Glossary of Wind Terms Used in Construction

7/28/2019 Glossary of Wind Terms Used in Construction

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Loss Prevention Council

Glossary ofWind Terms

used inConstruction

In association withthe WindEngineering Society

Edited byPaul Freathy, PF Consultants

Julian Salt, Loss Prevention CouncilLPR 15: l !

I D N D R1 9 9 0 - 2000suudhp I C u b Pmumlh

99

-ENGINEERINGWINDSOCIETY


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Glossary of Wind Terms

FOREWORD

During the1990s the United Nationshas been promoting the InternationalDecadeofNatural Disaster Reduction (IDNDR) as ameansbetter tounderstandandmitigate the effects of extreme natural hazards. As part of its effort in this area theUK Wind Engineering Society (WES) and the Natural Perils Advisory Committeeof the LPC became aware of the gap that sometimes exists between the providersand users ofechnicalnformation.This an lead to xpert advice eingmisinterpreted so that wrong actions are taken and can increase the risk of damage,and injury. As astep owardsbridging his gap, the Wind Engineering Societydecided to prepare this glossary of erms used inwind engineering. The LossPrevention Council kindly agreed to provide a large part of the funding to rite andpublish the document.As the title suggests, the terms contained within are mainly wind terms related toconstruction. It is not a meteorological reference, nor is it a extbook on windengineering. The intention is to define common terms hat engineers, designers,insurers, local authorities and others may come cross in the context of wind actionon structures.Theseerms xtendrom ome meteorological terms,hroughengineering ermsdescribingstructural response, to some of the terms used todescribe building elements. The definitions have been written in a way that tries tobalance technical correctness with the need to convey understanding to those notimmersed in this subject. We hope we have got the balance about right but ask foryour understanding where you may disagree. We would welcome feedback fromusers so that this document may, in the future, be revised and improved.Finally, as editors we would like to thank all those in insurance companies and inthe WES who gave freely of their time to read through the early drafts and correctthe most glaring of the errors. Any that remain are ou r fault not theirs.Paul Freathy, PF Consultants Julian Salt, Loss Prevention CouncilChairman,WES Working Group on ID NDR Manager, Natural Perilse-mail: [email protected] e-mail: [email protected] 1999

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Glossaty of Wind Terms

ACKNOWLEDGEMENTS

The following deserve thanks for contr ibutions to earlier drafts of the glossary:1.John Pelican and Ray Facer: Legal and General2. DavidCrichton:CommercialGeneralUnion3. Alan Aldridge: CommercialGeneralUnion4 . JimBatten: Royal & SunAlliance5 . Members of theWind Engineering Society includingDick Barnard, Chris

Baker, Stephen Ledbetter, Brian Lee and Tom Wyatt.

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Glossay, of Wind Terms

ACCELERATION DUE TO GRAVITYTh e standard acceleration of an object due to theforce of gravity is 9.81m/s2 at sealevel which equates to a force of 9.81N/kg.AERODYNAMIC INTERFERENCEThis describes the effect that onebuilding or structuremay have on another. Strictly itseffect can be both beneficial and adverse but it is most commonly used when the effectworsens the load or response on the other structure. Examples include wake buffeting,funnelling and enhanced dynamic response. A beneficial example would be shelter.AERODYNAMIC RESPONSEThe response of a structure s generally used o describe the motion that it akes underthe action of wind loading. This may be a static response, where the structure deflectsunder a steady load and stays in that deflected position until the load is changed orremoved. It may also refer to a dynamic response where the deflection of the structurechanges continuously with the short: term fluctuations in the applied loading.AEROELASTIC RESPONSEThis is a particular example of a dynamic response where the behaviour of hestructure as it responds to he load modifies the effect of the wind leading to achanged, often more severe, response. The famous Tacoma Narrows Bridge failure isa good example of an aeroelastic response. Some types of aeroelastic response lead toan increasing response until failure occurs, while others reach a limit. Even a limitedresponse can cause problems due to fatigue damage.Aeroelastic behaviour can also lead to aerodynamic damping which limits response- see Damping.AEROFOILAn aerofoil is another word for a body shaped ike an aircraft wing.AIR DENSITYThe standard density for air in the UK, in SI units, is 1.23kg/m3 at sea level. Airdensity varies with temperature, pressure and altitude. It can be significantly lowerat highaltitude sites. This will affect the loads on structureswhich are giventypically by F = /ZpV.C,.A where p is air density, V is the wind speed, C, is a non-dimensional pressure coefficient and A is the area of the structure or element.SeeUnitsAIR PERMEABILITYAir permeability is used to describe the leakiness of a structure or element. Thehigher the permeability he easier it is for air o pass through he structure. Forexample, an array of interlocking roof tiles would tend to have a relatively highpermeability whereas a solid brick wall would not.ALTITUDEAltitude describes the heightofaparticular site above a datum level. It is notgenerally used to describe the height of the structure above the local ground level.Usually alti tude is given in metres above mean sea level (mAMSL).ALTITUDE FACTORThis is the factor used in the British Standard for wind loading (BS 6399-2) toaccount for the fact that wind speeds increase with altitude. The altitude factorincreases basic wind speeds by 10% per 1OOm altitude. In the Standard, the altitudefactor is in fact a combined altitude and topography factor thatlso accounts for theeffects of local ground slopes that may accelerate the wind speed.

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Glossary o f Wind Terms

ANEMOMETERThis is a device used to measure wind speed. There are several different types ofanemometer, each with their own characteristics:

cup anemometer - the most common type, uses shapedcups, usuallythree, connected by horizontal arms to a central axis. The speed of rota-tion is the measure of wind speed. This type of anemometerhas a degreeof inertia related to the size and weight of the cups which means that itcannot respond quickly enough tomeasure very short gusts (typically lessthan 1-3 sec).rotating vane anemometer - similar to a cup anemometerusing propeller-style vanes ather than cups, measuring air low parallel o the xis of the rotor.ultrasonic anemometer - expensive and highly accurate anemometer, usedfor research, that sends ultrasound waves between electrodes. The timetaken for the wave to traverse the distance between the electrodes is affectedby thewind speed and this is used as the measure. Normally, eachanemometer has three pairs of electrodes set at different angles so that allthree components of wind speed can be measured.hot wire, or thermal, anemometer works by passing a current through awire or film and adjusting the current tomaintain a constant temperaturein the wire as the passing wind cools it . The current drawn is a measureof the windspeed. Mostly used in laboratory and other test applications.laser Doppler anemometer (LDA) - works by reflecting laser light backfrom particles in the airstream. Mostly used in laboratory applications.

ANTI-CYCLONEAn anti-cyclone is a region of high atmospheric pressure (marked as HIGH on aweather map). In the Northern hemisphere the winds tend to rotate clockwise aroundthe centre of the high pressure. In the Southern hemisphere this is reversed.

Ant i -cyc lone

ASPECT RATIOAspect ratio is commonly defined as the ratio between two orthogonal dimensionsof an area - normally the longest divided by the shortest. So, for a tall building, theaspect ratio would be height + breadth. It is often used as a way to describe therelative ease with which air can flow around thesides and endsof the structure.Forstructureswithonlyone free end most buildings) it is morecorrect to use(2 x height) +breadth but this definition is not always employed.ATMOSPHEREA generic term used to describe the air and weather systems around heearth.Sometimes also used as a descriptionof pressure - for example, one atmosphere woulddescribe a pressure as being equal to that exerted by a columnof mercury 76cm highat 0C under standard gravity; i.e. at sea level.

