Windload example - This an additional tutorial Should help in design for wind effect on a structure PDF

Preview

Summary

This an additional tutorial Should help in design for wind effect on a structure in accordance to the newly modified code SANS 10160- 3 2013...

Description

Table 5 Calculation procedure 1 Description Fundamental basic wind speed Basic wind speed Terrain category Reference height

2 Symbol

3 Reference

vb ,0 vb

Figure 1

A, B, C, D

Table 2 Section 7

ze

Equation (1)

Topography coefficient

c0  z 

Clause 6.3.3

Roughness / height coefficient

cr  z 

Clause 6.3.2

Peak wind speed

vp  z 

Equations (3) and (4)

Peak speed pressure

qp  z

Equation (6)

Internal pressure coefficient

c pi

Clause 7.3.9

External pressure coefficient

cpe

Section 7

Internal wind pressure

wi

Equation (7)

External wind pressure

Equation (8)

Wind force calculated from pressure coefficient

we Fw

Wind force calculated from force coefficient

Fw

Equations (9) and (10)

Internal forces

Fw,i

Equation (11)

External forces

Fw,e F fr

Equation (12)

Frictional forces

Equations (11) and (12)

Equation (13)

Calculate the wind loads on the walls and roof of a house that is in one of the suburbs of Pretoria. The height above sea level is 1400 m.

Pitch of the roof is = 15°

Fundamental Wind speed vb,0: 28m/s

32m/s

36m/s Polokwane

24

Nelspruit Pretoria

Witbank

Mmabatho

26 Johannesburg

Vryburg

Standerton

Klerksdorp

Sishen

Kroonstad

Upington

Bethlehem

Ladysmith

Ulundi

28

Kimberley Bloemfontein

Port Nolloth Springbok

Durban

30

De Aar

Brandvlei

Port Shepstone

Victoria West Umtata Queenstown

Calvinia

Vredendal

Beaufort West

32

Cradock Bisho

Saldahna

East London Worcester

Cape Town

Swellendam

Uitenhage

Oudtshoorn Mossel Bay

34

Port Elizabeth St Francis

Cape Agulhas

32

28

24

20

Basic Wind speed vb  c prob  vb,0 Vb,0 is for a 1:50 return period. If you want to change that, calculate Cprob n

c prob

1 K  ln(  (ln(1 p ))    1 K  ln(  ln( 0 ,98 )) 

If not, Cprob = 1,0 Vb = 28 m/sec

Terrain Category: Table 2 Terrain categories 1

2

3

Category

Description

Illustration

A

Flat horizontal terrain with negligible vegetation and without any obstacles (for example coastal areas exposed to open sea or large lakes)

B

Area with low vegetation such as grass and isolated obstacles (trees, buildings) with separations of at least 20 obstacle heights

C

Area with regular cover of vegetation or buildings or with isolated obstacles with separations of maximum 20 obstacle heights (such as villages, suburban terrain, permanent forest)

D

Area in which at least 15 % of the surface is covered with buildings and their average height exceeds 15 m

NOTE 1 : A certain amount of a reduction in loading for category D can be obtained (Clause 6.3.5) by using a procedure described in Annex A.5 which takes into account the vertical displacement of the peak wind pressure profile, within an environment with closely spaced obstructions.

In this case the terrain category is C

Reference Height, Ze, Section 7 Fig 7:

building face

reference height

shape of profile of velocity presure

b h

qp(z)=qp(ze)

ze=h

(a) h ≤ b

b qp(z)=qp(h)

h-b h

qp(z)=qp(b)

ze=b

b

ze=h

(b) b < h ≤ 2b

b ze=h qp(z)=qp(h)

b

h

hstrip

ze=zstrip qp(z)=qp(zstrip) ze=b

b

qp(z)=qp(b)

(c) h ≥ 2b

h < b, Therefore using (a) Ze = 2,5 m

Topography Coefficient C0(z), Clause 6.3.3 This is only adjusted in special cases. C0(z) = 1,0

Roughness/Height Coefficient; Use either Clause 6.3.2.1 or Table 3

 z  z0  cr  z   1,36   z g  z0   



Or Table 3 Variation of the cr ( z ) factor with height above ground level 1

2

3

5

C

D

Category

Elevation m

4

A

B

0 2

0,92 0,97

0,85 0,85

0,73 0,73

0,71 0,71

4 6

1,02 1,05

0,90 0,94

0,73 0,77

0,71 0,71

10

1,09

0,98

0,85

0,71

15 20

1,12 1,14

1,02 1,05

0,91 0,95

0,78 0,83

30 40

1,17 1,20

1,09 1,12

1,00 1,04

0,90 0,95

50 60

1,22 1,23

1,15 1,17

1,07 1,10

0,98 1,01

70 80

1,24 1,26

1,18 1,20

1,12 1,14

1,04 1,06

90

1,27

1,21

1,15

1,08

100

1,28

1,23

1,17

1,10

Terrain Category C, Elevation 2,5 m. Therefore Cr(z) = 0,73

Peak Wind Speed:

v p  z   cr  z  c 0 z  v b.peak

(3)

vb, peak  1, 4vb

(4)

where

Vb,peak = 1,4 x 28 m/s = 39,2 m/s Vp = 0,73 x 1,0 x 39,2 = 28,6 m/sec

Peak Pressure: qp ( z ) 

1    vp2 ( z ) 2

Density of the air at 1400 m

 1400 

( 1500   1000)  400   1000 500

(6)

Table 4 — Air density as a function of site altitude 1 2 Site altitude above sea level Air density



m 0 500 1 000 1 500 2 000

kg/m3 1,20 1,12 1,06 1,00 0,94

NOTE 1 : A temperature of 20° has been selected as appropriate for South Africa and the variation of mean atmospheric pressure with altitude is allowed for in the above table.

NOTE 2 : intermediate values of ρ may be obtained from linear interpolation.

1400 = 1,012 kg/m3 Peak Pressure:

p p ( z)  1  1,012 28,6 2  414 N 2 2 m EXTERNAL PRESSURE COEFFICIENTS – SECTION 7

d

e=b or 2h whichever is the smaller D

WIND

E

b

b : crosswind dimension

elevation

a) Plan

h A

B

C

h

A

B

C

WIND

WIND

e/5

4e/5

e/5

e

d-e

e

4e/5 d-e

b) Elevation for e...