Variable actions wind loads

The procedure for determining wind load to BS EN 1991-1-4 is presented below.

This presentation is a very simple interpretation of the Code intended to provide a basic understanding of the Code with respect to rectangular-plan buildings with flat roofs. In general maximum values are given: with more information a lower value might be used. The user should be careful to ensure that any information used is within the scope of the application envisaged. The user is referred to more specialist guidance [22] or the Code[23] and UK National Annex[23a]. Note that at the time of writing the UK National Annex had not been published: the information is taken from the DCLG document and a 2006 draft of the Annex.

Determine the fundamental value of the basic wind velocity, vb;0, from Expression NA.1.

vb,0 - vb,mapcalt where

Vb,map = fundamental basic wind velocity from Figure 2.2

calt where

= altitude factor = conservatively 1 + 0.0014

<BS EN 1991-1-4 4.2(1) Note 2 NA 2.4 Exp. (NA.1)> <BS EN 1991-1-4 4.2(1) Note 2 NA 2.4 (Fig. NA.1 )>

<BS EN 1991-1-4 4.2(1) Note 2, NA 2.4, NA 2.5>

A = altitude of the site in metres above mean sea level a.m.s.l.

Calculate basic wind velocity, vb vb - CdirC season Cprobvb,0

where cdir - directional factor. Conservatively, cdir = 1.0 (cdir is a minimum of 0.73 or 0.74 for wind in an easterly direction 30° to 120°)

cseason - season factor

For a 6 month return period, including winter, or greater, cseason = 1.00

cprob = probability factor <BS EN 1991-1-4 4.2(1) Note 4,

= 1.00 for return period of 50 years Note 5 NA 2.8>

Calculate basic wind pressure, qb qb = 0.5 p Vb2

where p = density of air

Calculate peak wind pressure, qp qp = Ce,fiat(z) qb for z < 50 m where ce,flat = exposure factor ce,flat is obtained directly from Figure NA.7 then modified by applying a factor from Figure NA.8 for sites in town terrain. Dependent on distance from shoreline and town perimeter, respectively, and on z - hdis where z and hdis are explained below When orography or dynamic effects are significant (i.e. the structure is on a hill or at the top of an escarpment) refer to BS EN 1991-1-4

z = reference height For windward wall,

<BS EN 1991-1-4 4.5(1) Note 1, NA 2.17, Figs NA.7 & NA.8>

<BS EN 1991-1-4 7.2.2(1)( Fig. 7.4), NA.2.26>

Wind Zone Map 1991National Annex Wind Loads Fig Na9

Figure 2.2 Map of fundamental basic wind velocity, vb,map, (m/s)

Figure 2.3 Exposure factor ce,fiat(z) for sites in country or town terrain

Figure 2.4 Multiplier for exposure correction for sites in town terrain

Note

Subject to altitude correction.

<BS EN 1991-1-4 4.2(1) Note 2, NA 2.4 (Fig. NA.1)>_

<BS EN 1991-1-4 4.5(1) Note 1, NA 2.17 (Fig. NA.7)>_

<BS EN 1991-1-4 4.5(1) Note 1, NA 2.17 (Fig. NA.8)>_

> b high < 2b high, the area may be divided into horizontal strips with ze = height above ground where h = height of building b = breadth of building For leeward and side walls, z = height of building hdis = displacement height

For country sites hdis = 0

<BS EN 1991-1-4 4.3.2 (1) Note, NA 2.11 (Fig. NA.3)>

6 have

where x = distance between buildings have = obstruction height.

In the absence of more accurate information the obstruction height may be taken as have = 15 m for terrain category IV (town centres)

Calculate peak external wind load, We

We = Qp(z)Cpe where cpe = (external) pressure coefficient dependent on size of area considered and zone. For 1 m2 and above cpe,10 should be used For the walls of rectangular-plan buildings, cpe,10 is determined from Table NA.4a and Table NA.4b. See Table 2.3.