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ATMOSPHERIC BOUNDARYLAYERThe region of the atmosphereclosest to thesurface, where the windspeed is reducedand turbulence is increased due o surface obstructions. This is what causes thevariation of wind speed with height, the wind profile.See alro Velocity profileATMOSPHERIC PRESSUREThere is a lot of air above us in the atmosphere and this exerts a pressure on everythingcalled atmospheric pressure.Although we are nor consciousf this pressure t always existsand can be measured by barometer. It is the value of pressure given on weather maps.Standard atmospheric pressure is 1 O 13bar = 1O 13 x 1 05Pa= 76Omm of mercury= 10.33m of water = 14.696psi.See UnitsATMOSPHERIC STABILITYThe atmosphere is said to be stable if when a parcel of air is displaced its temperaturerelative to its new surroundings is such that t will tend o return o its originalposition. For example, this would be the case if after an upwards displacement theparcel of air was colder than its surroundings and would therefore sink back to itsoriginal position. When there is no tendency for the displaced air either to continuemoving up or toreturn to its original position the atmosphere is said to be neutrallystable. This is generally the case in strong winds whenhere is so much mixing of theair through turbulence that the temperature profile with height is uniform - at leastwithin the region of interest close to the ground.ATTACHED FLOWThis erm is used to describe the typeof flow thatwould be seen around astreamlined body, such as an aerofoil or other smooth shape. It implies that thewind is passing smoothly over the body without separating and without causinglarge amounts of turbulence in thewake of the body.

AVERAGING TIMEFor time varying quantities, such as wind speed, it is important to specify over whattime period a meanvalue is averaged. The longer the averaging time, the more likelyit is that the shorter duration peaks of wind speed will be less significant and theresulting wind speed quoted will be lower. Commonly used averaging times in windengineering are one hour (for so-called mean wind speeds) and 1-15sec for gustspeeds, depending on the size of the structure being considered. Note that meanspeeds are also sometimes quoted as 10-min averages (especially in Europe). There isonly a small difference between 10-min and 1-hr averages typically. In the past theUSA has used an alternative definition of wind speed - the Fastest mile wind speed.See Fastest mile wind speedAZIMUTHTh e direction of the wind or of the buildingelative to north.

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BACKINGA meteorological term used to describe an anticlockwise change in wind direction- e.g. from northerly to westerlyBAROMETERAn instrument used to measure atmospheric pressure.BAROMETRIC PRESSURESee Atmospheric pressureBATTENA piece of timber commonly used in roofing construction which is fixed to therafters and runs parallel to the ridge and eaves. Tiles and slates are hung and /ornailed on to the battens.BATTEN SPACEIn tiled or slated roofs, the gap between the outer roof covering, hanging on thebattens, and the inner layer.BEAUFORT SCALEA method ofdescribing wind speed that originated from observations of theeffectof the wind on different tems.Nowadays here are specific wind speed bandsascribed to each Beaufort Force.

Force 0: less than 1 knot (32.4m/s)

BERNOULLISLAWIn wind engineering, for smooth, steady flow a simplified form of Bernoullis Lawstates that the sum of the tatic pressure and the dynamic pressure (=total pressure)remains constant between positions (if there is no significant change in height). Thisis approximately true for many wind applications, except in turbulent flows behindbuildings or otherroughness.With zero wind speed the totalpressure equals the staticpressure (in this case the atmospheric pressure). When the wind blows the dynamicpressure (= / l pV) ncreases so the static pressure must reduce. This effect explainswhy there are suctions on building surfaces where the local flow is accelerating.

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BLUFF BODYThe opposite of astreamlined body.Au does not pass smoothly around a bluff bodybut will separate at some point, eading to areas of reversed flow and high turbulence.Most buildings and structures are bluff bodies, often with sharp edges. Separation,when it occurs will always take place at those sharp edges, which makes the behaviourof the windery predictable and independent of the windpeed. This is an importantreason why results from wind tunnel models can be scaled up to full size.BOUNDARY LAYERAs the wind flows over a surface the roughness of that surface causes the air close toit to slow down. Eventually, this will cause the flow to separate from the surfaceleaving a boundary ayer beneath which the flow may be reversed and turbulent. Thishappens on all surfaces, including the earth's surface where the roughness of trees,buildings and other obstructions helps to create the atmospheric boundary layer.

BUFFETTINGBuffetting describes the time varying loading of structure by a turbulent wind. Innatural wind there will normally be no dominant frequency so that little dynamicamplification of the response occurs. However when the turbulence is caused ormodified by the wake of an upwind obstruction there may be significantly moreenergy at frequencies close to those of the downwind structure. Thiss called WakeBuffetting and i t may cause a significantly amplified response, especially when theupwind and downwind structures are similar in size and shape.BUILDING DIMENSIONSThe dimensions described below have specific meanings in the British Standard forwind loading (BS 6399-2) in addition to their more common general meaning.

HeightH) : Th e heightfheuilding, or partf auilding,above its local surroundings.

Length (L): The longest plan dimension of the building.Width (W): Th e shortest plan dimension of the building.Downwind length (D): The plan dimensionof hebuilding n hewind

direction.

m WandDL and B

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Crosswindbreadth (B): Th e plandimensionof hebuildingperpendicularto the wind direction.

Effective height (H e) : The heightof the building, or part , adjusted for theeffects of shelter from upwind obstructions (whereapplicable).

Diagonal dimension (a): A dimension taken diagonally across that part of astructurewhich is contributing o he load effectbeing calculated. This is used to allow for the effectof the spatial correlation of gusts across the surface.

BUOYANCYThis describes the tendency of air or other fluid to rise relative to its surroundings.For example, air warmer than its surroundings has positive buoyancy and tends torise, whereas colderair has negative buoyancy and ends osink.This is mostimpor tant in assessing the behaviour of chimney plumes or ventilation systems.

CALMRefers to a Beaufort Scale Force 0. In tables of wind speed from the MeteorologicalOffice it refers to mean wind speeds of less than 1 knot.CDFSee Cumulative distribution functionCFDSee Computational fluid dynamicsCILLA horizontal structural framing member at the base of a curtain wall and usuallysupporting the lowest run of glass areas, windows, panels or doors.COLD FRONTA front is described as a cold front when, as i t passes any ground location, colderair replaces the warmer a ir previously there. The usual symbols for a cold front ona weather map are solid triangles shown along the line of the front . They pointtowards the direction the front is moving. As the front passes a location winds willchange direction - always veering (e.g. from west to north).

Isobars*Cold frontCOMFORT CRITERIAWhen assessing the suitability of the wind environment around a development it isnecessary to set some guidelines to define what is acceptable - so-called comfortcriteria. There are many different sets of criteria in use but thebest will include windspeed ranges that take account of gustiness and define limits for different situations.For example, stronger winds can be tolerated if people walk briskly through an areathan if they are sitting down. Comfort riteria must also be flexible enough to reflectthe location. In warm climates some wind is desirable to keep temperatures down.

