2_1._Introduction_to_Wind_Design.pdf.pdf

wind design

1. INTRODUCTION TO WIND DESIGN
Dr. Henry LUK

2. Wind Load on Structures
•Wind loads represent the most critical kinds of loads in the
design of a typical high-rise building in Hong Kong.
•The taller a building gets, the more the wind loads become the
controlling factor in the design.
•Wind pressure on a building surface depends many factors:
•E.g. velocity, the shape and surface texture of the building, the protection
from wind offered by surrounding natural terrain or man-made
structures and the density of air.
2

3. Wind-induced Motions
3
•Wind-induced building motion can essentially be divided into
three types:
1.Along-wind motion
2.Across-wind motion
3.Torsional motion
•In tall buildings, the across-wind
and torsional motions are usually
more important.

4. 4
Translation
Translation + Twisting
Symmetric building Asymmetric building

5. Nature of Wind
•Wind is the term used for air in motion and it is usually applied
to the natural horizontal motion of the atmosphere.
•Winds are produced by pressure differences in the atmosphere and
rotation of the earth.
•Air flowing over the earth’s surface is slowed down and made
turbulent by the roughness of the surface.
•Flow of wind unlike that of other fluids, is not steady and
fluctuates in a random fashion. The sudden variation in wind
speed, called gustiness or turbulence, is an important factor in
determining dynamic response of tall buildings.
•Because of this random nature, wind loads for building design are studied
statistically.
5

6. Characteristics of Wind
•Wind flow is complex because numerous flow situations arise
from the interaction of wind with structures.
•In wind engineering, simplifications are made to arrive at the
design wind loads :
•Variation of wind velocity with height (velocity profile);
•Wind turbulence;
•Statistical probability;
•Vortex shedding;
•Dynamic nature of wind-structure interaction;
•Cladding pressure.
6

7. Wind Velocity Profile
•The wind speed profile depends mainly on the degree of
surface roughness, caused by the overall drag effect of
buildings, trees, and other projections.
•The zone of wind turbulence due to surface roughness is often
refereed to as surface boundary layer.
•How wind effects are felt within this zone, where human construction
activity occurs, is of concern in building .
•The height at which the slowdown effect ceases to exist is
called gradient height, and the corresponding velocity, gradient
velocity.
•At height of approximately 500 m above the ground, the wind
speed is virtually unaffected by surface friction.
7

8. Wind Turbulence
•Air has a very low viscosity. Any movement of air at speeds
greater than 0.9 – 1.3 m/s is turbulent, causing particles of air
to move randomly in all directions (turbulent).
•The wind speeds can be decomposed into two components
•Quasi-steady mean wind speed that increase with height;
•Turbulent speed (Gust wind speed) remains the same over height.
8
)(
~
)( tuutu
Wind speeds )(
~
)( tpptP
t

Total pressure

9. Probabilistic Approach
•The speed of wind is considered to be a function of the
duration of recurrence interval, i.e. return period.
•A 50 year return-period wind of 30 m/s means that on the
average, we will experience a wind faster than 30 m/s within a
period of 50 years.
•A return period of 50 years corresponds to a probability of
occurrence of 1/50 = 0.02 = 2% per year.
•Consider a building designed for a 50 year service life. The
probability of exceeding the design wind speed 30 m/s is
9
%6464.036.01)02.01(1
50
P

10. Vortex Shedding
•Along wind is used to refer to drag forces, while transverse
wind is the term used to describe cross-wind.
•In tall building design, the cross-wind motion is often more
critical than along-wind motion.
10

11. •The originally parallel upwind streamlines are displaced on either side of
the building. This results in spiral vortices being shed periodically from the
sides into the downstream flow of wind, called the wake.
•When the vortices are shed, that is, break away from the surface of the
building, an impulse is applied in the transverse direction. At higher speeds,
vortices are shed alternately on both sides of building, causing vibration of
structures in the transverse direction.
•This phenomenon is called Vortex Shedding.
11

