Comparative Study of Major International
Transcript of Comparative Study of Major International
COMPARATIVE STUDY OF MAJOR INTERNATIONAL
STANDARDS
Rachel Bashor1 and Ahsan Kareem
2
1 Ph.D. Candidate, NatHaz Laboratory, University of Notre Dame,
Notre Dame, IN, USA, [email protected] 2Robert M. Moran Professor, NatHaz Laboratory, University of Notre Dame,
Notre Dame, IN, USA, [email protected]
ABSTRACT
Globalization of the construction industry and the development of unified international codes and standards
intensifies the need to better understand the underlying differences between the major international wind loading
standards. A comprehensive comparison of the wind loads and their effects on tall buildings is conducted
utilizing six major international codes and standards: ASCE 2005 (USA), AS/NZ 2002 (Australian and New
Zealand), NBCC 2005 (Canadian), AIJ 2004 (Japanese), Eurocode 2004 (EU), and ISO 2009. The key areas of
comparison include the provisions for strength design as well as the serviceability requirements in the alongwind,
acrosswind, and torsional directions. As the standards utilize a common theoretical framework, the equations are
re-written in a general format in order to compare the individual parameters.
KEYWORDS: WIND CODES AND STANDARDS, TALL BUILDINGS, WIND LOADING
Introduction
Globalization of the construction industry and the development of unified international
codes and standards, i.e. ISO [2009], intensifies the need to better understand the underlying
differences between the major international wind loading standards. Previous studies have
found that the varying definitions of wind field characteristics, including mean wind velocity
profile, turbulence intensity profile, wind spectrum, turbulence length scale, and wind
correlation structure, were the primary contributors to the scatter in predicted response
quantities [Zhou et al. 2002; Tamura et al. 2005]. As nearly every major building code has
been updated in the last few years, it is necessary to update previous code comparison work.
A comprehensive comparison of the wind loads and their effects on tall buildings is
conducted utilizing six major international codes and standards. These codes/standards are:
the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other
Structures [ASCE 2005], the Australian and New Zealand Standard [SAA 2002], the National
Building Code of Canada [NRC 2005], the Architectural Institute of Japan Recommendations
[AIJ 2004], the European Standard [Eurocode 2004], and the International Organization for
Standardization 4354 [ISO 2009]. All utilize the traditional displacement-based gust loading
factor for assessing the dynamic along-wind loads and their effects on tall structures but
incorporate different provisions for the acrosswind and torsional loads [Tamura et al. 2005].
The key areas of comparison include the provisions for strength design in the
alongwind, acrosswind, and torsional directions as well as the serviceability requirements. As
the standards utilize the same basic theory, the equations are re-written in a general format in
order to compare the individual parameters. These parameters are investigated and several
examples are presented. Finally, the deviations in the predictions are discussed and
suggestions are made to improve agreement between the standards.
Wind Characteristics in Codes and Standards
Although these standards determine wind loading in the along-wind direction using a
random-vibration-based gust factor approach, the parameters are defined differently. These
parameters are re-written in a consistent format and compared with each other. Some of the
difficulties in using international standards is the use of different terminology and the
The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
incorporation of factors within other terms, making it hard for designers to work in a global
environment. Rewriting the basic equations in a general format will help designers decipher
the nuances of the different codes/standards and understand the resulting differences in the
response. Note that the scope of this analysis is limited to dynamically sensitive buildings of
regular shape. All the standards recommend that extremely tall and irregular shaped structures
be designed using wind tunnels.
Alongwind Wind Loads
In all six standards, the alongwind loads are determined by multiplying the wind
pressure by the tributary area of the building. The general expression for pressures on a
building for all the standards can be expressed as:
pqGCp = (1)
where =q velocity pressure; =G gust factor; and =pC pressure coefficient. The following
investigates both internal pressures and external pressures, acting in the windward and
leeward directions. The loads are then determined by combining the pressures acting on a
wall and the corresponding tributary area. Moments are determined by multiplying the load at
a given height by the corresponding height. Base shear forces and moments are then
determined by the sum of the loads and moments at each level.
The velocity pressure can be expressed as:
otherimportancedirectionterrainosure CCCCCVq ⋅⋅⋅⋅⋅= exp
2
021 ρ (2)
where =ρ air density; 0V = basic wind velocity; =osureCexp velocity profile or exposure
factor; =terrainC terrain and topography factor; directionC = directionality factor; importanceC =
building importance factor; and otherC = a factor accounting for other things such as hurricane
zone, shielding, or mean recurrence interval. The effects of terrain, directionality, building
importance, and other factors are not considered in this study. However, the definitions of
velocity profile are analyzed in detail and compared between the standards.
