The Eighth Asia-Pacific Conference on Wind Engineering,
December 10–14, 2013, Chennai, India
Recommendation on topographic effect correction factor for applying of
Chinese National wind load code
Zheng-wei Zhang1, Alex P. To2
1
Assistant Engineer, Wind Engineering, Ove Arup International Consultants (Shanghai) Co. Ltd., Shanghai,
PRC, [email protected]
2
Associate, Wind Engineering, Ove Arup & Partners Hong Kong Limited, Hong Kong, PRC, [email protected]
ABSTRACT
Complex terrain changes the wind field characteristics significantly, especially on the mean wind speed and
turbulence intensity during the separation zone of steep hills and escarpments. With the acceleration of
urbanization process in China, a large number of super tall buildings, large-span and high-rise structures will be
built in western and southern parts of China where the topography effects cannot be ignored in the wind-resistant
design. Based on the Chinese Code GB 50009-2012, Japanese Code AIJ-2004, American Standard ASCE 7-10,
Australian/New Zealand Standard AS/NZS 1170:2:2011, Euro code EN 1991-1-4:2005, ISO Code 4354:2005
and Canadian code NBC 2005, the topography influences of typical terrain are analyzed in details and the main
parameters on topographic effect are given. On this basis, the improved topographic correction factor formulas
for mean wind speed and peak wind speed are obtained. By comparing with the numerical examples, wind tunnel
test and field measurement results in literatures, the applicability of the improved formulas are verified and
provides a reference for future wind loading code amendments and wind-resistant design in complex terrain
areas.
Keywords: Topography effect, Wind code, Correction factor, Wind speed, Turbulence Intensity
1 Introduction
70% of China's land is mountainous and a large number of super tall buildings, largespan and high-rise structures will be built in western and southern parts of China during
urbanization process. For structures situated in hilly terrain, the speed up of the wind velocity
over hills and escarpments and the turbulence intensity during the separation zone of steep
hills and escarpments are important considerations (Holmes, 2007). In some low seismic zone,
wind loads always play a important role in structural design.
Mean and peak wind speeds can be increased considerably by natural and man-made
topography in the forms of escarpments, ridges, cliffs and hills. The topography effects were
the subject of considerable research in the 1970s and the main methods used by scholars are
wind tunnel test (Jackson 1979; Gong and Ibbetson 1989; Miller and Davenport 1998; Kim
1997, 2000; Cao 2006; Lubitz and White 2007) and numerical simulation (Jackson and Hunt
1975; Kaimal and Finnigan 1994; Taylor and Lee 1984; Weng 2000). Research objects are
two-dimensional ridge, two-dimensional escarpment and three-dimensional axisymmetric
hills (shown in figure 1). The effects of geometric dimensions of the mountain (different
slopes and height) and terrain roughness categories on mean wind speed, turbulence intensity
and peak wind speed are studied and the occurrence of flow separation conditions was
analyzed. These research results have been reflected in the load specification of various
countries. Chinese scholars do few researches on topography effects. Hu (2006) used wind
tunnel test to study the wind characteristics on the bridge site at the canyon terrain; Sun(2010)
gave the empirical formulas of mean and fluctuating wind speed distribution from wind
Proc. of the 8th Asia-Pacific Conference on Wind Engineering – Nagesh R. Iyer, Prem Krishna, S. Selvi Rajan and P. Harikrishna (eds)
c 2013 APCWE-VIII. All rights reserved. Published by Research Publishing, Singapore. ISBN: 978-981-07-8011-1
Copyright doi:10.3850/978-981-07-8012-8 187
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Proc. of the 8th Asia-Pacific Conference on Wind Engineering (APCWE-VIII)
tunnel test of 10 different slope and height of the three-dimensional axisymmetric mountain
model; Wei et al (2010) studied the speed-up mean wind speed used Fluent software; Li et al
(2010), Li et al (2010) and Wang and Sun(2012) have carried on the wind resistance design
on high-rise building, tower and transmission tower located on mountainous areas and the
topographic correction factor for wind pressure on Chinese wind load code GB50009-2012 is
based on developed countries wind codes.
The influence of wind field in typical terrain close to ground and the latest wind
loading codes adopted in some of the developed countries and regions are compared and
analyzed in depth. On this basis, the improved topographic correction factor formulas for
mean wind speed and peak wind speed are obtained. And by comparison with the numerical
examples and wind tunnel test and field measurement results in literatures, the precision and
applicability of the improved formulas are finally verified and can provide a reference to the
load specification amendments and wind-resistant design during complex terrain areas.
Fig. 1 Definitions for Wind speed-up effect and separation zone over hill and escarpment
2 Parameters analysis about topography effects
With flow normal to the upstanding face of a shallow escarpments and hills, the wind
speed near the surface first decelerates from the approach value Vz to the minimum value at
the foot of the escarpments and hills. Next the flow accelerates to a maximum near the crest,
before decelerating again to a constant value downwind. On steep escarpments and hills
(slope greater than 0.3), flow separation will occur. Separations may occur at the start of the
upwind slope, immediately downwind of the crest and on the downwind slope for hills and
ridges. The “topographic multipliers” can be defined as follows for the mean and peak
velocity.
U ( z) U 0 ( z)
, Mt
U 0 ( z)
'S
Uˆ ( z )
>1 2 gIU ( z )@U ( z ), IU ( z )
'Sˆ
Uˆ ( z ) Uˆ 0 ( z ) ˆ
, Mt
Uˆ ( z )
(1)
1 'S
VU ( z)
U ( z)
1 'Sˆ
(2)
(3)
0
Where, U ( z ) and Uˆ ( z ) are the mean wind speed and peak wind speed at height z above the
topography respectively; U 0 ( z ) and Uˆ 0 ( z ) are the mean winds peed and peak wind speed at the
height z above the upwind flat ground respectively.