12. •There is a simple formula to calculate the frequency of the transverse
pulsating forces caused by vortex shedding:
12
•When the wind speed is such that the shedding frequency becomes
approximately the same as the natural frequency of the building, a
resonance condition is created.
•Further increases in wind speed by a few percent will not change the
shedding frequency, because the shedding is now controlled by the natural
frequency of the structure.
•The vortex-shedding frequency has, so to speak, locked in with the natural
frequency.
D
SV
f
u

where
f = frequency of vortex shedding (in Hz)
V = mean wind speed at the top of the building
S = a dimensionless parameter for the shape
D = diameter of the building

13. Dynamic Nature of Wind
•Wind loads associated with gustiness or turbulence change
rapidly, creating effects much larger than if the same loads
were applied gradually.
•The action of a wind gust depends not only on how long it
takes the gust to reach its maximum intensity and decrease
again, but on the period of the building itself.
•If the wind gust reaches its maximum value and vanishes in a time much
shorter than the period of the building, its effects are dynamic.
•On the other hand, the gusts can be considered as static loads if the wind
load increases and vanishes in a time much longer than the period for the
building.
13

14. Cladding Pressures
•When air flows around a structure, the resulting pressures may
damage the local components such corner windows, eave and
ridge tiles, etc.
•The expense of replacement and hazards posed to pedestrians
is of major concern.
14
1.Positive pressure zone on the
upstream face (Region 1)
2.Negative pressure zones at the
upstream corners (Region 2)
3.Negative pressure zone on the
downstream face (Region 3).
The net load on cladding is the difference
between the external and internal pressures.

15. CODE OF PRACTICE ON WIND
EFFECTS IN HONG KONG

16. Code of Practice on Wind Effects
16

17. Scope
•This code of practice gives general methods for calculating the
wind loads to be used in the structural design of buildings.
•The code does not apply to buildings of an unusual shape or
buildings situated at locations where complicated local
topography adversely affects the wind conditions.
•Experimental wind tunnel data may be used in place of the values given
in the Code.
•The design wind pressures have been determined from the
hourly mean wind velocities and peak gust wind velocities
having a return period of 50 years.
•Appendix B in wind code provides mortification factor for design wind
pressure with return period greater than 50 years.
17

18. Calculation of Wind Loads
•Two conditions:
18
•The building is considered to be one with significant resonant
dynamic response if it has either of the following properties:
1.The height exceeds five times the least horizontal dimension.
2.The height of the building is greater than 100 m.
•Unless it could be justified that the fundamental natural
frequency of the building is greater than 1 Hz.
Without significant resonant dynamic response
With significant resonant dynamic response
Section 3

19. 19
•Breadth (b) means the horizontal dimension of the building normal to the direction of wind.
•Depth (d) means the horizontal dimension of the building parallel to the direction of the
wind.
•Height (H) means the height of the building above the site-ground level in the immediate
vicinity of the building up to the top of the building. Masts and other appendages on top of
the building should not be include.
•Frontal projected area means the area of the shadow projection of the building on a plane
normal to the direction of the wind.

20. Method 1: without significant resonant dynamic response
1.f
n > 1 Hz; or
2.H <= 5 x min(b,d); and
3.H <= 100 m
Method 2: with significant resonant dynamic response
1.f
n <= 1 Hz; and
2.H >= 5 x min(b,d); or
3.H > 100 m
Method 3: complex structures
1.Open frame with significant dynamic response; or
2.f
n < 0.2 Hz; or
3.Significant cross wind or torsional resonant response

20
Section 3
Equivalent static load
method (Clause 4 and 5)
Gust factor method
(Clause 7)
Recommendation from literature
or wind tunnel test (Appendix A)

21. Design Wind Pressure
•For building without
significant resonant dynamic
response, the design wind
pressure q
z at height z shall
be taken as the value given
in Table 1.
•Topography effect (see
Appendix C).
21
Section 4
(open sea terrain)