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See also Win d environmentCOMPUTATIONAL FLUID DYNAMICSComputational Fluid Dynamics is often abbreviated to CFD. It is a generic title forcomputer models that seek to predict the behaviour of fluid flows around andthrough structures. At the t ime of writ ing hese models are developing rapidly andtheir ability to deal with complex flows is improving. However, there are limitationson the accuracy of results, especially in complex turbulent flows such as wouldoccur around typical sharp-edged buildings.As with all forms of esting and analysis, the accuracy of results depends on the killand experience of the user. Potential users are strongly advised to seek expert helpto ensure meaningful results.CONVECTIONConvection, in this context, efers to the vertical motion of air caused by heating atground level or cooling due to precipitation. Convective activity in the atmospherecan give rise to higher gust wind speeds at ground level than would be expectedfrom the average wind speed existing at a site. The most striking examples of thistype of weather are thunderstorms.CORNER VORTEXWhen thewind strikes the corner of a building it must divide and pass around and overthe building. This can give rise to strong vortices on the roof, leading away from thecorner. These vortices can be particularly strong where the roof is flat or of low pitch.Strong rotating windspeeds in thevortices create very strong suctions (seeBernoullisLaw). These suctions can be the main design consideration for flat or low pitch roofs.

COUNTERBATTENSCounterbattens are used on a roof where the rafters have been covered - for exampleby insulation or sarking board. They are usually strips of timber laid from eaves toridge and normally fixed through to the rafters beneath. They are required to beable to fix the battens while still retaining a clear drainage path for any water thatmay occasionally get through the main roof covering.CRITICAL VELOCITYThis term can have a range of applications but most will refer to a wind speed atwhich some serious response will occur in a structure.A good example would be thewind speed at which the vortex shedding frequencyequals the structural frequency,leading potentially to large amplitude oscillations.See Strouhal numberCROSSWINDCrosswind describes a building response in the direction across (or perpendicularto) the wind direction.CUMULATIVE DISTRIBUTION FUNCTIONA probability distribution that escribes the likelihood of occurrence for vents thatare less than a particular value.CURTATN WALLINGA form of wall construction that only bears the loads directly acting on it and not anyof the other structural loads such as floor dead loads etc. It is normally supported fromthe primary building structure at each floor evel. Different types of curtain wall include:

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Stick wall - a curtain wall assembled at site from aluminium (normally) profiles toform a grid that carries glazing and opaque infill panels.Unitised wall - a curtain wall pre-assembled into units of storey height and one or twoglazing units in width.It is supported from the floors of the primary building structure.Panelised wall - a curtain wall pre-assembled into panels the size of a structural bayand supported from the columns f the primary building structure.CYCLONEA region of low atmospheric pressure, marked with the word LOW on a weathermap. Winds rotate anti-clockwise around a cyclone in the northern hemisphere.Cyclones tend to produce thetrongest wind storms in Europe. ropical cyclones area particular form of cyclone, more severe than in the temperate regions.See also Tropical cyclone

Cyclone or depression

DAMPINGDamping describes the ability of a structure to absorb energy and thereby to controlany large response to a dynamic oad. Examples that contribute to damping aremicro-cracks in materials that lead to rubbing between surfaces creating heat ormovement in, for example, bolted connections.Damping is most often quoted either as a damping ratio (usually written as 5 oras 6) or as a logarithmic decrement (6). Both parameters describe the speed atwhich the oscillations of a structure wouldie down f left to do so naturally. Criticaldamping is said to be present if a displaced structure returns to its rest position withno oscillation and no overshoot. Typical values of damping ratio are a few percentof critical. For such low values of damping ratio there is a direct relationship withthe logarithmic decrement: 6=27~5 or 27~5).DENSITYDescribes the mass per unit volume of amaterial - for the value for air, seeAit density.May also describe he coverage of buildings in an area (plan area density) where it wouldbe in the form of the ratio of the total plan area of all buildings to the overall land area.DEPRESSIONSee CycloneDESIGN RISKThe design risk describes the notional probability that failure will occur. It will be acombination of the probability of an extreme load occurring and the probability ofstructural resistance being inadequate for that load. The notional risk should varyaccording to the type of structure as well as the economic and social consequences offailure. Typical target valuesn the UK range from about 10-3 er annum to 10-6per annum.DESIGN WIND SPEEDThe standard design wind speed for buildings in the UK has an annual probability

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of exceedence of 2% er annum. That s, there is a 2% chance of the specified speedoccurring each year. Other values of exceedence probability are used in particularfields of activity; for example, 0.83% per ann um for bridges and 0.01% per annumfor nuclear installations.See also Return periodDIVERGENT OSCILLATIONA divergent oscillation is one in which here is no natural limit to the magnitude ofthe oscillation. If not checked by removal of the load or other restraint, a divergentoscillation will ultimately lead to structural failure.DOMAINThe area being used in a computational model o represent the whole area (orvolume) of a flow problem.DOMINANT OPENINGThis term is used in the British Standard BS 6399-2 to describe an opening in abuilding that, by virtue of its large size relative to all the other openings, cracks andgaps, greatly influences the pressure inside the building.DOUBLE F A W EA multi-layer wall typically of glass construction. A deep cavity is provided behindthe outer layer to allow ventilation through openings in the inner ayer.DOWNBURSTA strong downdraft resulting in an outward burst of damaging windsn ornear theground.Downburst winds anproducedamage imilar to a trong ornado.Although usually associated withhunderstorms,downbursts anoccurwithshowers too weak to produce thunder.DOWNSTREAMIn the direction that the wind is blowing.DRAGDescribes a force in thedirection of the oncoming wind.The term drag comes fromaeronautical applications where the oncoming flow is the result of the forwardmotion of the ircraft so that the dragorce tends to holdback that forward motion.

DRIVING RAINTh e combination of wind and rain tending to rive the rain on to a building facadeand potentially into any gaps or cracks in thesystem. Prolonged exposure to drivingrain may also cause long-term soaking of masonry or other claddings with hepossibility of water migrating through the building interior.DYNAMIC AUGMENTATIONTh e enhancement of the response of a structure dueto structural characteristics ordue to interference from other structures.

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Glossary of Wind Tprms

DYNAMIC HEADAnother way of describing dynamic pressure. When described as a head it may begiven in units related to the height of column of some luid - for example, inchesof water or mm ofmercury.See Dynamic pressureDYNAMIC PRESSUREThe pressure that is felt when in a moving airstream. Normally defined as q =/2pV2,where p is the density of air (kg/m3) andV is the wind speed (m/s) givingq in N/m2 orPa.See Units, Head, Dynamic headDYNAMIC RESPONSEA response where the motion of the building caused by the wind varies significantlyover short timeperiods. The dynamic response can be greater than would be xpectedfor a similar static load, especially near the natural frequency of the structure.DYNAMIC STRUCTUREA structure whose characteristics willend to esult in a dynamic response to wind loading.EAVESThe lower (horizontal) edge of a roof, parallel to the ridge line.

