abstract introduction

ALONG WIND LOAD ON TALL BUILDINGS – INDIAN CODAL PROVISIONS Bodhisatta Hajra1 and P. N. Godbole2 Department of Applied Mechanics Visvesvaraya National Institute of Technology Nagpur 440 011, India

ABSTRACT Most international codes and standards have kept pace with the changing scenario in wind engineering and have updated their codes and standards. The IS-875 (part-3)-1987 still makes use of hourly mean wind speed and cumbersome charts to arrive at the Gust Factor for calculating Along Wind response on a tall building. A document “Review of Indian Wind Code-IS-875 (part-3) 1987”,prepared by the Indian Institute of Technology, Kanpur suggests revision in the present IS-code to make it consistent and bring it close to the available international standards. This paper discusses the present IS-code, the revisions suggested by IIT Kanpur together with other international codes for computing Along Wind response on a tall building with the help of three examples of tall buildings. Key Words: Tall Buildings, Along – Wind Load, Building Codes, Gust factor.

INTRODUCTION Most international codes and standards utilize the “gust loading factor” (GLF) approach for assessing the dynamic along-wind loads and their effects on tall structures. The concept of the GLF for civil engineering applications was first introduced by Davenport (1967), following the statistical treatment of buffering in aeronautical sciences (Liepmann 1952). Several modifications based on the first GLF model by Davenport followed, which include Vellozzi and Cohen (1968), Vickery (1970), Simiu and Scanlan (1996), and Solari (1993a,b). Variations of these models have been adopted by major international codes and standards. The present Indian Standard for Wind Loads on Buildings and Structures (IS-875 (part-3)1987) also recommends Gust Factor (GF) or Gust Effectiveness Factor (GEF) for calculating along wind load or drag load on flexible slender structures which includes tall buildings. The procedure makes use of hourly mean wind speed and cumbersome charts to arrive at the Gust Factor. The proposed revision for IS-875 (part-3)-1987 [prepared by Department of Civil Engineering, IIT Kanpur under GSDMA project on building codes] simplifies the computation of along wind load on flexible slender structures using a 3 second gust speed prevalent in many other international codes and avoids use of charts which were difficult to interpolate. In this paper, the procedure recommended by the present IS code and the proposed revisions have been reviewed in comparison to other international codes of practice. To highlight the comparison examples on tall building have been selected and the response has been obtained from present IS code, proposed revision and from other international codes of practice.

1. Post- Graduate Student

2. Visiting Professor

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GUST LOADING FACTOR (10) Following the concept of the GLF approach (Davenport 1967), the peak Equivalent Static Wind Load (ESWL) on tall buildings provided in codes and standards is described by a product of the mean wind force and an appropriate amplification factor. T



P ( z )  G  .P ( z )

(1)

Where T

P (z) = peak ESWL at height z during observation time T, usually one hour (1 h) or 10 minutes (10 min) for most civil engineering applications; superscript  = averaging time used 

to evaluate the mean wind velocity; and P (z) = mean wind force with averaging time . 

P (z)  q (z ).C d .B

(2)

In which Cd = drag force coefficient; B = width of the building normal to the direction of wind; and q (z) = 1/2  V (z) 2 = mean wind velocity pressure, where  = air density and

V (z) = mean wind velocity evaluated at height z above ground. The gust factor G is given by G = G TY / G q (T)

(3)

In which G TY = GLF for displacement and G q (T) = gust factor (GF) for wind velocity pressure. The displacement GLF takes into account the correlation structure of random wind field, wind structure interaction, and the dynamic amplification introduced by the structure. Following the current practice in the GLF approach (Davenport 1967), it can be evaluated by T

ˆ T (z) / Y (z) G TY  Y

(4)

Where

ˆ T And Y T = peak and mean wind-induced displacement response, respectively. Y The above discussion clarifies the important role of averaging time in this comparative study. On the one hand, when  = T, the wind load model in Eq. (1) reduces to the general GLF model by Davenport (1967). On the other hand, when using mean wind velocity with a shorter averaging time, G in Eq. (1) may be significantly less than the GLF in Eq. (4). Therefore, it is important to compare the results based on similar averaging times. A summary of the averaging time for the basic wind velocity or pressure and GLF, employed in codes and standards is given in Table 1. All procedures for estimating GLF in major codes and standards are based on the preceding expressions, but differ in their modeling of the wind field and structural dynamic characteristics. These details have led to a large scatter in the predicted values of the GLF and wind load effects based on distinct formulations. Furthermore, as a result of several mathematical manipulations that have been introduced by individual codes and standards, the final expressions for the GLF do not follow exactly the same form.