22. Forces on Buildings
•The total wind force F on a building shall be taken to be the
summation of the pressures acting on the effective projected
areas of the building
22
zzf
AqCF?
where
C
f is the force coefficient for the building (Appendix D);
q
z is the design wind pressure at height z (Table 1);
A
z is the effective projected area of that part of the building
corresponding to q
z.
•Every building shall be designed for the effects of wind
pressures acting along each of the critical directions.
Section 5

23. Appendix D
•The force coefficient C
f for an enclosed building is given as


•where the height aspect factor C
h and the shape factor C
s given in Table
D1 and Table D2 respectively; or
•International Codes acceptable to the Building Authority may be used.
23
shf
CCC ?

24. 24

25. •If the frontal projected area is greater than 500 m
2
, the force
coefficient may be multiplied by a reduction factor R
A given in
Table D3 .
•This is applicable for structures without significant resonant
dynamic response.

25

26. •The force coefficient C
f for an open framework building shall
be the value given in Table D4; or the appropriate value
specified in other International Codes acceptable to the
Building Authority.
26

27. Example 1
Determine the design wind pressure distribution along the
height of building. Compute the base shear and overturning
moment. Assume that the building is sitting on a smooth surface
(open sea terrain).
27
10
20
40
Along wind direction
Unit: m

28. Solution
Checking for resonant effect
28
10
20
40
Along wind
direction
?
?
?


5410/40/
m 100m 40
dH
H
Without significant resonant
dynamic response (Clause 3.3)
Topography effect
Insignificant

29. Compute design wind pressure
29
q
z = 1.82 kPa
q
z = 2.01 kPa
q
z = 2.23 kPa
q
z = 2.37 kPa
q
z = 2.57 kPa
5m
10m
20m
30m
40m

30. Height/Breadth aspect factor
30 220/40/ bH 0.1
h
C

31. Shape factor
31 210/20/ db 1.1
s
C
Force coefficient factor 1.1
1.10.1

u
?
shf
CCC
Frontal projected area = 20 x 40 = 800 m
2
> 500 m
2
Wind load reduction may be considered.

32. Determine base shear and moment
32
F
z1 = 182 kN
F
z2 = 201 kN
F
z3 = 446 kN
F
z4 = 474 kN
F
z5 = 514 kN
5m
10m
20m
30m
40m
Base shear force
Overturning moment
kN 1999
)182201446474514(1.1

u
? zzf
AqCF
kNm 42342
)5.21825.7201
154462547435514(1.1

uu
uuuu
? zzzf
zAqCM
(
??LM
??#
??

33. Dynamic Effects
•For building with significant resonant dynamic response, the
total along-wind force F on an enclosed building with
significant resonant dynamic response shall be determined by
33
zzf
AqGCF ?
where
G is the dynamic magnification factor, or gust factor (Appendix F)
C
f is the force coefficient for the structure (Appendix D);
M$
? is the design hourly mean wind pressure at height z (Table 2);
A
z is the effective projective area.
Section 7

34. 34

35. Appendix F
•The dynamic magnification factor G may be taken as the
values from Table F1 or Table F2, or by
35

36. •Alternatively, the dynamic magnification factor G may be taken
as follows
36

37. 37

38. 38

39. 39

40. •Special cases
•an open framed building with significant resonant dynamic response; or
•a building for which the fundamental natural frequency is less than 0.2
Hz, or
•the cross wind resonant response / torsional resonant response may be
significant.
•The dynamic effects should be investigated in accordance with
recommendations given in published literature and/or through
dynamic wind tunnel model studies.
•The combination total response of such a building would
usually be calculated from the of the response in the three
fundamental modes of vibration.
40

41. Wind Tunnels
•Wind tunnels are used to provide accurate distributions of
wind pressure on buildings as well as investigate aero-elastic
behaviour of slender and light weight structures.
41