Averaging times for wind velocity vary between the standards and within the
standards. For example, in Eurocode, the velocity is adjusted from 10 minute to one hour for
calculations of response. In addition, the reference height at which the gust factor and other
parameters are calculated is different between the codes/standards, as summarized in Table 1.
These differences between averaging time and reference heights affect the intermediary
parameters and resulting responses, making a simple comparison between the standards
challenging. Throughout this analysis, the effect of differing averaging times has been
minimized as much as possible.
Table 1: Averaging Times and Reference Heights
ASCE AS/NZ NBCC AIJ Eurocode ISO
Averaging time for basic
wind velocity 3-s 3-s 1-hr 10-min 10-min
3-s
10-min
Averaging time for design
velocity at reference height 1-hr 3-s 1-hr 10-min 1-hr 10-min
Reference height for gust
factor 0.6h h h h 0.6h h
NOTE: ISO provides two procedures for determining loads: one for peak response and one for mean response.
The wind velocity in each code is described by a profile law, either power or
logarithmic. Eurocode, AS/NZ and ISO both use the logarithmic law, whereas the others use a
power law. ISO also provides power law fits to the profiles. The velocity profiles are
dependent on the exposure category. Each standard uses three to five exposures categories,
and can be described by six general exposure categories. For the purposes of this paper,
The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
exposure category (EC) 4 is defined as open land, category 3 is defined as suburban, and
category 2 is defined as urban. To compare the profiles, it is important to account for the
differences in averaging time. In the following, the velocity profiles are compared based on
their respective averaging times and exposure categories. In Figure 1, the profiles are
compared in terms of the general exposure categories in either mean-hourly or 10-minute
averaging time. It is noted that the standards have similar EC3 and EC4 profiles while the
“urban” profiles differ considerably. The divergence in the profiles at upper heights can be
attributed to the effects of using a power law versus the logarithmic profile [e.g., Simiu and
Scanlan 1978].
The gust factor for the six standards may be written in terms of a general format as:
qG
GLFG = (3)
where GLF is the gust loading factor originally defined by [Davenport 1967] which is
expressed as:
RgBgrGLF RB
221 ++= (4)
In the above equations, gB,gR are the peak factors for response, r describes the turbulence
intensity, qG is the gust factor for the wind velocity pressure, B is the background factor and
R is the resonant factor. A summary of the peak and gust factors for the standards is provided
in Table 2. The parameter r in Table 2 describes the turbulence intensity of the wind in terms
of the turbulence intensity profile and a multiplier. The general form of r is:
hIr 2= (5)
where Ih is the turbulence intensity profile. The AS/NZ, Eurocode, ISO and NBCC standards
utilize the turbulence intensity as defined in Equation (5). ASCE uses a factor of 1.7 instead
of 2, and AIJ uses a factor ranging from 2.13 to 2.418 based on the exposure category. As
shown in Figure 2, there is significant variation in the turbulence intensity profiles, especially
for the Urban exposure category. This leads to variations in the resulting gust factor [Zhou et
al. 2002].
0 0.5 1 1.5 20
50
100
150
200
250
300
350
400
EC2 - Urban
Velocity Profile
Heig
ht
(m)
0 0.5 1 1.5 20
50
100
150
200
250
300
350
400
EC3 - Suburban
Velocity Profile
0.5 1 1.5 20
50
100
150
200
250
300
350
400
Velocity Profile
EC4 - Open
AIJ Eurocode NBCC ASCE AS/NZ ISO
Mean (1-hr or 10-min) Velocity Profiles
Figure 1: Mean Velocity Profiles of All Codes/Standards for Exposure Categories 2, 3, and 4
The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
Figure 2: Turbulence Intensity Profiles of for Exposure Categories 2, 3, and 4
0 0.5 10
100
200
300
400
500
EC2 - Urban
Turbulence Intensity Profile
Heig
ht
(m)
0 0.2 0.4 0.6 0.80
100
200
300
400
500
EC3 - Suburban
Turbulence Intensity Profile
0 0.2 0.4 0.6 0.80
100
200
300
400
500
Turbulence Intensity Profile
EC4 - Open
AIJ Eurocode AS/NZ ASCE NBCC ISO
Table 2: Comparison of Peak and Gust Factors
gR gB vg Gv T ν
ASCE ( )( )T
Tgν
νln2
577.0ln2 += 3.4 3.4 rgv+1 3600 f0
AS/NZ ( )Tg νln2= 3.7 3.7 rgv+1 600 f0
AIJ ( ) 2.1ln2 += Tg ν gR – 1 600 f0C
Eurocode ( )( )T
Tgν
νln2
6.0ln2 += gR 3.5 rgv+1 600 f0C
NBCC ( )( )T
Tgν
νln2
577.0ln2 += gR – 1 3600 f0C
ISO ( )( )T
Tgν
νln2
5772.0ln2 +=
3.4 3.4 rgv+1 (peak)
1 (mean) 600 f0
NOTE: gR is peak factor for resonance, gB is peak factor for background, gv is peak factor for wind velocity, f0
is the natural frequency of the building and C refers to a factor which is a function of the background and
resonant responses.
