BS8100-Appendix a-Measurement and Interpretation of Wind Data

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BS 8100-1:1986

Appendix AMeasurement and interpretation of wind dataA.I Measurement and calibrationA.I.I Site for reference measurementsReference measurements of wind speed for use with this part of BS 8100 should be made at sites and inlocations such that they may be reliably interpreted. A suitable site for records should be:

a) in open terrain of category I, II or III as defmed in Table 3.1;b) not closer than 2 km to hills or mountains of height greater than 100 m above the level of the site.

A.l.2 Calibration of siteThe wind flow characteristics for all directions of wind should be determinedroughness factor KR may be derived from the relationships plotted in Figure A.l using the ratio:

is the maximum gust velocity of short duration, averaged over t seconds;velocity for the

VtVwhere

VtV is the mean hourly

These characteristics should beWhen calibrations such as these have not been carried out,the terrain characteristics of category III (see Table 3.1).for compatability with the observed or expected terrain characteristics.A.l.3 Location of measuring instrumentThe instrument used for speed measurement should be mounted atground level and well clear of obstructions interfering with the naturalmount the instrument at some greater level, records of theequivalent values appropriate to the 10 m level inA.l.4 Response and averaging periodThe response characteristics of the measuring instrument and the recording equipment should be knownand the effective averaging period for the recorded speeds defined. When this period is unknown, it shouldbe assumed to be 2 s. A procedure for determining the averaging period is given in A.2.Measurements of maximum speed over the effective averaging period for an instrument may be convertedto equivalent mean hourly speeds, 'iT, by reference to Figure A.l once the site has been calibrated. Whenrecords have been analysed to give equivalent mean speeds, Vt, averaged over t seconds at 10 m height,these may be converted to mean hourly values by division by the ratio Vt/Vappropriate to the value of KRfor the site of the instruments given in Figure A.l.A.l.5 Statistical interpretationThe maximum value of mean hourly speed at Df 10 m above grounddetermined, independent of direction, for each shoulddetermine the 50 year return maximumtheir coefficient of variation v assuming anat least 15 consecutive years of records are available. Where fewer records exist,determined but the coefficient of variation should be taken as 0.2.When predicting the extreme mean speeds of hurricanes or typhoons, referencerecords of at least three stations in the region concerned to ensure that thefollows the Gumbel (Fisher-Tippett type 1) form.

for the site. The terrain

speeds and directions as possible.recording station should be assumed to haveKR thus obtained should be checked

a level of at least 10 ill above theair flow. When it is necessary tospeeds should be converted to

should bebe analysed tomean of these maxima and(Fisher-Tippett type 1), whenthe mean value should beshould be made to theextreme value distribution


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BS 8100-1:1986

)1(/~;;:;:11ry~" 1" .-T/ ""I t/4.S' /C ,/ / fil f Ierrain roughness category ~ ;frOu.:2' N m II I2.2

2.12.01.91.8 [,6/1; z /.o,Zl1.7 'I"~ ("1,)1.6Vt ...

V 1.5 t (seconds) / L'1 d,. "., j4./ t7 < ""-'1.4 35101.3 3060 I..,.",h1.21.1 600 ,0"""4/..,(1.0 /11.41 ,--- L.0.6 0.7 0.8 0.9 1.0 1.1 1.2 ;I

KRFigure A.l- Derivation of terrain roughness factor, KR, from wind speed records

A.1.6 Application of results to other sitesIn applying the values of the mean hourly speeds derived in accordance with A1.l, A.1.2, A1.3, A.1.4and A.1.5 to sites other than those in the observations, due allowance should_be made for any differenceswhich may exist in altitude and surface roughness at the site. The values of VB given in 3.1.2 are relatedto an equivalent open inland site having a terrain roughness of category III (see Table 3.1). When the siteof the records has any other roughness category the values ofVmax,derived in accordance with A.1.5 should "-be divided by KR to obtain the basic wind speed, VB, to be used in 3.1.2.A.1. 7 Velocity profileThe index of variation of mean wind speed with height, may be taken to be related to the terrain roughnessfactor KR in accordance with Figure 3.3.A.2 Averaging period of an anemometerThe averaging period of an anemometer may be considered as the least duration of gust that can be reliablyrecorded by the instrument. For a rotating cup instrument, the averaging period should be taken as twicethe time constant, Ti. The time constant, Ti, should be taken as the sum of the individual time constants ofthe instrument, Til, and of its associated recorder, Ti2'The time constantTil of the instrument should be obtained from that calibrated in a steady air flow, Tib hisbeing equal to the time taken for the speed of rotation of the cups to reach 63 % of the maximum speed ofrotation starting from a stationary condition in flow of velocity Vb. For any other velocity, V, the timeconstant may be taken as:

Til = Tib (~The time constant for the associated recorder, Ti2,may be found by applying a voltage step to the inputterminals when the constant should be obtained as the time taken to record 63 % of the full voltage.


