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IEEE Transactions on Power Delivery, Vol. 10,No . I, January 1995 445EFFECT OF TORNADO LOADS ON TRANSMISSION LINES

Mag& F. Ishac, Member, EEEOntario Hydro393 University Ave nueToronto, OntarioCanada MSG2I.6

Abstract - Of all the populated areas in Canada,southwestern Ontario has experienced the highesttornado incidence and faces the greatest tornado dam age.About 1 or 2 tornadoes per 10,000 km2 can be expectedthere annually. The probability of a tornad o strike at agiven point is very small but the probability of atransmission line being crossed by a tornado issignificant. The purpose of this paper is to review theliterature related to tornadoes in Ontario and toinvestigate the effect of tornado loads on transmissionlines. Based on this investigation a design basis tornadoloading for transmission towers is proposed.1.0 INTRODUCTIONTornadoes are short-lived, randomly occurring, severestorms that cover small areas. Thes e narrow front winds,which seldom register at meteorological stations, are themost comm on cause of weathe r related transmission towerfailures in Ont ario Hydro. Th e probability of a torna dostrike at a given point is very small but the probability ofa transmission line being crossed by a tornado issignificant. However, the fact that the width of a tornadopath is very narrow, gives the possibility for improvingtransmission line resistance to tornadoes at reasonablecost. Th e purpose of this paper is to review the literaturerelated to tornadoes in Ontario and to investigate theeffect of tor nad o loads on transmission Lines. Based onthis investigation a design basis tornado loading fortransmission towers is proposed.

This paper was presented at the 1994 IEEE PES Transmission andDistribution Conference and Exposition held in Chicago, Illinois,April 10-15, 1994.

H. Brian White, Fellow, EEEConsultantPO Box 939Hudson, QuebecCanada JOP 1HO

2.0 TORNADOESTornadoes may be classified by their intensity. Th eFujita F-scale for classification of tornadoes is based ontornado wind speed, path length and path width. O n theFujita scale of 0 to 5, FO is the category for the weakesttornadoes and F5 is the category for the strongest. Only8% of all Canadian tornadoes are rated as F3 or strongerand no F5 tornadoes are known (Newark 1983).The first Canadian attempt to formulate a designbasis tornado was made by Collingwood (1977) to meetthe needs of nuclear power plants located in Southern

Ontario. By using a larger and more recent set of U.S.and Canadian data, Newark (1991) proposed a Canadiandesign basis tornado appropriate to each Fujita-scalecategory. The re are still many problem s in gathering andinterpreting tornado data but this data can still form thebasis for bette r inform ed judgem ent in structural design.A number of attempts have been made tocharacterize the physical dimensions of tornado damage.In Canada, Newark (1983) calculated mean, median andmode values of damage length, width and area. In theUnited States, the work of Schaefer et a1 (1986) isconsidered to be the best indicator of the magnitude oftornado damage parameters for North America. Their

median values of damage length, width and area, areshown in Table 1. These values are smaller than thosereported by Newark (1981).Given that there is no reason to suspect thatCanadian tornadoes should have larger damagedimensions than those in the U S . , the larger U.S. samplesize and the fact that not all parameters are available forall tornadoe s in the Canadian set, Newark (1991) hasused Table 1data as the basis for recommending severalparameters of a Canadian design basis tornado. Table 2gives the design parameters for a tornado in each F-scalecategory recommended by Newark (1991).

0885-8977/95/$04.000 994 IEEE

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4463.0 TORNADO FREQUENCY IN ONTARIO

In O ntario, the average starting da te of the tornadoseason is mid May and the average end date is the end ofAugust. Peak tornado activity occurs during the lateafternoon or early evening (Newark 1981). In O ntario, themajority (76%) of tornad oes travel from the southwest andwest.Table 3shows the return periods of the mo re severetornadoes in Ontario. During the 1950 to 1979 period astrong tornado, F3 or F4, was experienced somewhere inOntario within a return period as sho rt as 3 years (Newark1984).