As discussed in [Tamura et al. 2005], the energy factor of the standards takes one of
three forms: von Karman, Kaimal, or Davenport. Specifically, ASCE and Eurocode use a
form of the Kaimal normalized wind velocity spectrum, AIJ and AS/NZ both use the von
Karman definition, and NBCC uses the Davenport definition. The choice of spectra definition
impacts the value of the energy factor and the resulting resonance response factor. The final
parameter in Equation (1) is the pressure coefficient. For wind design, there are both external
and internal pressures. While the external pressure coefficients are reasonably consistent at
building height, the internal pressure coefficients vary considerably between the
codes/standards.
Acrosswind and Torsional Loads
Although the standards are fairly consistent with respect to alongwind loading, the
treatment of the acrosswind and torsional loading differs amongst the codes/standards. For
example, ASCE, Eurocode, and NBCC utilize partial loading to account for acrosswind and
The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
torsional loads, although ASCE provides an alternative method in the Commentary
(Aerodata). The partial loading technique simply applies fractions of the alongwind pressures
in different combinations. Torsion is introduced either by asymmetric loading, as in NBCC
and Eurocode, or by an applied moment defined as a combination of the alongwind load
multiplied by a defined eccentricity (ASCE). Aerodata, AS/NZ, AIJ, and ISO provide
procedures for determining the acrosswind and torsional loads as a function of the base
bending moment of the resonant response. As these procedures typically rely on databases,
the results vary to a higher degree than the alongwind comparisons.
Accelerations
In addition to strength requirements, the serviceability requirements, in terms of
acceleration, are assessed. All codes and standards provide equations for defining alongwind
accelerations, however acrosswind and torsional accelerations are not included in every code.
The alongwind acceleration can be generally defined as:
)()(ˆ1
1
zm
bhKCGqzx
fxRhφ=&& (6)
where qh is the velocity pressure at the reference height, GR is the resonant component of the
gust factor, Cfx is the force coefficient, b is the building width, h is the building height, K is
the mode shape correction factor, m1 is the generalized mass in the first mode, and
( )k
hzz =)(1φ is the first mode shape evaluated at height z. Note that both NBCC and ISO
define the alongwind acceleration in terms of the displacement.
Example Analyses of Tall Buildings
To compare the wind loading standards, several example buildings are analyzed with
each code. The first example building is a square building with height of 200 m, width and
depth of 33 m, natural frequency for alongwind and acrosswind of 0.2 Hz, damping of 1% in
all directions, linear mode shapes, building density of 180 kg/m3, air density of 1.22 kg/m
3,
basic wind velocity for strength of 40 m/s (3 second) and 35 m/s (3 second) for serviceability.
To convert the velocity to different averaging times, the relationship developed by Durst
[ASCE 2005] is utilized. The building is analyzed using first an urban exposure then an open
exposure. Factors accounting for wind direction, importance, etc. are assumed to be 1. While
the parameters are re-written in terms of a general form so as to accurately compare the
various parameters, the analysis performed calculates the loading in the same manner stated
by the codes/standards. The comparison of the individual parameters reveals several areas of
disagreement between the codes/standards, leading to differences in the wind loads.
