Incarcari din vant /wind loads

conf normativ NP082-2004/according to Romanian Code

NP082-2004

Vantul este un agent climatic dat de

miscarea unor mari mase de aer in

raport cu suprafata pamantului.

Miscarea este datorata incalzirii neuni

forme a atmosferei si scoartei

terestre de catre soare

H

H= inaltime de gradient

Wind is a climatic agent given by the

movement of large air masses with

respect to Earth level. The nonuniform

solar heating of the atmosphere is

responsible for the air masses

movement

z (m) Laminar flow umed= constant u(z)=cst.in time

Turbulent flow umed = logarithmically

varies with height

u(z)= varies in time

z (m)

u(z)

Miscare laminara

umed(z) =constantu (z) =constant in timp

Miscare turbulentaumed =variaza logaritmic cu inaltimea

u(z) =variaza in timp

u(z)

in zona de interes pentru domeniul

cladirilor viteza vantului poate fi ex-

primata ca suma de 2 componente:

componenta statica – constanta in

timp u(H)med= viteza medie a vantului

in intervalul de timp considerat la

inaltimea H deasupra terenului

componenta dinamica – dependenta

de timp u*H(t)= viteza de rafala,

introduce variatia in timp a vitezei

vantului ,in raport cu viteza medie

u (H,t)= u(H)med + u* H(t)

uH(t)

u* H(t) u(H)med

for building design purposes, the wind

speed may be expressed as the sum

of two components:

the static component – constant in

time, uHmed=mean wind velocity within

the considered interval evaluated at the

height H above the ground.

the dynamic component – time dependent

u*H(t)= gust speed, introducing the time

variation of wind velocity with respect to

the mean velocity

u (H,t) = u(H)med + u* H(t)

uH(t)

u* H(t) u(H)med

Valoarea vitezei medii si cea a vitezei de rafala la o inaltime

H, depind de rugozitatea mediului natural si construit pe

zona de inaltime H. In cazul zonelor cu rugozitate mica, umed

la o inaltime data H are valoarea mai mare decat in cazul

zonelor cu rugozitate mare la aceeasi inaltime H.

Amplitudinea de variatie a vitezei de rafala este mai mica in

zonele cu rugozitate mica.

Raspunsul unei c-tii la actiunea vantului este, in general,

suma a doua componente:- una cvasistatica provocata de viteza medie a vantului- o componenta dinamica provocata de rafalele cu

frecventa mare

• The value of average speed and the one of the gust speed at a certain height H, depends on the rugosity of the surroundings at a height H. In the case of the zones with reduced rugosity umed has an greater value than in the case of the zones with increased rugosity at the same height H.

• The amplitude of the variation of the gust speed is smaller in the zones with reduced rugosity.

• The response of a construction to the wind action is, generally, the sum of two components:

- a quasistatic one of due to the average speed of wind

- a dynamic component given by the gusts with big frequency

Cladirile curente sunt relativ rigide, din acest motiv componenta dinamica a raspunsului la actiunea vantului este neglijabila si poate fi considerata de tip static.

Energia cinetica de miscare a maselor de aer se transforma in energie potentiala la contactul cu anvelopa rigida a constructiei, si se materializeaza intr-o presiune exercitata pe respectiva suprafata.

Relatia de legatura dintre viteza vantului si presiunea exercitata de acesta pe o suprafata normala directiei sale:

q=0,613u2

In proiectarea cladirilor, modelarea actiunii vantului se face pornindu-se de la o “presiune de referinta” qref, asociata unei macrozone teritoriale.

Ea este stabilita pe baza unei viteze medii determinate in urmatoarele conditii:

• The current buildings are rigid, for this reason the dynamic response to the wind action is negligible and can be considered of static type.

• The kinetic energy of the air masses in motion change in to potential energy at the contact with rigid envelope of the building, and is materialized by a pressure exerted on the respective surface.

• The relationship between the wind speed and the pressure exerted on a surface normal to its direction is:

q= 0, 613u2.• In structural design of the buildings, the wind actionis

modelled starting from a reference pressure qref, associated to a territorial macrozonation.

• It is established on the basis of an average speed determined under the following conditions:

• medierea este facuta cu valori instantanee masurate in

interiorul unor intervale de timp de 10 min• masuratorile sunt facute in camp deschis la o inaltime

conventionala de 10 metri• probabilitatea anuala de depasire in sens nedorit pentru

siguranta cladirii este de 2%

Valorile lui qref , pe langa faptul ca se refera la o inaltime

conventionala de 10 m, se refera si numai la suprafete de

constructie plane, dispuse perpendicular pe directia

vantului si situate in camp deschis.

Pentru trecerea la situatia concreta a unei suprafete de

constructie data, reglementarile romanesti in vigoare

folosesc mai multi coeficienti de corectie

- the intermediation is done with instantaneous values moderated within a 10 min interval

- the measurements are done in the open field to a conventional height of 10 meters

- the annual probability of exceeding in the undesirable sense for a building is of 2%.

• The values of qref, besides the fact that they are refering to a conventional height of 10 m, are also refering to plane building surfaces, disposed perpendicular to the direction of the wind and situated in open field.