4.0 T O R N A D O L O A D I N G E F F E C T O NTRANSMISSION LINESIt is impossible to design a transmission line towithstand all tornadoes. However, the fact that 92% ofcategorized tornadoes in Ontario are rated as F2 or lessshows the possibilities of economical designs that mightresist most tornadoes.Many of the tornadoes of F2 or less arecharacterized by intense winds acting over a very smallpath width, sufficient to create large loads acting on thestructu re with relatively much lower c onductor loads actingon the full line span . Field investigations of struc turesdamaged by tornadoes support the concept of shearfailures within the tower body of a 4-leg tower or fromtransverse/longitudinal mast bending and possible shearfailures at the ends of the masts, causing failures of guyedtowers.Ontario Hydro loading criteria derive both wind onconductor loads and wind on structure loads from the

same basic wind speed. For med ium to long span lines,this usually produces total wind on cond uctor loads m uchgreater than the wind on structure loads and the resultantlocation of the total transverse loading is closer to thelevel of the conduc tor loads. T he result of this loadingphilosophy is a larger overturning moment and a largerfoundation reaction which contribute to a conservativedesign of the tower legs. Th e tornad o loading is a verysevere wind load to be applied to the tower from anydirection in combination with a very small or even zerohorizontal cond uctor loading. Th e shift in emphasis towind on structure loading will change the sheardistribution within the body of the latticed 4-leg towerswith a possible reduction in the overturning moment andthe foundation uplift reaction. With respect to guyed

towers, the tornado loading will increase the bendingmoments in the masts of the V-guyed towers and changethe shear distribution in the mast of the Y-guyed and theDelta-guyed towers. Th e structural changes, if needed,will be limited to increased capacity of small secondarymemb ers which may increase the tower weight by less than3%. It should b e noted that some tower designs have suchcapacity built in when these towers are designed forrelatively shorte r span lines which produc e total conductorwind loads of the same order of magnitude as structurewind loads.5.0 PROPOSED TORNADO LOADING FORONTARIO HYDRO

The proposed tornado loading for O ntario Hy dro isshown in Table 4. Also shown is a comparison betweenthe actual extreme wind loading used in our tower designversus the proposed tornado loading.The proposed tornado loading is a wind loadcorresponding to moderate tornadoes F1 or F2 fo rNorthern or Southern On tario applied only on the towers.

The load o n the condu ctors and groun d wires is neglectedbecause of the narrow tornad o path widths. Th emaximum tornado wind speed proposed is 240 km/hrwhich corresponds to a Southern Ontario F2 tornado to beused on the lines that are being designed to withstand anextreme wind of 160 km/hr.6.0 TOWER DESIGN LOAD EVALUATION

The tornado loading effect on transmission towersdescribed in section 4.0 has been evaluated on two typesof Ontario Hydro tower designs, (a) 4-leg towers and (b)V-guyed towers. For each tower design, three d ifferentfamilies were evaluated by applying a tornado load on suchtowers. For the 4-leg towers, the 1-cct 500 kV Z6S, Z l l Sand Z15S were used, and for the V -guyed towers, the 1-cct500 kV Z2S, Z5 S an d Z7S were used in this investigation.The tower outline sketches are shown in Figure 1. Theiummary of the 4-leg basic tower design evaluation istabulated in Table 5.a and the s umm ary of the 4-leg tallesttower design evaluation is shown in Table 5.b. Theevaluation is carried out by comparing the effect of highwind loading to the tornado loading in each case and howthese different types of loadings are affecting the towerdesign, i.e.. Transverse Design Load, Overturning M omentand Foundation Reactions for the 4-leg towers.

Similar analysis was carried o ut on the guyed towersby comparing the Transverse Design Load, the Guy/Mast

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447Design Load and the Mast Bending Mom ent for both highwind loading and tornad o loading. Th e summary of theV-guyed basic tower design and the V-guyed tallest towerdesign evaluations are shown in Tables 6.a and 6.b,respectively.7.0 ANALYSIS OF RESULTS

The 4-leg towers were chosen to cover a spectrumof different spans and tower heights and the analysis wascarried out for the basic structure and for the talleststructure of each tower family. For the basic tower thetornado load effect was within the design values of thehigh wind load, i.e. the basic tower can resist the tornadoload without modification. For the tallest structures, thetransverse load 'due to tornadoes is exceeding thetransverse load due to high winds, which results in extrashear reinforcement for the tower. The overturningmoments due to tornado loads aie within the designvalues. To reinforce the tower to withstand the extratransverse load, due to tornadoes compared to the highwind load, additional tower weight is limited to 2.5%.These comparisons are shown in Tables 5.a and 5.b.