For flat roofs, cpe,10 is determined from BS EN 1991-1-4 Table 7.2. See Table 2.4.

For other forms of roof refer to BS EN 1991-1-1 and the UK NA.

<BS EN 1991-1-4 7.2.2(1) Note 1, NA.2.27 (Tables NA.4a & NA.4b) >

Table 2.3

External pressure coefficient, cpe,10 ,for walls of rectangular-plan buildings

<BS EN 1991-1-4 7.2.2(2) Note 1, NA 2.27, Tables NA.4a & NA.4b >

Table 2.3

External pressure coefficient, cpe,10 ,for walls of rectangular-plan buildings

Zone

Description

cpe,10

max.

min.

Zone A

For walls parallel to the wind direction, areas within 0.2min[b; 2h] of windward edge

-1.6

Zone B

For walls parallel to the wind direction, areas within 0.2min[b; 2h] of windward edge

-0.9

Zone C

For walls parallel to the wind direction, areas from 0.2min[b; 2h] to min[b; 2h] of windward edge

-0.9

Zone D

Windward wall

+ 0.8

Zone E

Leeward wall

-0.5

Zones D and E

Net

+ 1.3

Notes h = height of building b = breadth of building

Table 2.4

External pressure coefficient, cpe,10 min, for flat roofs

Table 2.4

External pressure coefficient, cpe,10 min, for flat roofs

Zone

Description

cpe,10 min

Sharp edge at eaves

With parapet

Zone F

Within 0.1min[b; 2h] of windward edge and within 0.2min[b; 2h] of return edge (parallel to wind direction)

-1.8

-1.6

Zone G

Within 0.1min[b; 2h] of windward edge and outwith 0.2min[b; 2h] of return edge (parallel to wind direction)

-1.2

-1.1

Zone H

Roof between 0.1min[b; 2h] and 0.5min[b; 2h] from windward edge

-0.7

-0.7

Zone I

Remainder between 0.5min[b; 2h] and leeward edge

± 0.2

± 0.2

Notes h = height of building b = breadth of building

Where necessary, calculate pressure coefficient, cpi cpi = internal pressure coefficient dependent on size of openings.

For no dominant openings cpi may be taken as the more onerous of +0.2 and -0.3

Calculate resultant wind force, Fw

Fw = CsCdiWAref where cscd= structural factor, conservatively = 1.0

or may be derived where cs = size factor cs may be derived from Exp. (6.2) or Table NA.3. Depending on values of (b + h) and (z - hdis) and dividing into Zone A, B or C, a value of cs (a factor < 1.00) may be found cd = dynamic factor cd may be derived from Exp. (6.3) or Figure NA.9. Dependent on values of 4 (logarithmic decrement of structural damping) and h / b, a value of cd (a factor > 1.00) may be found. cd may be taken as 1.0 for framed buildings with structural walls and masonry internal walls and for cladding panels and elements iW = c f q p( z e)

where cf = force coefficient for the structure or structural <BS EN 1991-1-4 7, 8 & NA >

element (as described above) qp(ze) = peak velocity pressure at reference height ze <BS EN 1991-1-4 7, 8 & NA > (as described above)

Aref = reference area of the structure or structural element <BS EN 1991-1-4 5.3.2, Exp.

<BS EN 1991-1-4 6.1(1) Note & NA.2.20 > <BS EN 1991-1-4 6.2(1) a), 6.2(1) c) & NA> <BS EN 1991-1-4 6.2(1) e) & NA.2.20 >

<BS EN 1991-1-4 6.3(1), Exp. (6.2), NA.2.20 (Table NA3) >

<BS EN 1991-1-4 6.3(1), Exp. (6.3), NA.2.20(Fig. NA9) >

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Responses

  • pinja vesisaari
    How to calculate the wind foece and concrete?
    8 years ago
  • mareta
    How to get wind loads given distancr from the sea and distance from edge of town using eurocode?
    2 years ago
  • fulvia
    How to get altitude factor in eurocode?
    10 months ago

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