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- BS 8100-1:1986

A.3 Advisory and references sourcesA.3.1 GeneralFor interpretation of reference measurements and procedures for calibration of site wind speeds advicemay be obtained from sources given in A.3.2 and A.3.3.A.3.2 Address of the advisory service of the Building Research EstablishmentThe Advisory ServiceBuilding Research StationBucknalls LaneGarstonWatfordWD27JRTel: 0923 67 6612A.3.3 Addresses of advisory offices of the Meteorological OfficeFor England and Wales:

Meteorological OfficeMet 0 3London Road c~::~~::~G 12 2SZ I;.~~~,.cTel' 0344 420242 Extn 2299

FO~~:'::~calffice'231 Corstorphme Road t"'"~~f.~~.~"'4W\""

Appendix BTerrain parametersB.1 Terrain roughness parameter,The terrain roughness parameter, zo, s a measure of theon the wind speed near the ground. It is a parameter thatover the range of terrain found in nature. Forcategories, each with a given value of Zo,andgiven in Table 3.1. recognizable terrain type, asThe values given are average values applicable to sites of comparatively uniform roughness where thegeneral description of the terrain applies for a distance of at least several kilometres upwind. A sixthcategory with Zo= 0.8 m is referred to in other documents corresponding to city-centres, comprising mostlyfour-storey buildings or higher, typically between 30 % and 50 % of the terrain area for several kilometresupwind. Since almost no urban or suburban area exists in the UK of sufficient extent to enable the fullbenefit of the slower equilibrium profile to be obtained, this category VI should not be used as a basis fordesign, unless special studies of the wind profile at an urban site are made. For this reason category VI hasnot been included in Table 3.1.

'~o .~~ci.r,"" ;'" ~~,. ~"' ,-,'..T 0"1 ""~~ that the surface in question haschanges by more than two orders of magnitudethis range is divided into five


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There is a continuous gradation of aerodynamic roughness in nature, but these tabulated values should besufficient to define the character of a site. If the roughness of a particular site appears to lie between twoof these categories, then an intermediate value of Zomay be assumed; however, the adoption of the lessrough category will ensure a conservative result.The meteorological standard, or basic terrain in the UK, corresponds to the category of aerodynamicroughnessZo= 0.03 m. Even though the anemometersmay be on airfield sites where the aerodynamicroughness is in the range 0.003 m < Zo< 0.01 m, the extent of this lesser roughness is small compared withthe very much larger extent of the surrounding typical UK farmland, and not sufficient to modify the windparameters.The aerodynamic roughness of a large expanse of water is a special case, since the water surface respondsto the wind by becoming rougher with increasing wind speed. The available data on wind/wave interactionhave been correlated to produce the approximate relationship:Zo water) = 4.9 x 10-6 viwhere

VB is the basic mean hourly wind speed (in mfs).Giving:

Zo =0.002 for VB =21 mfs;Zo =0.004 for VB =28 mfs;Zo= 0.006 for VB = 35 mfs.

The value of Zo= 0.003 m for terrain category I corresponds to the middle of the range of UK mean hourlywind speeds, VB, and would produce slightly conservative results for sites with higher basic wind speeds.It should therefore be sufficient for design at coastal sites in the UK. It should be emphasized, however,that this code is not applicable to offshore sites where account should be taken of the appropriateturbulence characteristics and effective heights to determine the relevant wind structure.B.2 Effective height of surface obstructions, heThe flow between obstructions near ground level is very complex and no prediction of wind speed can bemade with any certainty. The relationships given in this code apply only to heights above the general levelof the surrounding obstructions and it is necessary to define a displacement plane to which all heights arereferred. The height, he, of this zero plane above the ground is generally less than the general height, h, ofsurrounding buildings.For open-country terrain where Zo ~ 0.03, the zero-plane displacement is not significant and may be takenas zero. Values of the zero-plane displacement which are typical of the rougher terrain categories in the UKare given in Table 3.1.Appendix CGradient wind speedsC.l GeneralWhere meteorological records are not available, or the nearest meteorological stations are remote from thesite, gradient wind speeds, Vg , may be used as a basis for deriving the site reference wind speeds, V , asdetailed in 3.1.5 in accordance with C.2 and C.SoC.2 Gradient wind speedThe gradient wind speed, Vg, should be obtained from official Meteorological Office wind maps of theaverage wind speeds over the period of 1 h, independent of direction, at the top of the boundary layer of airflow above the site of the structure, and having an annual probability of occurrence of 00020 n approximatemap for the UK is shown in Figure Co , based on an assumed gradient wind height of 900 mo


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C.3 Site reference wind speedThe site referencewind speed o be used forspeed,VB, derived rom:VB =Kg VggiVIng:

Vr =Yv~KRKgVgwhere

Yv is the partial safety factor on wind speed to be obtained from Figure 2.1, appropriate to thestructure (see 3.1.5);Kd is determined in accordance with 3.1.3;KR is determined in accordance with 3.1.4;Kg is a gradient wind speed re~distance upwind of the siteV is the gradient wind speed,

design, V , should be based on a value of the basic wind

type of


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BS8100-Appendix a-Measurement and Interpretation of Wind Data

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