Results from the analysis of Example 1 are presented in Table 3. This example
highlights important similarities and differences between the standards. For instance, ASCE
and AS/NZ both determine the response using peak velocity pressure thus yield very similar
results despite the variations in the intermediate parameters. While the GLF for Eurocode B
and C agrees with AS/NZ and ASCE, respectively, the large velocity pressure contributes to
the increase in the overall loads for Eurocode. Note that although Eurocode defines the
velocity as 10-min, the velocity pressure is based on gust response. The velocity pressure for
ISO using the peak method is smaller than that for Eurocode, thus the larger GLF contributes
to larger overall loads. For the standards based on mean response (AIJ, NBCC, and Mean
ISO), the ISO velocity pressure is the smallest leading to the smallest overall load while AIJ
has the largest velocity pressure and overall load. The coefficient of variation (CoV) for the
resulting loads is 0.10. The acceleration predictions follow similar trends as compared to the
strength results, with Eurocode providing the largest accelerations.
The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
Table 3: Alongwind Results for Example 1 using Urban Exposure
Eurocode ISO ASCE
Aero-
data AS/NZ
B C AIJ NBCC
Peak Mean
0V (m/s) 40 40 40 28.1 28.1 26.4 40.0 28.1
refh (m) 120 200 200 120 200 200 200 200
refhV (m/s) 27.5 32.6 46.4 31.5 36.4 33.1 50.8 31.6
hq (kN/m2) 1.41 – 1.31 1.73 0.81 0.76 1.57 0.61
B 0.583 – 0.633 0.512 0.409 0.491 0.422 0.663 0.701
R 0.526 – 0.601 0.735 1.015 0.810 1.67 0.853 0.853
GLF 2.69 – 2.49 2.50 2.61 2.18 2.84 3.085 3.112
Base Shear
(MN)
9.95 8.10 9.65 10.81 11.44 11.82 11.23 10.59 9.87
Base Moment
(MN-m)
1,084 1,112 1,049 1,257 1,328 1,303 1,271 1,268 1,279
x&&σ (milli-g) 3.44 3.98 3.3 5.37 6.39 3.96 – – –
x̂&& (milli-g) 13.03 15.06 10.44 17.21 20.49 12.72 – – –
NOTE: Aerodata refers to Commentary section of ASCE. Eurocode and ISO have two procedures.
0V ,refhV are basic and design wind velocity;
refh is reference height; hq is velocity pressure; B, R are
background and resonance factor, GLF is gust loading factor; and x&&σ , x̂&& are rms and peak acceleration.
As shown in Table 4, the results for Example 1 using the open exposure category EC4
follow similar trends to the results using exposure category EC2, with a CoV of 0.2. However,
ISO using the peak methods agrees very well with ASCE. Also, the NBCC loads are
considerably higher than the other loads. As indicated in Figure 1, the large variation in the
velocity pressure, which is a function of the velocity profile, contributes to the scatter in the
resulting loads. To reduce the scatter and dependence on velocity pressure, Example 2
analyzes the same building with a modified velocity pressure distribution, external and
internal pressure coefficients for all the codes/standards. In order to obtain a velocity pressure
consistent between the codes/standards, the basic velocity is adjusted so that the velocity
pressure at building height is the same. To account for varying averaging times, the gust
factor for wind velocity pressure is determined in accordance with [Zhou et al. 2002] and
[Solari 1993] and defined as:
( ) 12 −
=
TV
VTG
q
ττ (7)
where τ ,T are averaging times of interest. Thus, ( ) 92.1103 =mins
qG and ( ) 06.213 =hr
s
qG . As shown
in Table 5, the modifications made to the velocity pressure and pressure coefficients brought
the wind load amongst the codes/standards closer together. The CoV was reduced to 0.07 and
the mean velocity pressure based codes/standards are now more in line with the peak based
codes/standards. The discrepancies can be contributed to turbulence intensity variances and
the differences in determining the GLF.
For the third example presented in Table 6, a rectangular building is analyzed. For this
example the following properties are assumed: h = 180 m, b = 30 m, d = 60 m, natural
frequencies in both sway directions = 0.25 Hz, building density = 180 kg/m3, damping of 2%,
and basic wind velocity of 60 m/s (3 second) for strength and 45 m/s (3 second) for
serviceability. The building is assumed to be in EC3. Based on the discussions above, velocity
pressure and pressure coefficients are modified to be consistent. The results are similar in
The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
trend to those noted previously, with a CoV of 0.07. Note that NBCC is markedly smaller
than the other results. This is likely attributed to the lower value of turbulence intensity for
NBCC as compared to the others. When the turbulence intensity for NBCC is modified to
correspond to the other turbulence intensities, the resulting wind loads are more consistent.
