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Dynamic Analysis of TelecommunicationMonopole underWind Loads.................................................................................................................................................................................................

By : Engr. Lum Peng Fatt M.I.E.M., P.Eng andEngr. Jeyananda Chinniah Ahnantakrishnan M.I.E.M., P.Eng

INTRODUCTIONTelecommunication transmission stru c t u rehas been traditionally using lattice spaces t ru c t u re, similar to the electricitytransmission towers. The aggre s s i v eg rowth of the telecommunicationsindustry means an equal surge in thenumber of telecommunications stru c t u re son the urban and rural landscape.H o w e v e r, while “pro g ress is inevitable,ugliness is not”. Monopoles can be usedw h e re aesthetic concerns are of primeimportance and also where usable space islimited. Monopoles can also be disguisedto resemble trees or advertisementb o a rding, where ever- i n c reasing eco-logical awareness concerns have to betaken into consideration, in order top rotect our enviro n m e n t .

Recently, in an attempt to save landspace and to shorten transmissioncommission lead time, steel monopolesare being used frequently.

Being a tall slender cantileverstructure, it is naturally susceptible towind excitation when subjected to wind-attributed loading.

Various wind load codes anddesign standard had been written togive guidelines on the derivation ofwind loading.

This paper discusses the variousa p p roaches in deriving dynamicresponse from the static consideration ofwind, as recommended in each Code.

STRUCTURE AND ANTENNAREQUIREMENTSTwo cantilever monopoles at 30m and45m height were studied in detail. Table 1lists the antenna loading for each polew h e reas Table 2 shows the height atwhich platform is required.

The design wind speed specifiedwas 120km/hr 3-sec gust. For thesimplicity of comparison, no shieldingof the pole from antenna and/orplatform is considered. Te r r a i ncharacteristics, topographic category

and importance of stru c t u re wereselected for urban areas and as far aspossible, to be comparable amongd i ff e rent Standard s .

CODE REQUIREMENTS INWIND LOAD DERIVATIONThe traditional wind code used was CP3:Chapter V Part 2 (5). This was deemednot applicable when one looks at thenatural frequencies of a monopolestructure of the order of 0.6 ~ 0.9 Hz,which was thus excluded as per thecomments given in the Wind LoadingHandbook (6).

F u r t h e r m o re, CP3 had been off i c i a l l ywithdrawn on 15 October 2001, andreplaced by BS6399:Part 2: 1997.

Wind Load to BS6399:Part2:1997This Code placed a limit to itsapplicability to structures with dynamicaugmentation factor, Cr, not exceeding0.25 (Clause 1.6). If one assumes that amonopole structure as a single memberwelded steel unclad frame, thenaccording to Table 1 and Figure 3 of thisCode, the limiting height of applicabilityis only about 20m.

Nevertheless, as stated in Annex C ofthe Code, the wind loading onto thes t ru c t u re may still be derived using the

p rovisions of the Code but the finalresponse of deflection (and stresses) needto be amplified by a factor of (1+Cr), whicha c c o rding to the Code, would be lessaccurate and generally more conservative.

The dynamic augmentation factor, Crfor the two pole heights is shown in Table 3.

The Institution of Lighting Engineers(UK) in its Technical Report Number 7 (ILE-TR7) provides guidelines for the estimationof wind forces on high mast for lighting andCCTV applications.

In this report, the provisions inderivation of wind load according toBS6399:Part 2:1997 were followed andf i n a l l y, to account for the dynamic nature ofthe high mast, a magnification factor calledRESPONSE FA C TOR Beta is applied.

The response factor is a function ofthe natural frequency of the structure, themean hourly wind speed at 10m aboveg round level, and the logarithmicdecrement of damping in the structuralsystem. The response factor variesconsiderably according to thelogarithmic decrement factor. The rangegiven was from 0.1 to 0.4. The reportrecommended that the logarithmicdecrement of damping to be used shall beassumed to be 0.1, unless evidence can bep roduce to justify the use of highervalues. Harris (7) had indicated that the

Diameter of parabolic Distance measured upwardfrom the bottom of tower ( m )

30m Monopole 30m 27m 24m 21m

1.8m 1 no. 1 no. 1 no. 1 no.

45m Monopole 45m 42m 3 9m 36m

1.8m 1 no. 1 no. 1 no. 1 no.

Table 1: Antenna requirement

Ht of monopole Distance measured upwardfrom the bottom of Monopole ( m )

30m 29m 26m 23m 20m

45m 44m 41m 38m 35m

Table 2: Platforms to be provided

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normal range of damping for lightingtowers was 0.01 to 0.18. In view of theserecommendations, there f o re, a value of 0.1was adopted in the present calculations.