EKMAN SPIRALDescribes the change of wind direction that occurs with height above ground as aresult of the diminishing nfluence of surface friction and the continuing nfluenceof pressure gradients.EXTERNAL PRESSUREThe pressures that exist on the outside surfaces of a building. These will normallybe a function of the wind speed, wind direction, building shape and size.FASTEST MILE WIND SPEEDA definition of wind speed used in the USA that records the time taken for a mileof air to travel past a point. The drawback to this definition is that the effectiveaveraging time varies with wind speed - from 6Osec at 6Omph down to 30sec at120mph. This makes it an unsuitable reference wind speed and recent changes toAmerican codes are moving away from this definition.FATIGUEFatigue damage is caused to a structure or its fixings when subjected to cyclic loadsover a long period of time.The damage normally occurs as cracks within the materialof a structure grow as a result of the repeated loading and may, over time, progresssufficiently to cause failure. The size and number of the loading cycles determinewhether fatigue will cause failure within the lifetime of a structure.Normally, damagewould be most severe when a resonant response occurs because of the enhanced

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dynamic response and the large number of cycles that would take place. However, insome structures the general buffeting due to turbulence may also be important.Repeated loading may also cause the displacement of structural components - suchas seals and gaskets in glazing or curtain-wall panels.FETCHDescribes the terrain away from a site and/or its extent. For example, the upwindfetch was suburban or a 1Okm downwind country fetch.FISHER-TIPPETT DISTRIBUTIONA family of probability distributions used to describe the extreme occurrences ofwind speed. More recently i t has been found better to fit such a distribution todynamic pressure (velocity squared) in order to better predict extreme events.FLAT ROOFSee Roof pitchFLOW FIELDA term used to describe the whole area in which a fluid is flowing around anobstruction. Most commonly used in computer modelling of lows (CFD ).FLOW VISUALISATIONA technique used by wind engineers to visualise what a flow is doing.Thesetechniques nclude ntroducing smoke nto he airstream, attaching ufts o hesurface that indicate the surface wind direction or using fluorescent pigments onthe surface that are blown into streaks that indicate surface flow patterns.

FLUTTERA particular form of aeroelastic response in which two separate structural modes ofvibration interact to cause a serious oscillation. A typical example might combinedtorsional movement and up/down movementofa bridge deck.Flutter is oftendivergent - i.e. there is no natural limit to the magnitude of the oscillations. If notchecked it will lead to structural failure.FOOTPRINTSee Storm footprintFRESHENA meteorological term using to indicate an increasing wind speed.FRONTFrontal zones define the boundary between two large air masses. The two air masseswill typically have different air temperatures and the rontal zone will extend upwardsfrom the ground at anngle so that the colder air mass underlies the warmer, forminga shallow wedge. The term front is used where the frontalzone intersects the ground.

Height i Warm airmassl Distance


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Glossary of Wind Term5

Frontal zones are often associated with vertical motion, particularly in the warmer airmass, and this can lead to cloud formation and precipitation.

F0

F1

F2

F3

F4

F5

FUJITASCALEThis is a scale used to describe the relative strength of tornadoes:

Gale tornado (40-72mph). Some damage to himneys; breaks branches offtrees; pushes over shallow-rooted trees; damages sign boards.Moderate ornado 73-112mph). Th e lower limit is thebeginning ofhurricane wind speed; peels surfaces of roofs; mobile homes pushed offfoundat ions or overturned; moving ehicles pushed off roads.Significant tornado (113-157mph). Roofs torn off frame houses; mobile homesdemolished; large trees snapped or uprooted; light-object missiles generated.Severe tornado 158-206mph). Roofs and ome walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavycars lifted off the ground and thrown.Devastating tornado 207-260mph). Well-constructed houses levelled;structures withweak foundations blown off some distance; cars thrown andlarge missiles generated.Incredibleornado261-3 8mph).Strongrame housesifted fffoundations and carried considerable distance to disintegrate; automobile-sized missiles ly through the air in excess of 100m; trees debarked; incrediblephenomena will occur.

See also TornadoFUNNELLINGFunnelling occurs when the proximity of two structures cause wind to acceleratethrough the gap between them - for example, between two building side walls. Theseincreased speeds create higher suction loads on the side walls than would have existedon an isolated building. They may also cause problems for pedestrians using the area.

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GABLEA building with a imple two-slope (duopitch) roof that meets at the tophas two endwalls where the brickwork, o r other cladding, comes to a point at theoof ridge. Theseare the gable walls. Gable walls arewell-known points of failure in windstorms, oftenresulting from the stronguctions caused near their edges and the ailure to adequatelytie the wall into the roof structure. Wind uplift loads on roof coverings are alsostronger close to the gable end than they would be a similar roof with hipped ends.See Hip

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GALEOften used generically for a strongwind buthas a specific meaning in theBeaufortScale of wind speeds.See Beaufort ScaleGALLOPINGA type of aeroelastic response where the structurehas an increased dynamic responsecaused by the motion of the structure ffecting the wind load. Fatigue damage mayresult from the ncreased dynamic response. A good example is the dynamicresponseof stranded cables (e.g. guys on a tower) that can oscillate as a result of ice accretionon one side giving a unsymmetrical aerodynamic shape. Th e resulting crosswindforce and the nature of the motionnteract to create an ovalling motion.GRADIENTWINDA meteorological term for the wind speed at a high level in the atmosphere, wellabove the influence of the surface roughness.GUSTA term used to describe short duration wind speeds such as in the fluctuations ofwind speed about a mean value. To be meaningful it must be clear over what timethe gust speed has been averaged. Typical durations would be l-3sec, but 5sec or15sec gusts may alsobe quoted in certain contexts. The averaging time shouldreflect the size of element being considered and its ability to respond to the load.This is because of the possibility of spatial averaging.GUST FACTORTh e ratio of the peak gust speed to the mean windpeed in a particular time period.HEADAnother term for pressure - often quoted in units elated to the height of a columnof some fluid - for example, inches of water or mm of mercury.See PressureHELMHOLTZ OSCILLATIONThis describes the pressure fluctuations that can occur when the air inside a cavity(such as an open building or room) resonates.HERTZThe SI measure of frequency equal to 1 cycle/sec.See UnitsHIPA line running from theeaves to the ridge (normally) where two roof slopes meet.Hipped roofs do nothave such abrupt sharp dges as would occur on a gable-endedroof. For this reason the wind uplift loads on tiles are lower for hipped roofs.

L

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Glossary of Wind Z r m s

HOURLY-MEAN WIND SPEEDThe value of wind speed averaged over a period of 1 hour.HURRICANEAnextreme ormof cyclonic windstorm.Thisname is typically used in thesouthern partof the North Atlantic and Caribbean. lso used in the UK to describeBeaufort Force 12. The wo are not the same thing.See Tropical cyclone.INDUSTRIAL AERODYNAMICSTheiscipline of applyingerodynamicsondustrial,ommercialndconstructional problems rather than aircraft. Similar to wind engineering.INTENSITY OF TURBULENCESee Turbulence intensityINTERNAL PRESSUREThe pressures that exist inside a building. These will depend on the porosity of thebuilding envelope and will typically be related to an average of the external pressuresacting on the building.INVERSIONMeteorogically, inversion refers to a relatively shallow layer in the atmosphere ( < l k m )where the air temperature increases with height (i.e. the lapse rate is negative).Pollutants can be trapped below an inversion layer because rising air will encounterwarmer surroundings and will not longer be more buoyant than the surrounding ir.See Lapse rateISOBARLine of constant pressure o n a weather map.JAMBA vertical structural framing member ositioned at theextreme side of a curtain alladjacent to an end vertical run of glass areas, windows, panels or doors.JET STREAMA region of very high wind speed, often of limited height and widthbut long in thewind direction. They occur in the upperevels of the atmosphere.JOULEThe uni t of energy in the SI system of units. lJ=lNm.See UnitsKNOTA unit of wind speed equal to I nautical mile/hr. Strictly the nautical mile variesaccording the latitude of the location but the knot is conventionally defined usinga nautical mile of 6OSOft. 1 knot =0.51477m/s.LAPSE RATEThe term apse rate is used to describe the temperature profile of the atmosphere withheight. The lapse rare is positive when temperature reduces with increasing height.Three terms are commonly used: Dry Adiabatic Lapse Rate (DALR),SaturatedAdiabatic Lapse Rate (SALR) and Environment Lapse Rate (ELR).The first tworelate to the change in temperature of a parcel of air as it rises. They refer, as the

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names suggest, to dry air and moist air respectively and, for typical temperatures areabout 10 deg C per km and 6 deg C per km respectively. The ELR refers to thetemperature profile of the atmosphere itself. It is the difference between the DALR(or SALR) and the ELR that determines whether the atmosphere is stable or not.See Atmospheric stabilityLEEWARDO n the side of a body away from the wind, the downwindside.LIFTLift describes a force perpendicular to the oncoming wind low direction. It comesfromaeronauticalapplicationswhere i t describes the force on awingand isgenerally used in wind engineering to imply a vertical force rather than a side force.