)U
tan DU
LU
, )D
H
tan D D
LD
H
Where )U and ) D are the upwind slope H/Lu in the wind direction (see Figure 1).
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Proc. of the 8th Asia-Pacific Conference on Wind Engineering (APCWE-VIII)
From Table 1, most countries use formulas for topography correction factor; only AIJ2004 provides the correction factor for turbulence intensity and AA/NZS:2011 gives the
correction factor during the separated zone. From Table 2, the effect parameters on
topography correction factor are hill upwind slope, height of the hill, upwind horizontal
distance X and the reference height on the structure above the average local ground level z.
Table 1: Main research contents in main wind codes / standards
Main wind Codes / Standards
Considered items
Formulas
Charts
Mean wind
speed
wind
pressure
Correction
item
Turbulence
intensity
Peak wind
speed
2-D ridges
3-D axisymmetrical
Hill
hills
shapes
2-D
escarpments
2-D Valleys
Separation zone
Terrain roughness
categories
Correction
factor
GB
500092012
ASCE
7-10
AS/NZS
1170:2:2011
ISO
4354:2005
NBC
2005
AIJ
2004
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
BS 63992:1997, EN
1991-14:2005
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
Ĝ
(a) Hills and ridges
(b) Escarpments
Fig. 2 Definitions for horizontal topographic speed-up zone
Table 2: Correction factors of upwind (X d 0) section for all topography
Main wind
Codes /
Standards
GB50009-2012
Topographic correction factor
Mt
ȘB 1
LU
L U X , ȘB
z ·
§
1 ț)U ¨1 ¸ , if )U >0.3, )U =0.3; ț =2.2 for hills and ridges;
© 2.5H ¹
ț =1.4 for escarpment
AS/NZS
1170:2:2011
X·
·§
H
¸
¸ ¨1 © 3.5(z L1 ) ¹© L 2 ¹
§
ĭU < 0.05, M̂ t 1 ;0.05 ”ĭu, M̂ t 1 ¨
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Proc. of the 8th Asia-Pacific Conference on Wind Engineering (APCWE-VIII)
s
EN 1991-1-4˖
2005
Mt
)U 0.05
­ 1,
°
s
)
d
)U d 0.3 ; A
1
2
,
0.05
®
U
° 1- 0.6s,
)
U ! 0.3
¯
B
ª Xº
«B »
Lu ¼
A·e ¬
, 1.5 d
X
z
d 1.0,0 d
d 2.0
Lu
Le
4
3
2
§ z ·
§ z ·
§ z ·
§ z ·
0.1552 ¨ ¸ 0.8575 ¨ ¸ 1.8133 ¨ ¸ 1.9115 ¨ ¸ 1.0124 ,˗
© Le ¹
© Le ¹
© Le ¹
© Le ¹
2
§ z ·
§ z ·
0.3542 ¨ ¸ 1.0577 ¨ ¸ 2.6465
© Le ¹
© Le ¹
Take s=0 unless the following conditions apply.
ISO 4354:2005
AIJ 2004
Vref ,m
Mˆ t
1
Mt
C1 1 ®C2 §¨
E gI
EI
, EI
Mt
Vref
k1)U s ( x, z ), M t
­
z
¯
©H
1 k1)U s ( x, z ), s x, z 1 x
0.75 LH
e
k3
z
LH
­
· ½
§z
·½
C3 ¸ 1¾ exp ® C 2 ¨ C3 ¸ ¾ 1, 'S t 1 (for wind speed)
¹ ¿
©H
¹¿
¯
­
z
¯
©H
C1 1 ®C2 ¨§
­
· ½
§z
·½
C3 ¸ 1¾ exp ® C2 ¨ C3 ¸ ¾ 1,E I t 1 (for turbulence intensity), E I is
H
¹ ¿
©
¹¿
¯
the topography factor for the standard deviation of fluctuating wind speed.
ASCE 7-10
Mt
1 K1K 2 K 3 , K 2
§
X ·
¨1 ¸ , K3
© 1.5L h ¹
e Ȗz/Lh K1 DQG J =0.725 )U and 3(for 2-D ridges or valleys),
0.425 )U and 2.5 (for 2-D escarpments), 0.5025 )U and 4 (for 3-D axi-symmetrical hills);
NBC 2005
§
X · Įz/L 1 'Smax ¨1 , 'Smax and D =1.1 )U and 3(for 2-D ridges or valleys), 0.65 )U
¸e
© 1.5L ¹
and 2.5 (for 2-D escarpments), 0.8 )U and 4 (for 3-D axi-symmetrical hills);
Mt
Note: 1) Le is the effective length of upwind slope, for shallow upwind slopes 0.05 < )U < 0.3, Le = Lu; for
steep upwind slopes )U > 0.3, Le = H/0.3;
2) L1 is length scale, to determine the vertical variation of , to be taken as the greater of 0.18 Lu or 0.4 H;
3) L2 is length scale, to determine the horizontal variation of , to be taken as 4 L1 upwind for all types,
and downwind for hills and ridges, or 10 L1 downwind for escarpments;
4) C1, C2, C3: parameters determining the topography factor, depend on the topography shape, inclination
and distance (m) from the top of the topographic feature to the construction site. When the inclination
is greater than 60 degrees, the topography factor is assumed to be the same as that at 60 degrees.
5) K1: Factor to account for shape of topographic feature and maximum speed-up effect; K2: Factor to
account for reduction in speed-up with distance upwind or downwind of crest; K3: Factor to account for
reduction in speed-up with height above local terrain.
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Proc. of the 8th Asia-Pacific Conference on Wind Engineering (APCWE-VIII)
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