WIND FRENCH NORM NV65

I – General points

1. Definitions

Hypothesis : for all buildings, the direction of the wind is horizontal..

Def1: Mater couple : orthogonal projection of the wind surface on a perpendicular surface to the wind direction.

Elements to determine :− dynamic pressure q− proportions (ratios λ et φ)− ν and c

Def2 : Dynamic pressure : pressure exercised on a flat surface perpendicular to the wind, so as

the wind speed become null.

The wind force is perpendicular to the structure and depends on the wind speed V, the properties of the structure, its orientation in relation to the wind and its form and dimensions.

In Dynamics is defined a normal pressure qn and an extremal pressure qext such as :

qextqn

=1.75

Def3 : Nominal pressure : for a classic location, without any area effect or mask effect, for every element which higher dimension is higher than 0.5m, the basic pressures qn and qext qre those calculated 10 m above the ground level. Those pressures are defined in tables and maps (see pp 47-52 NV 65 for France). They will be called qn10 and qext10 or more easily q10 when formulas will be set for both.

Def4 : Permeability of the walls : the permeability µ of a structure is defined as the ratio of the total surface opened to the wind on the global surface of the structure.A structure is called 'closed' if µ<5%, partly opened if 5<µ<35 and opened if µ>35.

2. Effects leading to changes

(a) Calculation according to the height

H in (0;500), we've got :

qHq10

=H 18H 60

qH is the dynamic pressure considering the height H.

This is valid as told : − if the slope <0.3, H is the height from the basis of the structure− else, H is the height from the lower part of the slope.

(b) Calculation according to the location : area effect

There is a table in which we can find the correcting coefficient ks according to the ratio area/situation, that is to say : − for a sheltered area ks = 0.8.− for a classic area (slope lower than 10%) ks =1.− for a windy area (mountain, beach) ks=1.2-1.3.

(c) Calculation according to the location : mask effect

If the structure is masked, 2 cases come :− if it makes the wind force higher, a scale model simulation is required.− If it makes the wind force lower, we can reduce the pressure up to 25%.

(d) Calculation according to the location : dimensions

There is a table (fig R-III-2 nv65) giving the δ coefficient affecting the dynamic pressure. For the big dimensions continuous structures, like pipes, a 0.9δ or 1.1δ correction may be applied, by conservating the worst result.

(e) Boundary counditions

− The cumulated reductions can't exceed 33%− The following limit values are imposed :

− Normal dynamic pressure : max = 170 daN/m2 ; min=30 daN/m2 − Extremal dynamic pressure : max = 297.5 daN/m2 ; min=52.5 daN/m2

3. Calculation of the needed coefficients

Here are the differents way to calculate the needed coefficients :• λ is the ratio between 2 characteristic dimensions, for a cylinder which diameter is D and

height H, λ = H / D .

• φ : ratio between 2 surfaces

• pressure coefficient c : it depends on λ ou φ, but for pipes & tubes, there are the following valoues :

d is the diameter in m.If the axis of the cylinder is perpendicular to the wind :

d q0.5 : c = 1 0.5≤d q≤1.5 : c=1.135−0.27×d×q d q1.5 : c = 0.73

If the axis makes an α angulus with the wind :

α <15° : cα = c× 0.5×α30

−0.25

15° < α : cα =c α >75° : cα =0

The force Rα is so cα×q

d

For an half-full sphere with the wind on the hollow side, c=0.4For a full sphere :

d q0.5 : c = 0.48 0.5≤d q≤1.5 : c=0.62−0.28×d×q d q1.5 : c = 0.20

4. Calculation of the dynamic &static actions

(a) Static forces

There are static actions inside % outside, caracterised with cext et cint knowing that the inside action are generally neglected.

Outside, the action on the wall is :

elementary action p=c×qon a wall : p r=c1−c2×q r

so the global action is : P=c×q×S

(b) Dynamic forces

There are two kinds of overload, the same way there are two kinds of dynamic pressures : normal and extremal.

Normal overload : we apply the majorating coefficient β=θ×1ξ×τ in which : ξ is the amortissment coefficient, function of the period T of the fundamental mode of the structure, τ is givent in the tables (p 83 nv 65), θ = 1 for the cylinder-shaped structures.

Extremal overload : same overload by β for the pipes (because θ =1).