The V-guyed towers were chosen also to cover arange of spans between 350 m to 500 m and the analysiswas carried out for the basic structure and for the talleststructures. The effect of the tornado load is basically onthe design of the mast to resist the local mast bendingmoment and shear.To reinforce the mast to withstand the extra mastbending moment and shear due to a tornado load,compared to the high wind load, additional tower weightis limited to the range of 0.7% - 2% of the basic towersand 2.1% - 2.5% of the tallest towers. The se comparisonsare shown in Tables 6.a and 6.b.

8.0 CONCLUSIONIn addition to the extreme wind loads used in theexisting design philosophy of transmiss ion lines, the towe rsmay be subjecte d to wind loads assoc iated with tornadoe s.The tornado load in On tario has been evaluated based ona literature review and the effect of tornado loading ontransm ission lines has been investigate d. Based on thisstudy, a design basis tornado loading for transmissiontowers is proposed . Th e struc tural changes on the existingtowe r designs, if nee ded , will be limited to an increase intower weight by less than 3%.

REFERENCEST.T. Fujita, "Proposed Characterization ofTornadoes and Hurricanes by Area and Intensity",SMRP Research Paper No. 105, University ofChicago, Chicago, IL, 1971.T.T. Fujita and A.D Pearson, "Results of FF'PClassification of 1971 and 1972 Torn adoe s",Preprints, Eighth Conference on Seve re Loc al Storms,American Meteorological Society, Boston, Mass.,GA. McKay, "Some Weather Hazards AffectingTransmission Line Design", Environment Canada,Atmospheric Environment Services, Downsview,Ontario, 1973.B.R. Collingwood, "Design Basis Tornad o", AtomicEnergy Control Board Ottawa, Ontario, Report No.77271, 1977.M J . Newark, "Canadian Tornadoes, 1950-1979,Atmosphere - Ocean, Vol. 22 No. 3, 1984, pp 343-353.J.T. Schaefer, D.L. Kelly and R.F. Abbey Jr., "AMinimum Assumption Tornado Hazard Model",Joumal of Climate and Applied Meteorology, 25:1934-1945, 1986.ASCE "Guidelines for Electrical T ransmission L ineStructural L oading", ASCE Manual 74, New York,NY, 1991.M.J. Newark, "A Design Basis Tornado", CanadianJoumal of Civil Engineering, Vol. 18, 1991, pp 521-524.

1973, pp 142-145.

Dr. Magdi Zshac, IEEE(M) , ASCE (M) , has 15yearsExperience in the areas of nuclear power generation andtransmission lines design. He is currently a design engineerspecialist in the L ines Projects Division of the Grid Systemof Ontario Hydro. He has a broad range of euperience inanalysis, design, constructability and maintainability oftransmission lines. Dr. Ishac has completed his M.Eng.(Structures) in 1976 and his Ph.D. (Structural Dynam ics &Earthquake Engineering) in 1979from McMaster University,Canada.

H. Brian White, ASCE(M), IEEE (Fellow),Consulting Line Engineer, Hudson, Quebec, Canada, hasover 41 years ex perience in the design arid co nstniction ofoverhead transmission lines operating in 15 countries, atvoltagesfrom 69 kV to 1200 kl/. In addition to the moreconventional designs, he is responsible fo r the design of theGuyed V Structure now used widely throughout the world.He also designed the Cross Rope Suspension Towerinstalled on 3 ofthe 735 VJames Bay lines for Hydro Quebec.

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448I

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449

WIND LOAD O N COND/TOWER &pa)

TORNADO WIND SPEED @"r)TORNADO LOAD ON COND/TOWER (koa)

Table 1MEDIAN VALUES OF TORNADO DAMAGE PARAMETERS BY F-SCALE

F-SCALE

(after Schaefer et a1 19 86)

0 .29 10 .8 I 0.39 1 1 . 1 0.77 I 2 .1 0 .96 12 .6 I 1.15 13. 1F1 F1 I F 2 F2

120 I 1 4 4 192 224 1 2400 1 1.7 I 0 12 .4 0 1 4 . 8 0 1 6 . 0 I 0 1 6 . 5

Table 2

(after Newark 1991)Note:M edian damage area is not equal to median damage width times median damage length

Table 3

1950- 19591960-19691970-1979 3.31950-1979 1.3(after Newark 1984)

Table 4PROPOSED TORNADO LOADING VERSUS EXTREME WIND LOADING I

IEXTREME WIND SPEED (km/hr) I 80 I 96 I 128 I 1 4 4 I 160 I

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