LIMIT STATE DESIGNAphilosophyof design that defines different failure criteria(limitstates). Thedesign for each state often involves different acceptable risks of failure.See Serviceability limit state, Ultimate limit stateMANOMETERA device for measuring pressure using the height of column of fluid supported bythat pressure - e.g. water or mercury.MASS-DAMPING PARAMETERSee Scruton numberMEANAn average. For ime varying quantities, such as wind speed, it is important tospecify over what time period a mean value is averaged. The longer the averagingtime, the more likely i t is that the shorter duration peaks of wind speed will lesssignificant and the resulting wind speed quoted will be lower.MEAN-HOURLY WIND SPEEDSee Hourly-mean wind speedMONTE-CARLO SIMULATIONA numerical technique for estimating probabilistic events. Knowing the probability ofindividual inputs, asimulation can be run toestimate the probability of specific results.MULLIONA vertical framing member of a curtain wall, glazing system or other cladding. Itspans from floor to floor or cill to head and is normally the main structuralmember.NATURAL FREQUENCYAll structures have frequencies at which they would vibrate orscillate if loaded and

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Glossary of Wind Term3

then allowed to vibrate freely. This is similar to, for example, a tuning fork thatvibrates at a particular frequency when struck. It is the lowest (or fundamental)preferred frequency of vibration for the structure and if the loading excites i t at, orclose to, that frequency then greatly increased response can be expected and thestructure would be said to resonate. Other, highernatural frequencies exist fordifferent vibration modes.See also Dynamic response, Modes of vibrationNEWTONThe unit of orce in the SI system of units.See UnitsOVERTURNING MOMENTThe rotational force tending to overturn a structure or element.

PASCALOne definition of the unit of pressure in SI units. It is equivalent to N/m2 andpressure may be described in either terms.See UnitsPDFSee Probability density functionPERMEABILITYDescribes the relative leakiness of a body. Can be expressed as a ratio of open rea toenclosed area (=porosity) or as an effective area based on the relationship betweenflow rate through the body and the pressure difference across it. Effective area isnormally smaller than the actual area of gaps, because of the resistance to flowthrough small gaps and the effects of separated flow from the edges of openings.In building walls, permeability affects the internal pressures in the building. Forexample permeable roof coverings such as tiles may attract less direct load becauseof the ability of pressures to equalise above and below the tiles. In this case, moreload will be taken by the relatively less permeable underoof - underfelt or sarking.PITOT-STATIC TUBEAn instrument that, when connected to a manometer, records the total and staticpressures in a moving airstream. From this the dynamic pressure can be inferred byBernoullis Law.PLUMEGenerally used to describe an exhaustofsomefluidfromachimney or otheropening. By implication the exhaust is expected to have some properties differentto the surrounding ir - for example, i t may be smoke orgas, it may be warmer andtherefore buoyant.

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Glossuly of Wind Terms

PORTAL FRAMEA portal frame building is a typical example of commercial style building, so calledbecause the frames of the building resemble gateways. The frames are largely self-supporting with additional rigidity provided by wind bracing, a ridge beam andother lightweight bracing.

PRESSUREThe term pressure describes a force exerted over a unit area and has the units ofN/m2 (sometimes called pascals (Pa)). In a wind engineering context this wouldtypically be the pressure exerted by the air on the surface of a building. Note thatpressure is a scalar quantity and at any point it cts equally in all directions.See Units, HeadPRESSURE COEFFICIENTA non-dimensional way of describing the pressures on the surface of a structure.Themeasured pressures, often from model tests in a wind unnel, are divided by thedynamic pressure of the wind approaching the building. This allows the full-scalepressures to be determined by using the full-scale dynamic wind pressure multipliedby the pressure coefficient. Note that this technique is only reliable in situationswherethe behaviour of the flow around the body is not speed or size (Reynolds Number)dependent. This will normally be a safe assumption for sharp-edged bluff bodies butnot fo r curved o r streamlined bodies. In those cases the wind tunnelmodels will haveto take account of this Reynolds Number effect when formulating the coefficients.PRESSURE EQUALISATIONA technique used to try andensure equal pressures on either side of a panel o thatthe wind loads on that panel are greatly reduced. Commonly used on curtain wallsto reduce loads on the rainscreen and achieved by providing ventilation openings.The sameprinciple applies to any vented multi-layer system. For example, theprinciple explains why wind loads on roof tiles (which are highly permeable at thejoints) experience a lower wind load than the roof structure tself.PROBABILITY DENSITY FUNCTIONA PDF describes the probability (or likelihood) of an event occurring in the formof anequation. Probabilities range from 0 (definitely will nothappen) o 1.0(definitely will happen), although they are sometimes expressed as percentages inthe range 0 - 100%. The probability density function most often used to describewind speeds is the Weibull distribution.PROBABILITY FACTORA factor used in the British Standard BS 6399-2 to allow designers to design againsta different level of risk than the standard 2% per annum.PURLINA structural member in a commercial style roof, often using metal cladding, that

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runs perpendicular to the rafters and provides additional stiffness and support tothe roof covering.

l Rafter l

See Dynamic pressureRAFTERA principal structural element of a roof running from eaves to ridge. Rafters areoften constructed as A-framesor similar spanning between the walls of the building.RAILA structural member in a framed building running along the walls parallel to theeaves and which provides support for the wall cladding.See also PurlinRAJNSCREENThe oute r layer of a multi-layer wall designed primarily to protect the inner layersfrom rain penetration. It experiences some wind load but is ventilated to allowpressures either side of i t to equalise. The inner layers of the wall therefore take themajority of the wind load.REATTACHMENTWhen flow separates from the surface (see Separated flow) it may reattach if thebuilding is longenough n hedownwinddirection. Beyond the eattachmentpoint there may be frictional forces from the flow slumming over the surface andpressure coefficients are generally quite small positive or negative values. Wherereattachment occurs the area under the separated region is sometimes referred to asthe separation bubble.

