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,


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


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


0 0.2 0.4 0.6 0.80

100

200

300

400

500


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


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.


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


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.


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).

References

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ASCE (2005), Minimum Design Loads for Buildings and Other Structures, Reston, VA, American Society of

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Eurocode (2004), Part 1-4: Wind Actions, Eurocode 1: Actions on Structures, London, British Standards

Institute.

ISO (2009), 4354: Wind actions on structures, Switzerland, ISO.

NRC (2005), National Building Code of Canada, Ottawa, Associate Committee on the National Building Code,

National Research Council.

SAA (2002), AS1170.2 Part 2: Wind Forces, Australian Standards AS1170.2:2002, Sydney, Standards

Association of Australia.

Simiu, E. and R. Scanlan (1978), Wind Effects on Structures: An Introduction to Wind Engineering, New York,

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Tamura, Y., A. Kareem, G. Solari, K. C. S. Kwok, J. D. Holmes and W. H. Melbourne (2005), "Aspects of the

Dynamic Wind-Induced Response of Structures and Codification," Wind and Structures 8(4): 251-268.

Zhou, Y., T. Kijewski and A. Kareem (2002), "Along-Wind Load Effects on Tall Buildings: Comparative Study

of Major International Codes and Standards," Journal of Structural Engineering 128(6): 788-796.

Comparative Study of Major International

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