BS8100:Part1:1986This Code allows a quasi static orequivalent static method of analysis to becarried out on mast and tower structuresusing mean wind forces and a fluctuatingcomponent that accounts for thegustiness of wind. This code appeared tobe more appropriate to the slender anddynamically sensitive pole structure, as itaccounts directly for the gustiness ofwind, just like the telecommunicationmast and towers.

The Code imposed a limit for theapplicability of the equivalent staticmethod (clause 5.1) that the factor givet h e rein must be less than 1.0. It wasfound that for the cantilever monopoles t ru c t u re, the factor was much morethan 1.0 and hence some form ofdynamic analysis, if not a spectralanalysis, need to be carried out. Sincet h e re is no commonly availables o f t w a re to perform the spectralanalysis, it was proposed to use timehistory analysis to simulate thedynamic response of the monopole.

Dutt (8) proposed a simplified sharp-edge gust with a variation of Vav/4 aboutVav, and having a gradient of 200m.p.h.(89.4m/s) per second. Vav was the meanof ‘five second gust’.

For the present study, the 3 sec gustwind speed was used in lieu of thep roposed ‘5 sec gust’ wind speed.Although, Dutt indicated that thep roposed simplification worked wellwith natural frequencies between 1.7 Hzto 7 Hz, it was none the less adopted hereas one of the method of computingdynamic response of the monopoles t ru c t u res, whose natural frequencies arelower than 1 Hz.

ANSI/TIA-222-G-2005This Code uses the 3 sec gust as thedesign wind speed. Clause 3.6 of the

Code states that in order to account forthe dynamic effects of wind gusts, aseries of wind load patterns arespecified, which are similar to thepatched loading defined inBS8100:Part4:1995-Code of Practice forLoading on Guyed Mast (9).

For monopole stru c t u res, there willonly be one single load pattern, viz.,full loads throughout, that need to beanalysed. It must be pointed out that ino rder to comply with this Standard ,Clause 3.5 concerning P-Delta eff e c ta p p e a red to be mandatory since theheight to face width ratios is certainlyg reater than 10 for the two monopoles t ru c t u res considered here .

Nevertheless, it was found that forthe present assumed antenna andplatform loading, the second order orP-Delta effect was small.

COMPARISON OF DIFFERENTCODES/STANDARDWhile the British Standards set out aclear criterion for determining whetherthe stru c t u re is dynamically sensitiveby the limiting Cr factor in BS6399:Part2 or Equivalent Static limit factor inBS8100:Part1, the final methodadopted for dynamic analysis re s t swith the designer.

It is the authors’ opinion that theAmerican Standard TIA-222-G-2005a p p e a red to be silent in this aspect,although it has a pole gust factor of 1.10(clause 2.6.7.3) and a higher load factor(for strength analysis) of 1.6, which ishigher than 1.44 for BS8100:Part 1 or 1.4for ILE-TR7/BS6399LPart 2.

Incidentally, it was felt that the use ofwind velocity factor of 1.2 in BS8100,which results in a load factor of 1.44, is

Table 4: Dynanic factors for 30m and 45m monopole structures

Ht. of Natural Frequency Response Factor Dynamic Ratio (1+Cr)/BetaMonopole Beta Augmentation

(1+Cr)

30m 0.918 1.330 1.638 1.23245m 0.605 1.421 1.880 1.323

Table 5: Analysis results for base moments for 30m high monopole using different method of analysis

Method of Moment at ULS Ratio (as Compared Ratio (ComparingAnalysis with Lowest) Dynamic Analysis

Results)

BS6399:Pt2/ILE-TR7 1636kNm 1.46 1.23

Dynamic analysis 2091kNm 1.86 1.57using Time-HistoriesAnalysis

ANSI/TIA-2220G-2005 1330kNm 1.18 1.00

Equivalent Static 1123kNm 1.00 -Method BS8100:Pt1

Table 5: Analysis results for base moments for 30m high monopole using different method of analysis

Method of Moment at ULS Ratio (as Compared Ratio (ComparingAnalysis with Lowest) Dynamic Analysis

Results)

BS6399:Pt2/ILE-TR7 1636kNm 1.46 1.23

Dynamic analysis 2091kNm 1.86 1.57using Time-HistoriesAnalysis

ANSI/TIA-2220G-2005 1330kNm 1.18 1.00

Equivalent Static 1123kNm 1.00 -Method BS8100:Pt1

Ht of monopole Distance Augmentation Factor, Cr

30m 0.638

45m 0.880

Table 3: Dynanic augmentation factor, Cr

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more rationale than the simplefactor of 1.4 as proposed in ILE-TR7 and as such in the presentanalyses, a velocity factor of 1.2was adopted for both BS8100and BS6399/ILE-TR7 methodsof analysis.