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Table-1 – Averaging Time in Codes and Standards ASCE 7

IS 875-1987

AS1170.2-02

Revision IITK

Basic wind velocity or basic wind pressure

3s

3 sa

3s

3s

Gust loading factor

3s

1h

3s

3s

a

Although the basic wind velocity is defined as 3 s gust in IS 875 (part-3) – 1987 it is converted to the 1 h mean wind velocity to evaluate the gust loading factor and the wind-induced response of dynamic structures.

INDIAN CODAL PROVISIONS FOR ALONG WIND LOAD The Part 3 of present Indian standard ‘Code of Practice for Design Loads (other than Earthquake) for Buildings and Structures’, deals with the Wind Loads. This code on Wind Loads IS-875 (part 3) was last revised in 1987 and is due for revision now to incorporate the current state of knowledge and practice in the area of Wind Engineering. The last 2 – 3 decades have been witness to substantial progress in the understanding of procedures to determine design wind speeds from measured wind data as well as of the response to wind of various kinds of structures, particularly buildings. Most leading codes have tried to keep pace with this changing scenario. However, the relevant Indian Code, having been drafted in early eighties and implemented in 1987, has remained as such and there are apparently no moves to revise the same in near future. Noting that the Indian Standard Code of Practice on Wind Loads IS 875 (part-3) – 1987 needs revision to keep pace with the other International Standards, the Department of Civil Engineering at Indian Institute of Technology, Kanpur, under Gujarat State Disaster Management Authority Project (GSDMA project) took up this initiative of revising the present Indian Standard Code of Practice on Wind Loads. A document ‘Review of Indian Wind Code – IS 875 (part 3)-1987’, was released in 2004 incorporating up-to-date state of knowledge in Wind Engineering very similar to available in other international standards. This document will immensely help the Bureau of Indian Standards, as and when it takes up the revision of IS-875 (part 3)-1987. There are major differences in computation of along wind load between the present Indian Standard on Wind Loads (IS-875 (part 3)-1987) and the proposed revision document prepared by Indian Institute of Technology, Kanpur (Revision – IITK) which have been summarized below :(a)

IS – 875 – (part 3) – 1987 The code gives only the methods of calculating along wind or drag load by using gust factor method. Further, the code recommends the use of hourly mean wind speed for the computation of Gust factor and gives a procedure to obtain hourly mean wind speed from regional basic wind speed. The procedure to obtain the gust factor is rather involved as it requires use of four figures to obtain certain variables. The figures contain closely spaced curves which make the interpolation difficult and approximate. Along wind load on a structure on a strip area (Ae) at any height (z) is given by, Fz = Cf . Ae. p z . G

(5)

Where Fz = along wind load on the structure at any height z corresponding to strip area Ae, Cf = force coefficient for the building, Ae = effective frontal area considered for the structure at height z,

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p z = design pressure at height z due to hourly mean wind obtained as = 0.6 Vz2 (N/m2)

(6) and

V z = Vb k1 k 2 k3 Where

(7)

V z = hourly mean wind speed in m/s, at height z; Vb = regional basic wind speed in m/s k1 = risk coefficient

k 2 = terrain, height and structure size factor; and k3 = topography factor

 peak load   , and is given by :  mean load 

G = gust factor = 

G = 1+ gf r

 SE 2  B 1       

(8)

To compute Gust factor, the values of gfr, B, S and E are interpolated from the figures given in the code of practice. Also,  is damping coefficient as given in Table 2 and  = gf r B / 4 and is to be accounted only for buildings less than 75 m high in terrain Category 4 and for buildings less than 25 m high in terrain Category 3, and is to be taken as zero in all other cases. Table 2 – Damping Coefficient () Nature of Structure Welded steel structures Bolted steel structures Reinforced or prestressed concrete structures (b)

Damping Coefficient,  0.010 0.020 0.020

REVISION IITK In the document ‘Review of Indian Wind Code – IS 875 (part-3) – 1987’, the method to obtain the along wind load using gust factor method is very much similar to revised Australian Code AS/NZ 1170.2 – 02. The gust factor is renamed as Dynamic Response Factor (Cdyn) and is calculated using 3 sec gust speed and set of expressions which do not involve use of figures or charts. Thus the calculations are rather straight forward and simple. Along wind load on a structure on a strip area (Ae) at any height (z) is given by:

Where:

Fz = Cf Ae pz Cdyn

(9)

Fz = along wind load on the structure at any height z corresponding to strip area Ae, Cf = force coefficient for the building, Ae = effective frontal area considered for the structure at height z, pz = design pressure at height z due to 3–sec wind gust velocity obtained as pz = 0.6 Vz2 (N/m2), (10) Cdyn = Dynamic Response Factor (= total load/ mean load), and is given by:

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C dyn

2  2 H g SE  1  2 I h  g v Bs  s R      1  2 g v I h 

0.5

where Ih = turbulence intensity, obtained from Table 3 by setting z equal to h gv = peak factor for the upwind velocity fluctuations, which shall be taken as 3.5 Bs = background factor, which is a measure of the slowly varying background component of the fluctuating response, caused by low frequency wind speed variations, given as follows : 1 Bs  2 0.5 36(h  s) 2  64b sh 1 2L h





Hs = height factor for the resonant response = 1 + (s/h)2 gR = peak factor for resonant response (1 hour period) TABLE 3: TURBULENCE INTENSITY (IZ) Height (z) m 10 15 20 30 40 50 75 100 150 200 250 300 400 500 (c)

Terrain category 1 0.157 0.152 0.147 0.140 0.133 0.128 0.118 0.108 0.095 0.085 0.080 0.074 0.068 0.058




AUSTRALIAN AND AMERICAN STANDARDS ON WIND LOADS The Australian Standard AS/NSZ 1170.2 – 02 Structural design actions part 2 : Wind Actions and American Standard ASCE 7-02 American Society of Civil Engineers : Minimum Design Loads for Buildings and other Structures uses 3 sec Gust Speed for defining the basic wind velocity and for evaluating the Gust Loading factor. Table 1 gives the averaging time in various codes and standards while Table 4 summarizes the expressions for gust factors and other variables used in the various codes and standards.

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Table – 4 IS-875 (PART-3)-1987 G=1+gfr

B 1   SE /  2

REVISION IITK / AS 1170.2 – 0.2 Cdyn =

{1+2Ih [gv2Bs+(Hs gR2 SE/

ASCE – 7 – 0.2

0.5

 )] }

{1+2 gv Ih}

Gf=0.925(1+1.7 Iz(gQ2Q2+gR2R2)0.5)/(1+1.7 gV Iz)

“B” To be found from fig.9, page-50 “E” To be found from Fig.11, page52

Bs=1/ [[1+36(h-s)2+64bsh2]0.5/(2Lh)]

Q=(1/(1+0.63 ((B+h)/Lz)0.63))0.5

E=N/(1+70 N2)(5/6)

Rn=7.47 N1/(1+10.3N1)(5/3)

Fo=12 fo h/Vh

N= foLh{1+(gV Ih)}/Vh

N1=n1 Lz/Vz

“Iz”Not taken into account

Iz Obtained from Table 32, page-87

Iz=c (33/Z) (1/6), [Values of “c” from table-6-2]

From figure.8, page-50, found as a function of building height

gV=3.5

gV=3.4

G,Gf E,Rn Iz Vh/Vdes

= Gust factor = Gust energy factor = Turbulence intensity factor = Design wind speed

Bs Q = Background factor N1,N,F0 = Reduced frequency gV = Peak factor for upwind velocity

APPLICATION TO TALL BUILDINGS Three tall buildings have been analyzed to compare the estimates of along wind load effects as obtained from present IS Code, revision IITK (Australian) and American Standards. The building particulars are given in Table 5 and table of formulations are presented in Table – 4 for Gust factor. The shear force and bending moment plots for various buildings are presented in figures-1 through 6. Table – 5 - Building particulars PARAMETER HEIGHT BREADTH LEAST LATERAL DIMENSION TERRAIN CATEGORY BASIC WIND SPEED

BUILDING-1

BUILDING-2

BUILDING-3

60m 50m 10m 2 50m/s

96m 24m 12m 3 50m/s

200m 33m 33m 4 50m/s

NOTE: Breadth of the building is considered normal to the wind direction

6

70 60 50 40 30 20 10 0

BENDING MOMENT V/S HEIGHT

IS-875-PART-3,1987 IS-DRAFT/AUS. ASCE-7

0

2000

4000

6000

HEIGHT(m)

HEIGHT(m)