Table 4: Alongwind Results for Example 1 using Open Exposure
Eurocode ISO ASCE
Aero-
data AS/NZ
B C AIJ NBCC
Peak Mean
0V (m/s) 40 40 40 28.1 28.1 26.4 40 28.1
refh (m) 120 200 200 120 200 200 200 200
refhV (m/s) 38.1 41.2 52.6 41.6 44.0 40.2 54.4 43.9
hq (kN/m2) 1.84 – 1.62 2.21 1.18 0.98 1.81 1.177
B 0.624 – 0.633 0.524 0.428 0.528 0.422 0.625 0.636
R 0.889 – 1.107 1.177 1.536 1.274 2.466 1.513 1.513
GLF 1.854 – 1.939 2.083 2.171 2.062 2.347 2.028 2.031
Base Shear
(MN)
14.88 13.78 14.07 16.82 17.67 17.94 20.13 14.86 14.78
Base Moment
(MN-m)
1,572 1,705 1,468 1,862 1,955 1,918 2,136 1,683 1,846
x&&σ (milli-g) 3.85 6.63 4.51 7.28 8.46 6.06 – – –
x̂&& (milli-g) 14.57 25.09 13.95 23.53 27.34 19.57 – – –
Table 5: Alongwind Results for Example 2 using Suburban Exposure
Eurocode ISO ASCE AS/NZ
B C AIJ NBCC
Peak Mean
0V (m/s) 40 42 25.8 26.0 25.7 39.1 27.1
refh (m) 120 200 120 200 200 200 200
refhV (m/s) 33.5 52.1 33.3 37.6 36.2 52.0 37.6
hq (kN/m2) 1.65 1.65 1.65 0.86 0.80 1.65 0.86
B 0.607 0.633 0.517 0.417 0.513 0.422 0.637 0.653
R 0.729 0.967 0.801 1.095 0.907 2.010 1.147 1.147
GLF 2.228 2.177 2.231 2.329 2.055 2.491 2.385 2.392
Base Shear (MN) 12.79 13.66 11.81 12.33 12.54 11.76 12.89 12.08
Base Moment (MN-m) 1,370 1,450 1,312 1,370 1,360 1,317 1,457 1,512
NOTE: Aerodata database only has Urban and Open Exposures and is therefore removed from this example.
Table 6: Alongwind Results for Example 3 using Suburban Exposure
Eurocode ISO ASCE AS/NZ
B C AIJ NBCC
Peak Mean
0V (m/s) 60 62.6 38.5 39.2 38 58.7 40.8
refh (m) 108 180 108s 180 180 180 180
refhV (m/s) 48.9 76.9 48.8 55.5 53.5 76.9 55.4
hq (kN/m2) 3.61 3.61 3.60 1.88 1.75 3.61 1.87
B 0.617 0.651 0.523 0.427 0.519 0.459 0.645 0.660
R 0.530 0.737 0.633 0.820 0.738 1.590 0.931 0.931
GLF 2.153 2.158 2.193 2.249 2.037 2.389 2.370 2.377
Base Shear (MN) 18.03 19.66 17.29 17.73 18.05 16.14 19.19 18.22
Base Moment (MN-m) 1,759 1,900 1,731 1,775 1,789 1,656 1,942 2,044
NOTE: Aerodata database only has Urban and Open Exposures and is therefore removed from this example.
The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
Concluding Remarks
This paper examines the differences and similarities in major international wind
codes/standards. Although many parameters were examined, the scope is limited to
dynamically sensitive, regular-shaped buildings with flat roofs that are classified as enclosed.
To accurately compare the parameters, the various equations in the codes/standards are
written in a general format. While significant discrepancies are apparent in the comparison of
the intermediary parameters, the overall loads are reasonably consistent. However, with a few
modifications to specific parameters, the discrepancies between the standards are further
reduced. The parameters contributing to the most differences in the resulting wind load are
those associated with the wind velocity characteristics. Ultimately, the standardization of
wind loading standards is achievable with an understanding of the similarities and differences
between the major codes/standards.
Acknowledgements
The support for this paper was in part provided by the grant # CMMI 0601143 from
the National Science Foundation and the Global Center of Excellence, Tokyo Polytechnic
University funded by the Ministry of Education, Culture, Sports, Science and Technology
(MEXT).
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