code of Practice for tower and line design BS8100 British Standards 1986 The resulting scatter plot indicated that the Code tended to over predict the measured loads by about 14 especially at the higher wind speeds Because this conclusion was based upon a relatively limited analysis it was therefore somewhat tentative Hence it was decided to carry out the more detailed study reported here 2 Details of the tower and instrumentation The monitored tower was chosen for its exposed location 8km inland from the Dorset coast in South West England The type L6 free standing lattice tower shown in Fig 1 has ARTICLE IN PRESS Fig 1 The monitored L6 transmission line tower E Savory et al J Wind Eng Ind Aerodyn 96 2008 1103 11101104 a base of 9 1m 9 1m and a height of 50m to the wind speed anemometer and direction vane location It carries three quad bundles of conductors on either side of the tower with each conductor being 400mm2ACSR and a ground wire of the same material and size at the top of the tower The mean height of the conductors above ground is 30m with an effective span between adjacent towers of 341m The base of the test tower is 186m above mean sea level ARTICLE IN PRESS Strain gauges 203 2 x 203 2 x 12 5 mm equal angle Strain gauge locationsLine orientation 103 North 1 2 3 4 Line Tower leg Fig 2 Location of strain gauges and orientation of the transmission line Fig 3 PDF for site wind speeds E Savory et al J Wind Eng Ind Aerodyn 96 2008 1103 11101105 ARTICLE IN PRESS Fig 4 Wind rose for the site over the complete monitoring period Fig 5 Variation of tower leg strains with the square of the wind speed for a South Westerly wind direction of 2251 measured from the normal to the line E Savory et al J Wind Eng Ind Aerodyn 96 2008 1103 11101106 The four legs of the tower were instrumented with strain gauges immediately above the foundations with eight on each leg angle as shown in Fig 2 The gauges were aligned so that they measured only strains corresponding to axial tension and compression All of the instrumentation was powered from solar panels mounted on the fi rst level of plan bracing above the ground The alignment of the transmission line with respect to North is also shown in Fig 2 illustrating that the line is 131 offset from an East West direction The wind speed and direction together with the leg strains were measured every 10min during the monitoring period thereby building up a substantial database of over 200 000 sets of points 3 Results and discussion Before assessing the wind loading on the tower some aspects of the site meteorology will be presented fi rst Fig 3 shows the probability density function PDF for all the wind speed measurements at the site It may be seen that it fi ts a Weibull distribution quite well with a coeffi cient k 2 04 that is typical of parent winds in the UK Fig 4 presents the wind rose for the site winds with the data given in 51 bands The predominance of South Westerly and North Easterly winds is very evident Note that the low sampling frequency of the wind speed data prevented any analysis of the site wind turbulence characteristics In the analysis of the wind and tower leg strains the complete fi eld data set was divided into 51 interval wind direction bands with each wind direction data set then being ARTICLE IN PRESS Fig 6 Variation of tower leg strain wind speed 2for all four legs with wind incidence angle E Savory et al J Wind Eng Ind Aerodyn 96 2008 1103 11101107 processed separately For every direction band the strain in each tower leg was plotted against the square of the wind speed and the best straight line through the origin obtained to give a slope value of leg strain wind speed 2for that direction In all cases the data for wind speeds below 10m s were ignored to ensure reliable strain data Fig 5 shows a typical plot that was obtained from the leg strain data for a single wind direction of 2251 measured clockwise from the Northern normal to the line Typically the strain data were reliable to within 720m strain Considering the errors in the measurement of the wind speed and strains together with those associated with analysing 51 wind direction bands the overall accuracy in the leg strain wind speed 2parameter is 70 05 Bringing all the data together for the 4 tower legs and for the 72 different wind direction bands yields the plot shown in Fig 6 The curves in the plot show a high degree of consistency well within the error band given above and so may be used to estimate the wind induced strain at the base of any tower leg given the wind speed and direction at the top of the tower The UK Code of Practice for lattice towers BS8100 British Standards 1986 has a series of calculation procedures for computing the loads on tower sections insulators conductors and earth wires for a given wind speed and direction In the present work those procedures have been generalised to produce loading functions for each of these line system elements which give the tower foundation axial load expressed as leg strain wind speed 2 as a function of wind direction ARTICLE IN PRESS Fig 7 Code calculation of the variation of tower leg strain wind speed 2for all four legs with wind incidence angle measured from the normal to the line measured from the normal to the transmission line E Savory et al J Wind Eng Ind Aerodyn 96 2008 1103 11101108 The monitored tower and line were constructed in 1965 in accordance with the design procedures then in place within the former Central Electricity Generating Board Hence this was well before the introduction of the present Code of Practice British Standards 1986 The magnitudes of the various design factors taken from the Code for the calculations used in the present work were clearly defi ned for the type of rural site in which the monitored tower was located The Terrain Category was type II with the terrain roughness factor KR and the power law exponent for the atmospheric boundary layer a being 1 10 and 0 14 respectively The conductor earth wire and insulator drag coeffi cients CN were taken as 1 0 1 0 and 1 2 respectively whilst the tower load was derived using a drag coeffi cient of 3 07 and an average area solidity ratio f of 0 168 The wind speed used to quantify the leg strain wind speed 2ratio was factored to be that occurring at the anemometer height of 50m representing the same wind speed location used in the fi eld measurements The analysis using the functions derived from the Code produces the curves given in Fig 7 which show a remarkably good correlation with the fi eld data given in the previous fi gure Combining the leg strain wind speed 2data from these two plots to give a scatter plot of fi eld data against code calculated values yields the interesting graph shown in Fig 8 The regression analysis for all the data in the plot shows a slope of 1 0102 that is an overall agreement between the fi eld and calculated data of within approximately 1 and a reasonable correlation coeffi cient of R 0 965 Hence the analysis suggests that the Code of Practice provides a good indication of the wind induced ARTICLE IN PRESS Fig 8 Comparison of the fi eld data and the Code calculations BS8100 for leg strain wind speed 2 E Savory et al J Wind Eng Ind Aerodyn 96 2008 1103 11101109 foundation loadings that may be expected to occur on a transmission tower for any given wind speed and direction 4 Concluding remarks The results of the fi eld data analysis presented here together with the related wind loading Code of Practice calculations suggest that the L6 tower and line design is not particularly conservative in that the wind induced foundation tension and compression loads follow very closely those derived from the design code However it is important to note that it is only the ratio of the foundation loads expressed as leg strains and the square of the wind speed that has been verifi ed here and not the magnitudes of any extreme loadings that might actually occur in the fi eld due to especially high wind speeds Furthermore since the fi rst mode of vibration of the tower is 2 22Hz transversal defl