SHEAR FORCE V/S HEIGHT 70 60 50 40 30 20 10 0

8000

IS-875,PART-3,1987 IS-DRAFT/AUS ASCE-7

0

100000

SHEAR FORCE(kN)

200000

300000

BENDING MOMENT(kN-m)

Fig-1 Fig-2 SHEAR FORCE AND BENDING MOMENT FOR 60 m TALL BUILDING(TC-2)

BENDING MOMENT V/S HEIGHT

SHEAR FORCE V/S HEIGHT 120 HEIGHT(m)

80

IS-875,PART-3,1987

60

IS-DRAFT/AUS

40

ASCE-7

HEIGHT(m)

150

100

100 50

0 200000 400000 BENDING MOMENT(kNm)

0 2000

4000

6000

ISDRAFT/AUS

0

20 0

IS-875,PART3,1987

8000

ASCE-7

SHEAR FORCE(kN)

Fig-3 Fig-4 SHEAR FORCE AND BENDING MOMENT FOR 96 m TALL BUILDING (TC-3)

SHEAR FORCE V/S HEIGHT

BENDING MOMENT V/S HEIGHT 250

200 IS-875,PART-3,1987

150

IS-DRAFT/AUS

100

ASCE-7

50 0

HEIGHT(m)

HEIGHT(m)

250

200 IS-875,PART-3,1987

150

IS-DRAFT/AUS

100

ASCE-7

50 0

0

5000

10000

15000

SHEAR FORCE(kN)

Fig-5

20000

0

50000 1E+06 2E+06 2E+06 3E+06 0 BENDING MOMENT(kN-m)

Fig-6

SHEAR FORCE AND BENDING MOMENT FOR 200 m TALL BUILDING (TC-4)

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CONCLUSIONS The computation of along Wind Load as per the present IS 875 (part 3) – 1987 is cumbersome and difficult to program as it uses various charts. Further the present code makes use of mean hourly wind velocity, which is not consistent with other International Codes. The suggested Revision – IITK uses 3 second gust speed for computation of Gust factor (Dynamic Response factor) and gives a set of expressions to evaluate the Gust factor and hence the along wind load. For 60 m tall building in the terrain category-2, the present IS-code gives higher values of shear forces and bending moment as compared to Revision IITK (Australian) and ASCE-7 codes of practice (fig.1 and 2). While for the 96 m building in terrain category-3, all the three codes give comparable values of shear forces and bending moment (fig.3 and 4). For the 200 m tall building in terrain category-4 the present IS code gives much lesser values for shear force and bending moment than Revision IITK (Australian) and ASCE-7(fig.5 and 6)The Revision IITK (Australian) and ASCE give comparable values of shear force and bending moment for all the three buildings for respective categories, while present IS code gives higher values for 60 m building and much lower values for the 200 m building, suggesting need for revision to obtain values consistent with other international codes.

REFERENCES 1. Davenport (1967). “Gust loading factors.” J. Struct. Div., ASCE, 93(3), 11-34. 2. Liepmann 1952 “On the application of statistical concepts to the buffeting problem,” J. Aeronaut. Sci., 19(12), 793-800. 3. Vellozzi and Cohen (1968) “Gust response factors” J. Struct. Div., ASCE, 94(6), 12951313. 4. Vickery (1970) “On the reliability of gust loading factors” Proc., Technical Meeting Concerning Wind Loads on Buildings and Structures, Building Science Series 30, National Bureau of Standards, Washington, D. C., 296-312. 5. Simiu and Scanlan (1996) “Wind effects on structures: Fundamentals and application to design, 3rd Ed., Wiley, New York. 6. Solari (1993 a) “Gust buffeting. I: Peak wind velocity and equivalent pressure.” J. Struct. Eng., 119(2), 365-382. 7. Solari (1993 b) “Gust buffeting. II: Dynamic along-wind response.” J. Struct. Eng., 119(2), 383-397. 8. IS – 875 – (part 3) – 1987 – Wind Loads Bureau of Indian Standards, Manak Bhawan, New Delhi. 9. Review of Indian Wind Code – IS – 875 (part 3) 1987 Document No. IITK – GSDMA – Wind 01 – V 2.0, IITK – GSDMA Project on Building Codes, Department of Civil Engineering, IIT Kanpur, India (2004). 10. Yin Zhao, Tracey Kijewski and Ahsan Kareem ‘Along Wind Load Effects on Tall Buildings : Comparative Study of Major International Codes and Standards ASCE, Journal of Structural Engineering, Vol 128, No. 6, 2002, pp 788 – 796.

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