Separationbubble

L l

REDUCED VELOCITYUsed typically when describingvortex hedding, the reduced velocity is thereciprocal of the Strouhal Number andgiven by VlnD where V is the wind speed,n the natural frequency of the structure and the diameter of he structure. It is anon-dimensionalway f escribinghewind peedwhich is useful whencomparing the response of different structures.RESONANCEResonance occurs when the frequency of an pplied load is the same as the naturalfrequencyof the tructure. If hisoccurs here is a significant risk that he

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Glossay of Wind Term5

amplitude of response will increase very dramatically, possibly leading directly tostructural failure. The effect may, however, be ameliorated if there is sufficientdamping in the structure.RETURN PERIODThe return period is one way of describing the risk of an extreme event occurring.Its use is being discouraged now in favour of describing the risk of exceedancedirectly because there is the possibility of mis-interpreting return period. In theUKthe standarddesign wind speed for buildings has a probability of xceedence of 2%per annum. Thi s means there is 2% chance each year that the design wind speedwill occur. Over a 50 ear building life this means that there is a 63.6% chance thatthe design wind speed will occur at least once but, by the same token, a 36.4%chance that it will not occur.The 2% per annum exceedence probability equates to a return period of 50 years.This means that on average the design wind speed will occur once every 50 years.However, i t does not mean that i t will occur in any particular 50-year period, northat there will 50 years between occurrences. This is where thepotential formisunderstanding arises in the use of return periods. Stating the annualexceedenceprobability avoids this ambiguity.REYNOLDS NUMBERA non-dimensional parameter that describes the relative importance of inertial andviscous forces in aflowingfluid. For correct simulation n a wind unnel thisparameter should be the same at both model and full scales. This is difficult toachieve given the definition below. However, where the behaviour of the structuredoes not vary with Reynolds Number this requirement can be relaxed. Exampleswhen this is acceptable are for sharp-edged bluff bodies where the points at whichthe flow separates (the sharp corners) do not change with wind speed. For curvedsurfaces special measures must be aken to ensure the mis-match of ReynoldsNumber in the wind tunnel does not give erroneous results.Reynolds Number is given by:Re =V .D / V where V is the wind speed, D a suitable reference dimension of thestructure and V is the lunematic viscosity of air (= 1.46 x m2/s).RIDGEThe horizontal apex of a pitched roof where two rooflopes meet.

I I

RISKSee Design riskRISK OF EXCEEDENCESee Weibull distributionRIVET FIXINGSome slates, especially fibre-cement varieties, can be fixed using a rivet, normally

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made from copper. The disc of the rivet slides between and under the two slatesbeneath the target one tobe fixed. A vertical rod then passes up and through a holein the target slate and is bent over by hand. This is designed to hold the tail of theslate down.RMSRoot Mean Square - a statistical measure of the variability of a series of values.Often used in wind engineering to describe turbulence intensity, which is the rmswind speed divided by the mean. The use of this terminology is often loose so thatwhat is intended by the term rms is actually standard deviation. Stricrly, rms (orM S ) describes the fluctuations of the whole signal whereas standard deviationdescribes the fluctuations about the meanvalue (i.e. with the mean subtracted romindividual values).ROOF PITCHTh e pitch of the roof is the angle of the slopes relative to the horizontal. Note thatany roof with a pitch lower than 5" is normally considered to be flat. There aremany types of pitched roof:Monopitch - the roof comprises just a single sloped surface.Duopitch - the roof comprises two sloped surfaces generally pointing upwards sothe roof is convex and joined at the 'ridge'. Troughed duopitch roofs are thosewhere the slopes point downwards and they are joined at a 'trough.Multi-bay - normally found on long commercial or farm type buildings, the roofcomprises a series of pitched roofs.

ROOF TILE TYPESThere are a wide range of roof tile types available on the market but the mostcommon are briefly described below:Interlocking - these tiles overlap both o he side andat he opandbottom.Generally hereare design features, especially at he sides, which nterlock andincrease weathertightness. Tiles like this are sometimes called single lap productsbecause there is only ever one overlap, i.e. a maximum of two tile layers at anyposition.nterlocking tiles are typically made of concrete o r clay and areapproximately 300mm wide by 41Omm long. Variants on this do occur however andresin-based interlocking products also exist.

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Glouay of Wind Terms

Plain tiles - plain tiles are an example of double lap products. There s no interlockand instead weathertightness is achieved by providing more overlapping layers. Thejoints between tiles are staggered to further increase the resistance to rainpenetration. Plain tiles are made in clay or concrete and typically are smaller thaninterlocking tiles - lOOmm wide by167mm long.Slates - natural slate is provided in many sizes (typically 300m by 6OOm long, or 250mmby 500mm long). They too are laid in a double lap formation with staggered joints.Fibre-cement slates are similar in concept but are made from fibre-reinforced cement.ROUGHNESS LENGTHA parameter (20) used to describe the roughness of a surface when describing thewind velocity profile. Typical values for common terrain descriptions are:Country: .03mSuburban: 0.3mUrban: O.6mSee dlso Velocity profile

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SARKINGBoarding used in place of underfelt as the main structural load transmitting elementof the roof construction and as the second line of defence against rain or snowpenetration. Tiles are fixed either directly to the sarking board or onto battens thatare fixed to it.SCRUTON NUMBERA non-dimensional parameter used to describe the relative effectiveness of structuraldamping in comparison to the aerodynamic input of energy, taking account of thesize and mass of the structure. It is most commonly written in the form:Sc = 4nmC/(pB2)where m is the mass per unit length of the structure, is the damping ratio, p is thedensity of air and B is a cross-wind dimension.Using the logarithmic decrement this can also be written Sc=2m6/pD2.Th e higher the Scruton number, the less likely are severe dynamic oscillations ofthe structure.See D am pi ng for an explanation of 6 and 6.SFASONAL FACTORA factor used in the British Standard BS 6399-2 to allow for the reduced risk of failurewhen structures are temporary and will not exist during the most windy months.SEPARATED FLOWSeparation occurs when the main ody of the moving wind pulls away from the surface

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leaving beneath a re-circulating region of turbulent flow, often with reversed winddirection at the surface. The implication of using the term separated flow is that it is amore substantial feature than the normal boundary layer region. In wind engineeringthis most commonly occurs at sharp edges but it can also happen on the leeward partof a curved body. The location of the separation is called the separation point.

Seoaration Doint

SEPARATION, SEPARATION POINT, SEPARATION BUBBLESee Separated flow, ReattachmentSERVICEABILITY LIMIT STATEMany codes andstandardsnowadopt a imitstate design approach.Ultimatestructural failure is normally the main design criterion but theremay be otherserviceability limit states. One example might be for a communicationsower wheredeflection must be limited in order to maintain line-of-sight with a transmitter.Another could be a limit on accelerations at the top of an office block to ensureoccupant comfort. Serviceability limit states are generally designed to a higher risklevel than the ultimate limit stateecause although they are undesirable they can betolerated more often than complete failure.See Ultimate limit stateSHEAR FORCEThis is normally used to describe a force in the direction of the applied load thattends to separate layers in a material or structure.

SHEAR LAYERWhen flow separates from a body there is a boundary between the separated flowand the surrounding airstream. Tha t boundary is known as the shear layer. Therewill typically be large changes in wind speed as you pass through the shear layer.See Separated flowSI UNITSSee UnitsSIZE EFFECT FACTORSee Spatial averagingSLENDERNESSUsually, the ratio of a structural elements cross-sectional dimension to its length.Aspect ratio (see above) describes a similar ratio but is the more common term sedfor whole buildings.

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SPATIAL AVERAGINGSpatial averaging describes he way that individual gusts, of varying sizes, do not occursimultaneously across the whole face of a structure. Short duration gusts are smaller insize and, for large buildings, they will not be hlly correlated across he building face. So,it would give too high a design load if it was assumed that all gusts are simultaneously attheir strongest everywhere on the surface. This effect is accounted for in the UK designstandard using a size effect factorwhich reduces loads for large structures or elements.SPECTRUMA spectrum is a graphical representation of the amount of energy contained in afluctuating signal at different frequencies. They are used to describe, for example,the turbulence in wind and the pressures measured o n a building surface.This is analagous to the way that white ight can be decomposed according owavelength (or its reciprocal frequency) into the visible spectrum (a rainbow).STAGNATION POINTWhen flow approaches a building or other bstacle it must split and pass around andover the obstruction. The streamline which defines the division between flow goingto one side or the other actually comes to rest on the building and the flow speed atthat point will be zero. This point on the building is called the stagnation point. Atthe stagnation point all of the dynamic ressure in the approaching wind is convertedto static pressure so this is the point with the highest positive pressure coefficient.