42. •Services provided by a wind tunnel consultant typically offer the
following benefits:
•Provides an accurate distribution of wind loads, especially for structures in a
built-up environment by determining directly the impact of surrounding
structures.
•Provides predictions of wind-induced building motions (accelerations and
torsional velocities) likely to be experienced by occupants of the top floors,
and compares the test results to available serviceability criteria.
•Estimates cladding pressures and overall loads which can help the engineer,
the architect, and the facade engineer to develop a preliminary foundation
design and initial cost estimate for the curtain wall.
•Provides an assessment of expected pedestrian wind comfort along with any
conceptual recommendations for improvement to key pedestrian areas.
•The overall design wind loads are generally (not always) lower than code
wind loads resulting in lower cost.
42

43. •In determining the effects of wind for a particular building,
there are two main components to consider.
•The first comprises the aerodynamic characteristics of the building.
These are simply the effects of the wind when it blows from various
directions.
•This climatological information, in the form of a probability distribution
of wind speed and direction, is the second main ingredient needed for
determining wind effects for a particular development.
•Appendix A in wind code states the necessary provisions for
wind tunnel testing.
43

44. •Types of wind-tunnel test
•Rigid pressure model (PM)
•Obtains cladding design pressures, storey shear forces and base shear and
overturning moment.
•Rigid high-frequency base balance model (HFBB/HFFB)
•Determine the effects of wind load on a flexible building with the
consideration of mean wind, fluctuating wind and inertia effect.
•Aero-elastic model (AM)
•Investigate the instabilities of structure or capture the resonant behaviour of
building.
44

45. Appendix B
•The design wind pressures on buildings where the period of
exposure to wind is longer than 50 years shall be multiplied by
the following factor:
45
where R is the period of exposure to wind in years.

46. Appendix C
•Wind code states that local
topography is considered
significant for a site located
within the topography
significant zone as defined
in Figure C1.
46

47. •The relative dimensions of
the topography are defined
in Figure C2.
•For shallow upwind slopes
0.05 < α
u < 0.3


•For steep upwind slopes α
u
> 0.3

47
ue
DD
ue
LL
3.0
e
D 3.0/HL
e

where
α
e = effective slope
L
e = effective slope length

48. •The design wind pressure at height z shall be multiplied by a
topography factor S
a at that height.
•The topography factor S
a at height z above site ground level
shall be determined by
48
2
)2.11( sS
ea
D
where
α
e = effective slope
s = a topography location factor (Figure C3 for hills and ridges, Figure C4 for
cliffs and escarpments)

49. 49

50. Forces on Elements
•The total wind force F
p acting in a direction normal to the
individual elements such as walls, roofs, cladding panels or
members of open framework structures shall be determined by
50
mzpp
AqCF
where
C
p is the total pressure coefficient for individual elements (Appendix E);
q
z is the design wind pressure at height z;
A
m is the surface area of the element.
•Except for members of open framework structures, the design
wind pressure shall be a constant value over the lower part of
the building.
Section 6

51. Appendix E
•The total pressure coefficient C
p for individual elements in a
particular area of an enclosed building:
•where there is only a negligible probability of a dominant opening
occurring during a severe storm, the value given in Table E1; and
•where a dominant opening is likely to occur during a severe storm, the
value determined with the aid of other published materials acceptable to
the Building Authority or through the use of wind tunnel model studies.
•The total pressure coefficient C
p for individual elements of an
open framework building shall be
•2.0; or
•appropriate value specified in other International Codes acceptable to
Building Authority.

51

52. 52

53. Summary
•(Method 1) Without significant
resonant dynamic response
1.Calculate design wind
pressure (Table 1)
2.Determine topography factor
S
a (Appendix C)
3.Calculate force coefficients C
f
(Appendix D)
4.Calculate total wind force

53
•(Method 2) With significant
resonant dynamic response
1.Calculate design hourly mean
wind pressure (Table 2)
2.Compute gust response factor
G (Appendix F)
3.Determine topography factor
S
a (Appendix C)
4.Calculate force coefficients C
f
(Appendix D)
5.Calculate total wind force

zzf
AqCF?
zzf
AqGCF?