Slide 1

Incarcari din vant /wind loadsconf normativ CR1-1-4/2012according to Romanian Code CR1-1-4/2012

Vantul este un agent climatic dat de miscarea unor mari mase de aer inraport cu suprafata pamantului.Miscarea este datorata incalzirii neuniforme a atmosferei si scoartei terestre de catre soare

H

H= inaltime de gradientWind 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 laminaraumed(z) =constantu (z) =constant in timp

Miscare turbulentaumed =variaza logaritmic cu inaltimea

u(z) =variaza in timp

u(z)in zona de interes pentru domeniulcladirilor viteza vantului poate fi ex-primata ca suma de 2 componente:componenta statica constanta in timp u(H)med= viteza medie a vantuluiin intervalul de timp considerat la inaltimea H deasupra terenuluicomponenta 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 withinthe considered interval evaluated at theheight H above the ground.the dynamic component time dependentu*H(t)= gust speed, introducing the timevariation 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)medThe 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 metersthe 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 levelZe= 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 bwhere cr( z) = rugosity factor cpq(z)= gust factor qm(z)= mean value of dynamic wind pressure at a certain height zThe 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.