RESULTS OF ANALYSISThe computed natural frequen-cies and response factors Betaa c c o rding to the re c o m m e n-dations of ILE-TR7 for the twomonopoles are listed in Table 4.A comparison with the dynamicaugmentation according toBS6399:Part2:1997 is alsop re s e n t e d .

F rom the table, it can beseen that a direct magnificationof static moments by (1+Cr) isconservative as per thecomments in Annex C ofBS6399:Part 2:1997.

Figures 1 to 6 shows thetypical structural model of themonopole in finite element,loading and moment distri-bution along the height of thepole and mode shapes ofdeflection.

The results of analysis forthe 30m and 45m monopoles t ru c t u res are present in Ta b l e5 and 6 re s p e c t i v e l y. Theresults for equivalent staticanalysis from BS8100:Part1 isalso included for comparison.

From the above results, thefollowing can be noted:(i) Equivalent static method

BS8100:Part1 under esti-mate the base moment.

(ii) Time history analysis usingthe arbitrary velocityvariation of +/-Vav/4 appeared to beover-conservative.

(iii)Although A N S I / T I A - 2 2 2 - G - 2 0 0 5results were lower than thatcomputed using BS6399:Part2:1997with magnification Beta factor, theoverall results appeared to be withinpractical limits and could bec o n s i d e red similar, in view of thewide ranging assumptions inderiving various factors and windresistance of the pole and antenna.

CONCLUSIONDynamic response of two cantilevermonopole stru c t u res of height 30mand 45m had been computeda c c o rding to various methods andrecommendation by the British andAmerican Standards. Despite the factthat each Standard define its ownmanner in accounting for windp re s s u re computations for stru c t u r a lelements and appurtenances, theresulting base bending moments

a p p e a red to be fairly similar to eacho t h e r, with the American StandardANSI/TIA-222-G-2005 being lower byabout 20% as compared to the BritishS t a n d a rd BS6399:Part2:1997/ILE-TR7re c o m m e n d a t i o n s .

The use of simplified wind velocityp rofiles based on Vav +/- Va v / 4p roduce the largest base moment at thelowest mode of natural fre q u e n c y,which was as much as 50% higher thanestimation by Code method. ■

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Figure 1: Finite element of monopole structure withplatform

Figure 2: Loading application onto pole structure

Figure 3: Distribution of moment with pole height Figure 4: Dynamic moment response of pole

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Figure 5: First mode of deflection of pole undertime-history loading

Figure 6: Second mode of deflection of pole undertime-history loading

REFERENCES

[1] BS6399:Part 2 : 1997 BritishStandard for Loading forBuildings, Part 2: Code ofpractice for wind loads

[2] HIGH MAST FOR LIGHTINGAND CCTV The Institution of LightingEngineers (UK), Technical ReportNumber 7 Specification fordesign, manufacture, assembly,erection, painting, testing andmaintenance. (2000 Edition,Amended and reprinted 2003)

[3] ANSI/TIA-222-G-2005 TIA STANDARD, StructuralStandard for Antenna SupportingStructures and Antennas,Effective January 1, 2006,Telecommunications IndustryAssociation (TIA) August 2005

[4] BS8100:Part 1 : 1986British Standard for LatticeTowers and Masts, Part 1: Codeof practice for loading(Superseded and replaced withBS6399:Part 2:1997)

[5] CP3: Chapter V Part 2:1972Code of Basic Data for the designof buildings, Chapter V. Loading,Part 2. Wind loads(Superseded and replaced withBS6399:Part 2:1997)

[6] Wind loading handbookC.W. Newberry and K.J. Eaton,Building Research Establishment (1974)

[7] Harris, R IOn the spectrum and auto-c o r re l a t i o n function of gustinessin high winds.Electrical Research Association TR 5273

[8] Dutt, A JAn Approach to theSimplification of the DynamicCharacteristics of Wind Loadingon Tall Buildings.Proceeding, Conference on TallBuildings, Kuala Lumpur,Dec 1974

[9] BS8100:Part 4 : 1995British Standard for LatticeTowers and Masts, Part 4: Codeof practice for loading of guyedmasts

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