Incarcari din vant /wind loads

conf normativ CR1-1-4/2012according to

Romanian Code CR1-1-

4/2012

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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)

Click to edit the outline text format Second Outline

Level Third Outline

Level Fourth Outline

Level Fifth

Outline Level

Sixth Outline Level

Seventh Outline Level

Eighth Outline Level

• Ninth Outline LevelClick to edit Master text styles

– Second level

• Third level

– Fourth level

» Fifth level

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

• 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

• 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:

qb= 0, 625 nb2.

• In structural design of the buildings, the wind actionis modelled starting from a reference dynamic wind pressure qb, associated to a territorial macrozonation.

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

- 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 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” with respect to the surface of the ground.

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

• we=q p (ze)*c pe*giw [KN/m2] (interior pressures respect the formula with diff. coeff.)

where:

• we= the wind pressure /sucction on an envelope area at a hight „z” above the ground level

• Ze= the refference height for exterior pressure

• c pe= the aerodynamic coefficient of pressure of the considered envelope zone

• 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.

• q p = the peak value of the dynamic wind pressure at a certain height z above ground level is given by the formula:

qp(z) = cpq(z) * qm(z)= cpq(z) * cr2(z) * q b

where

• cr( z) = rugosity factor

• cpq(z)= gust factor

• qm(z)= mean value of dynamic wind pressure at a certain height z

• The factor cr(z) introduces in the expression the influence of the ground surface rugosity of the construction location, and is expressen function of 2 parameters:

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

 

• 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.