Introduction - The Behaviour and Design of Steel Structures to EC3 - page 19

dominate, it is common to replace the dynamic loads by static force equivalents
[14]. However, such a procedure is likely to be inappropriate when the dynamic
loadsformasignificantproportionofthetotalload,inwhichcaseaproperdynamic
analysis [37, 38] of the structure and its response should be made.
1.5.4 Wind loads
The wind loads which act on structures have traditionally been allowed for by
using static force equivalents. The first step is usually to determine a basic wind
speed for the general region in which the structure is to be built by using infor-
mationderivedfrommeteorologicalstudies.Thisbasicwindspeedmayrepresent
an extreme velocity measured at a height of 10 m and averaged over a period of
3 seconds which has a return period of 50 years (i.e. a velocity which will, on
average, be reached or exceeded once in 50 years, or have a probability of being
exceededof1/50).Thebasicwindspeedmaybeadjustedtoaccountforthetopog-
raphyofthesite,forthegroundroughness,structuresize,andheightaboveground,
and for the degree of safety required and the period of exposure. The resulting
design wind speed may then be converted into the static pressure which will be
exertedbythewindonaplanesurfacearea(thisisoftenreferredtoasthedynamic
windpressurebecauseitisproducedbydeceleratingtheapproachingwindveloc-
ity to zero at the surface area). The wind force acting on the structure may then
becalculatedbyusingpressurecoefficientsappropriatetoeachindividualsurface
of the structure, or by using force coefficients appropriate to the total structure.
Manyvaluesofthesecoefficientsaretabulatedin[15],butinspecialcaseswhere
these are inappropriate, the results of wind tunnel tests on model structures may
be used.
Insomecasesitisnotsufficienttotreatwindloadsasstaticforces.Forexample,
whenfatigueisaproblem,boththemagnitudesandthenumberofwindfluctuations
mustbeestimated.Inothercases,thedynamicresponseofastructuretowindloads
mayhavetobeevaluated(thisisoftenthecasewithveryflexiblestructureswhose
longnaturalperiodsofvibrationareclosetothoseofsomeofthewindgusts),and
this may be done analytically [37, 38], or by specialists using wind tunnel tests.
In these cases, special care must be taken to model correctly those properties of
thestructurewhichaffectitsresponse,includingitsmass,stiffness,anddamping,
aswellasthewindcharacteristicsandanyinteractionsbetweenwindandstructure.
1.5.5 Earth or ground-water loads
Earthorground-waterloadsactaspressureloadsnormaltothecontactsurfaceof
the structure. Such loads are usually considered to be essentially static.
However,earthquakeloadsaredynamicinnature,andtheireffectsonthestruc-
ture must be allowed for. Very flexible structures with long natural periods of
vibration respond in an equivalent static manner to the high frequencies of earth-
quakemovements,andsocanbedesignedasifloadedbystaticforceequivalents.