I I

STATIC PRESSUREThe pressure that is experienced on the surface of a body. In still air he staticpressure felt is simply atmospheric pressure. When air accelerates around a body,the increase in velocity (or dynamic) pressure results in reduced static pressure. Thisexplains how suctions arise on building surfaces. Conversely, as wind is sloweddown on the windward face of buildings, dynamic pressure is reduced and staticpressure must therefore rise. Static pressure is most often referred to relative toatmospheric pressure so that reduced static pressure (suction) would be describedas negative because it is lower than atmospheric pressure.STATIC RESPONSEThe response of a building s deemed to be static if it does not fluctuate significantlyover a short period of term, consistent with the duration of the gusts in the wind.STORM FOOTPRINTThis describes the area affected by a particular storm. It would ypically be definedin terms of contours of maxium gust wind speed plotted on a map of the area.

STREAMLINED BODYSee StreamlinesSTREAMLINESStreamlines indicate the direction of low on a diagram of a body with wind lowing

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past it. Strictly, they are defined so that wind flows between the streamlines and doesnot cross them. By implication, a streamlined body s one where the flow can alwaysbe described in this way. This is in contrast to a bluff body, where flow separationscause turbulence and disorderly flow so that streamlines can not be defined.STREET CANYONA term used to describe the streets of major urban areas where the tall buildingseffectively create canyons along the streets. These canyons can make it difficult forpollutants to escape or, in other circumstances, may cause accelerated wind flowsgiving rise to uncomfortable or dangerous conditions for pedestrians.STROUHAL NUMBERA non-dimensional parameter used to describe the frequency of vortex sheddingfrom a structure. It is defined as follows:S t = n . D / Vwhere n is the frequency of vortex shedding, D is the crosswind dimension of thestructure and V is the wind speed. For circular cylinders the value of St is 0.2 andfor rectangular cylinders, typically 0.12.When the sheddingrequency is at or close to the natural frequency of the structure(n) then large amplitude oscillations can occur. Thus it s possible to define a criticalwind speed at which this may occur:Vcrir= n .D / St or, for circular cylinders Vcrit= 5 . n .DSTRUCTURAL GLAZINGA means of bonding glass units onto an internal frame with minimal mechanicalretention to provide a flush-glazed shear wall. The structural loads are then largelytaken by the glass units.SUB-ANNUAL PERIODSSee Seasonal factorTERRAINThe description of the land use around a site. In standards terrain categories areused to describe the roughness of heupwind fetch and define he levels ofturbulence and change of wind speed with height.THUNDERSTORMThunderstorms develop from cumulo-nimbus clouds, which are formed by rising,moist air. When precipitation starts strong down-draughtsare created. These hit theground and spread out sideways, creating strong local wind speeds well in excess ofthose existing previously. These winds are often of relatively shor t duration and anlead to very high apparent gust factors. This is because, averaged over an hour thewind speed may be modest but the peak gusts in the hour are associated with theshort-term convective winds in the thunderstorm.

TILE CLIPSClips re used as ameans offixinganyypesfile,especially interlockingvarieties.They are generally pecified inareas whereindoadsreexpectedo be higherhan

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Glossay of Wind Terms

normal. A widevarietyareavailable but most clip into he nterlocking sideport ion of the tile, towards the bottom end, and are then nailed or otherwisefixed into hebatten.Theycan be more effective than nailsatpreventingoverturning of the tile because they are closer to the free bot tom edge.TOPOGRAPHIC EFFECTLocal changes in groundslope near to a site will affect the wind speed. Close to thecrest there will be an acceleration of the wind as it flows around the obstruction.The speed-up effect is important for slope gradients of more than 1 in 20 but theyreduce with height above ground.

Region ofacceleraledflow

I

TOPOGRAPHIC FACTORThe effects of opography are allowed for in British Standard BS 6399-2 by acombined altitude and topography factor.See Altitude factorTOPOGRAPHYThe degree of slope in the local terrain surrounding a site - for example, hills andvalleys. In BS6399-2 the wider effects of changing altitude are treated separatelyfrom the local effect of hills.TORNADOA tornado is a violent whirlwind, usually quite small in size with a core diameterof50-200m. They normally form here there is a very strong updraft, for example asseen in developing thunderstorms. Wind speeds of more than 1OOm/s can occurnear the centre of the tornado but in a narrow ring. At the centre the horizontalwind speed is zero but the large updraft is capable of lifting heavy objects. Also, inthe centre there is a substantial depression in pressure that can cause the suckingout of building walls etc.The st rength of tornadoes is described by the Fujita Scale.See also Fujita ScaleTORSIONA load that tends to twist a structure or structural element about its axis.

I I

TOTAL PRESSUREThe total pressure experience by a body n a fluid flow is madeupof twocomponents - the static pressure and the dynamic pressure.See Bernoullis Law

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TRANSOMA horizontal framing member of a curtain wall, glazing system or other cladding. Itnormally spans from mullion to mullion and allows infill panels o span in two directions.TROPICAL CYCLONEAn extreme form of cyclonic wind storm. Hurricanes and typhoonsare alternativeregional names.A tropical cyclone is a cyclonic storm deriving tsenergyprimarily rom theevaporation of water from warm tropical eas (particularly where water temperatureexceeds 26C). Tropical cyclones may cause winds considerably stronger han occurin temperate cyclones (such as experienced in the UK) but re smaller in extent andimpact less frequently on any single location. The wind strength decreases rapidlyas the storm runs onto land.The strength of tropical cyclones can be measured by the Saffir-Simpson Scale.See alro Saffir-Simpson ScaleTURBULENCEDescribes the fluctuations in the wind. These are important both for loading andthe wind environment around a building. For loading they represent short-termvariations about the mean wind speed so that a structure should be designed towithstand the largest gust (short term average wind speed) that could affect it. Inaddition to the effect of turbulence on structural loads it also affects the ability ofpeople to walk and stand safely in urban ocations. Turbulence ntensity is thestandard measure ofgustiness.TURBULENCE INTENSITYThe standard deviation of wind speed divided by the mean wind speed. Normallyexpressed in percent. Turbulence is greatest in rough terrain and least in smoothterrain. Typical values are:Country terrain: 10-20%Sub-urban errain:25-30%Urban terrain: 35%+TURBULENCE LENGTH SCALESThese describe the representative size of gusts in the along-wind, crosswind andvertical directions. The y are derived from the spectral equation describing windspeed fluctuations.See SpectrumTYPHOONAn extreme form of cyclonic wind storm. This name s typically used in the SouthWestern Pacific and China Seas.SeeTropical cycloneULTIMATE LIMIT STATE, ULTIMATE FAILUREMany codes and standards now adopt a limit state design approach. This meansthat different design limits, or limit states, can be defined and designed for. Theultimate limit state is the one where structural failure would occur. This is clearlythe major design criterion for most structures. However, there may be other lesserstates that also dictate the design such as deflection or acceleration.See Serviceability limit state

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UNDERFELTUnderfelt is amaterial that is laid under tiles. It is normallya hick fabric-likematerial that is draped over the rafter and held in place by the battens nailed on topof it. Its purpose is to resist the main part of the wind uplift oad on the roof(transmitting it into the main structure) and i t also acts as a second line of defenceagainst rain or snow penetration. The most common material used is a bitumenbased felt, although more modern types are now available, some of which includean element of entilation by being permeable to water vapour. Note that n Scotlandi t is more common to use a solid wooden deck (or sarking) in place of an underfelt.UNITSWhen describing terms in wind engineering or other technical field, i t is commonto use equations to describe the relationships etween ifferent arameters.Generally, theseequations will hold rue fo r any set of consistent units. Thestandard set of units used is the SI (Systkme Internationale) system in which thekey, consistent units are:length - metre (m)mass - kilogramme (kg)time - sec ( S )force - newtons (N =kgm/s2)pressure - (N/m2) also called pascals (Pa)UPWINDIn a direction opposite to that of the wind, the windward sideVALLEYA valley in a roof occurs where two roof slopes meet at an angle. O n the outside ofthe corner there will be a hip and on the inner (re-entrant) side of the corner therewill be a valley.