• For passing to the concrete situation of a given construction surfaces the Roumanian reglementation in force uses several correction coefficients

 

• In cazul constructiilor de cladiri de tip uzual, presiunea/

succtiunea vantului pe suprafata exterioara sau interioara a

unei zone de anvelopa este o functie de inaltime “z” fata de

suprafata terenului.

• Expresia acestei functii este, conform normativ,

w(z)=q ref*ce(z)*cp [KN/m2]

w(z)=presiunea /suctiunea pe suprafata unei zone de anvelopa aflata la inaltimea z deasupra terenului

ce(z)=factorul de expunere al suprafetei de anvelopa aflata la inaltimea z deasupra terenului

cp=coeficientul aerodinamic de presiune al zonei de anvelopa considerat

• In the case of constructions of usual type, the wind pressure/sucction on the exterior or interior surface of an envelope area is depending on height „z” wiyh respect to the surface of the ground.

• The expression of this functions is, as given in codes:

• w(z)=q ref*ce(z)*cp [KN/m2]

where:• w(z) = the wind pressure /sucction on an envelope

area at a hight „z” above the ground level

• ce(z) = the exposure coefficient of the envelope surface at a hight „z” above the ground level

• cp= the aerodynamic coefficient of pressure of the considered envelope zone

• Cand este vorba de o suprafata exterioara coefientul cp are notatia cpe, cand este vorba de o suprafata interioara coefientul are notatia cpi

• Pentru determinarea valorii factorului de expunere ce(z) normativul NP082 indica relatia

ce(z)=cr(z)*cg(z)*ct(z)

Unde cr(z)=factor de rugozitate

cg(z)=factor de rafala

ct(z)=factor topografic

Factorul cr(z) introduce in modelul de calcul influenta gradului de rugozitate al suprafetei terenului zonei in care se gaseste amplasamentul.Valoarea lui cr(z) este calculabila cu relatia:

cr(z)=q(z)/qref=kr2(z0)*(lnz/z0)2

• When the expression is applied to an exterior surface the coefficient cp has the notation cpe, when the expression is applied to an interior surface the coefficient cp has the notation cpi.

• For determining the value of the exposure factor ce(z), the NP082 code indicates the following relationship:

ce(z) = cr(z) * cg(z) * ct( z).

where

• cr( z) = rugosity factor

• cg( z) = gust factor

• ct( z) = topographic factor.

• The factor cr(z) introduces in the expression the influence of the ground surface rugosity of the construction location.

• The value of cr(z) is calculable with the relation:

cr( z) = q( z) /qref= kr2( z0) *( lnz/ z0) 2

 

Unde z0=caracteristica de rugozitate a terenului (in metri cu semnificatia unei lungimi de rugozitate)

kr(z0)=coeficient de rugozitate

• Valorile celor doua marimi sunt exprimate in normativ in

functie de tipul terenului sub aspectul rugizitatii suprafetei

sale.• Valorile maxime ale celor doua marimi se intalnesc in

cazul amplasamentelor neexpuse actiunii vantului,iar cele

minime in cazul amplasamentelor foarte expuse actiunii

acestuia.Valorile lui cr pentru o inaltime data sunt cu atat mai

mari cu cat amplasamentul e mai deschis.

Factorul de rafala introduce in modelul de calcul efectul

turbulentei vantului,caci reprezinta raportul dintre presiunea

de varf produsa de rafalele vantului si presiunea medie

produsa de vitezaa medie a vantului. Exprimarea celor doi

factori conduce la urmatoarea relatie de calcul:

Where

• z0= rugosity caracteristic of the ground (in meters with the significance of the rugosity length)

• kr( z0) = coefficient of rugosity.

 • The values of the two elements are given in codes

depending on the type of the rugosity of the ground below • The maximum values are met in the case of locations not

exposed to the wind action, and the minimum in the case of very exposed locations. The values cr for a given height are increasing in open spaces.

• The gust factor introduces in the model the effect of the turbulence of the wind, for it represents the ratio between the peak gust pressure and the average pressure of wind.

• Expressing the two coefficients leads to the following:

cg(z)=1+g(2I(z))

In care I(z) este intensitatea turbulentei vantului la inaltimea z calculabila cu relatia

I(z)=√β/(2,5*lnz/z0)g=factorul de varf a carui valoare recomandata pentru

calcule curente este 3.5√β=este o marime ce depinde de rugozitatea z0 a

amplasamentului si a carui valori sunt independente de inaltimea z

Factorul topografic introduce in modelul de calcul faptul ca intre viteza vantului de la poalele unui deal si cea din vecinatatea varfului acestuia exista diferente. Acest facor se ia in consideratie numai in cazul amplasamentelor situate pe terenuri in panta, mai aproape de creasta decat de poale.

cg(z)=1+g(2I(z))

Where I(z) is the wind turbulence intensity at the height zand given with the relationship:

I(z)=√β/(2,5*ln z/z0)g= peak factor, with recomended values for usual computation of 3.5√β= is a factor depending on the location rugosity and is independent of the design height z

The topographic factor introduces in the designmodel the difference in the wind speed values function of the location altitude.This coefficient is introduced in design only when there are sloped locations closer to the top than to the bottom of the slope.