VAN DER HOVEN SPECTRUMA measurement made by van der Hoven at Brookhaven, NY of he frequencycontent of winds over a long period. t identifies the major fluctuationsas being thediurnal changes of night and day, a 4-day cycle representing the typical passage timeof a temperate weather system, seasonal changes and longer term changes due tosun spot activity.VEERINGThis is a meteorological term used to describe a clockwise change in wind direction,e.g. from northerly to easterly

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VELOCITY PROFILETh e variation of wind peed with heightcaused by the friction of surface roughnesselements such as hedges, trees, buildings etc. Common ways to describe the profileare a power-law and a logarithmic law.In smooth terrain the wind speed increases with height rapidly and soon reaches itsmaxium value whereas for rough terrian greater surface friction slows the air downand the rise in wind speed with height is more gradual.

Height Smoothterrain

------+Wind speed

VENTILATED RAINSCREENSee RainscreenVERGEThe side edge of the roof, on the slope.

VORTEXA rotating mass of air with low pressure at its core and strong speeds around itsperiphery.VORTEX SHEDDINGWhen air flows separate romabody vortices are formedandshed nto hedownstream wake of the body. In some situation these are strong and can causesignificant loading of the structure. This is especially true for slender, prismaticstructures including circular cylinders such as chimneys where the vortices are shedalternately from each side resulting in a fluctuating cross-wind force. The sheddingfrequency is described by the Strouhal Numberandwhen t coincides with astructural natural frequency a large resonant dynamic response may result.WAKETh e region of flow behind a structureor element. Often his is a region of low windspeed and high turbulence.

WARM FRONTA front is described as awarm rontwhen, as i t passes any ground location,warmer ir replaces the colderirpreviously there. The usual symbols forawarm ront on aweather map are

Isobars

Warm frontI I

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solid semi-circles shownalong he ineof he ront.Theypoint owards hedirection the front is moving. As the front passes a ocation winds will changedirection - always veering (e.g. from south to west).WEIBULL DISTRIBUTIONThis is the equation most oftenused to describe the long-term probability of windspeeds occurring. In its probability density form the equations quite complex andit is more often used in its cumulative distribution form, where it describes thelikelihood of wind speeds at or below a given threshold value:P(v) = I - exp (-cVk)From this, the risk of exceedence of a particular threshold is even simpler:Q(.) = exp (-cVk)Th e parameters c and k are obtained from long data records taken from a localmeteorological office or by long ermmeasurementon-site. The value of k istypically in the range 1.6 - 2.3, while the value of c is approximately 90% of themean wind speed for a site.WIND BRACINGUsed in a portal frame building to transfer the shear loads caused by wind into thefoundat ion and prevent progressive (domino-like) collapse of the frames. Normallyin the end bays of the building.

WIND DIRECTIONThe direction that the wind s blowing from.fromnorth osouth.Winddirection may

For example, a northerly windblowsbe given in cardinalpointsof the

WIND ENGINEERINGThe discipline associated with all aspects of wind flow and their effects on theenvironment and on structures. Similar to Industrial aerodynamics.WIND ENVIRONMENTWind environment describes the conditions in or around a structure orevelopment.As opposed to wind loading, in which the prime concern is the pressure distributioncreated on the building by the wind, with wind environment it is the actual windspeeds and their effect on users of the development that are of interest. Examplesinclude keeping street level wind speeds low in cities to ensure the comfort and safetyof pedestrians, or reducing the possibility of pollutant build-up in urban areas.WIND SHEARWi nd shear is a term used to describe a variation of wind speed with distance,normally used to describe a vertical variation. For example, wind speed is known

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to generally increase withheight above ground nd this is wind shear. Inaeronautical terms it is often used specifically to describe a rapid change in windspeed such as might cause a pilot difficulty when landing.WIND SPEEDWind speed is described in many different ways that can cause confusion. It isimportant to distinguish between mean speeds (averaged over a long period) andshort duration gust speeds. Without a knowledge of the averaging time used thewind speed value is not very useful. Mean wind speeds are normally quoted for anaveraging time of 10min or one hour and, ecause of theenergy spectrum of windspeed fluctuations, there is only a relatively small difference between the two. Gustspeeds are usually quoted for l-3sec averaging periods and these are the smallestgust sizes that would affect typical structural components. For larger elements, agust speed based on 15sec averages might also be used.See Gust, Hourly-Mean WindSpeedWIND TUNNELA wind tunnel is a facility for testing the effects of the flow of air over obstacles,such as buildings. To be suitable for testing buildings he wind tunnel mustbe ableto adequately represent at least the vertical profile of wind speed with height as wellas the profile of urbulence. Normally, thisshouldbe possible fora range ofdifferent errain types and this will entail the wind unne l havinga ong flowdevelopment section upwind of the test area. Different facilities are required forspecialised testing such as the dispersion of pollutants wherevery slow wind speedsare often the key problem. In such cases there is also a need to simulate the thermalstratification in the atmosphere. There are a number of suitable test facilities in theUK and abroad capable of performing tests on buildings.The accurate use ofwind unnelsdependson he skill and experience of theoperator. Potential users are strongly advised to consult experts in theield to ensuremeaningful results.WINDWARDTh e side of the structure facing the wind, or upwind.

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SOURCES OF INFORMATIONTh e following are not intended as an exhaustive referenceor contacts. If you wish tofind out more please start with these o r contact the editors using the details iven inthe Foreword.Key References:1. BS 6399: Part 2: 1997

Loading for Buildings. Code of practice for wind loads.British Standards Institution

2.The designers guideowindoading f uildingtructures. art l:Background, amageurvey,wind ata ndtructural lassification,N J Cook, Butterworth, 1985

3 . Th e designers guide o wind oading of building structures. Part 2: Staticstructures, N J Cook, Butterworth, 1990.

Key Contacts:Wind Engineering Societyc/o The Institution of Civil Engineers, 1 Great George Street, London S W l P 3AATel: 020 7222 7722 Web site: http://www.homeusers.prestel.co.uk/gaylard/WESLoss Prevention CouncilLoss Prevention Council, Melrose Avenue, Borehamwood, Herts WD6 2BJTel: 020 8207 2345 Web site: http://www.lpc.co.ukBritish Standards Institution389 Chiswick High Road, London W4 4ALTel: 020 8996 9000 Web site: http://www.bsi.org.uk

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Glossary of Wind Terms Used in Construction