Comparative Study of Along-Wind Response of Major International Codes with Indian Code

Comparative Study of Along-Wind Response of Major International Codes with Indian Code

Indian Code (IS 875 Part-III (2015))

Shams Ahmed1

Civil Engineering Department, IIT (BHU),

Varanasi, India

Abstract The research paper discusses a comparative study of five major international codes and standards with the latest Indian Code for wind load i.e. IS 875 Part-III(2015) for along wind loads on tall buildings and other provisions for along wind response on tall buildings by Gust Factor Method (GFM). The major international codes and standards of wind loads included within the scope of this research paper are ASCE-7-98 (United States)[3], AS1170.2-89 (Australia)[5], NBC-1995 (Canada), RLB-AIJ-1993 (Japan)[4], Eurocode 1-4 (1993)[7]. The research work is basically an inclusion of latest Indian Code IS-875 Part- III (2015) in the comparative study published by Yin Zhou, Tracy Kijewski and Ahsan Kareem[1]. Major emphasis is put on the gust factor method approach for estimating along wind loads on tall buildings. A detailed example is also solved at the end so as to facilitate quantitative comparison.

Keywords Building, Wind Loads, Gust Factor Method, Along Wind Effects, Building Codes.

1. INTRODUCTION

   In modern era buildings are made very tall, slender and asymmetrical, with special architectural and aesthetic requirements. Due to developments of new building materials and construction techniques, these buildings are becoming much lighter and more slender than earlier. Buildings of modern era are very susceptible to wind load. These buildings are often built in groups as in case of a metropolis, and their responses are different from the response of an isolated building. This is due to complex interference effects induced by turbulent wind flow field.

   In order to estimate these wind loads on various types of structures, codes of practice of several countries are available. These codes have different approaches to estimate wind load such as gust factor method etc. This paper is mainly based upon the comparative study of dynamic response factor as determined using the gust factor method of latest Indian standard code for wind loads, i.e., IS 875 Part-III (2015) and five other major international codes and standards. All the values and data of five major international codes is obtained from the recent comparative study published by Zhou, Kijewski and A Kareem [1] (2002) whereas and all the values and data for Indian code is calculated by the author(s).

   For the evaluation of wind loads on buildings using gust factor method determination of Turbulence Intensity, Peak factor, Gust factor and Gust wind speed are required. In order to determine the aforementioned parameters, wind speed averaging is done with respect to time. This averaging times differs from code to code. Apart from averaging time, definition of several other wind characteristic parameters also

   Prof. S Mandal2

   Professor,

   Civil Engineering Department,

   IIT (BHU), Varanasi, India

   differ from each other. The salient parameters are category wise turbulence intensity vs. height curve, dynamic response factor, background factor, peak factor, size reduction factor etc.

   Many comparative studies have pointed out a large scatter in predicting the wind loads hence an in-depth investigation is required. As due to globalization and unification of construction industries around the globe unification of these standard is important and understanding the underlying differences is a must.

   Significant works have already been published by various authors on gust factor method. Gust factor application in civil engineering was first introduced by Davenport(1967)[6]. Several modifications to Davenport's work have been made, notably by Vellozzi and Cohen (1968)[8], Vickery (1970)[9]. Notable comparative studies focussing on Gust factor method have been done by Lee and Ng (1988), recently Kijewski and Kareem (1998) also compared few standard international codes. S. Behra and A. K. Mittal (2012)[11] also conducted a similar comparative study for buildings and towers.

   Averaging Time:

   The averaging time is basically the observation time during wind monitoring operation using anemometers. The averaging time varies from country to country depending on the way wind data is collected in different regions. In Indian Code IS-875 Part-III (2015) the averaging time used is 1Hour (3600 seconds). The ASCE 7 (1998) and AS1170.2 (1989) codes uses 3 seconds averaging time whereas Eurocode (1993) and RLB- AIJ (1993) uses 10 minutes (600 seconds) averaging time.

   Table 1 The given below highlights the various averaging time used in different codes under the consideration.

   TABLE I. AVERAGING TIME IN CODES AND STANDARDS[1]

   	IS 875 Part-III (2015)	ASCE 7 (1998)	AS1170.2 (1989)	NBC (1995)	RLB- AIJ (1993)	Eurocode (1993)
   Basic wind	1 Hour	3 Sec	3 Sec	1 H	10	10 Min
   velocity or					Min
   Basic wind
   pressure

   Published by : http://www.ijert.org

   International Journal of Engineering Research & Technology (IJERT)

   ISSN: 2278-0181

   Vol. 6 Issue 11, November – 2017

2. DETAILED COMPARISON OF RELEVANT PARAMETERS :

   Table 2 presents comparison of all the important parameters used in estimation of Gust Factor. Following is the list of terms: Cdyn : Dynamic Response Factor

   B : Background Factor

   s : Size Reduction Factor gr, gv, gf : Peak Factors

   1 + ( + )0.5

   1 + 3.5

   1 + ( + )0.5

   1 + ( + )0.5

   1

   0.925(1 + 2 + 2 )

   1 + .

   1 + 2.

   2 0.5

   TABLE II. PARAMETERS FOR CALCULATION OF GUST-LOADING FACTORS IN CODES AND STANDARDS[1]

   IS 875 Part-III (2015)

   ASCE 7(1998)

   AS1170.2(1989)

   NBC(1995)

   RLB-AIJ(1993)

   Eurocode (1993)

   Ga

   1 + 2 [2 + ]

   + (2(1 + )2 + 2)0.5

   T

   3600 s

   3600 s

   3600 s

   3600 s

   600 s

   600 s

   H

   0.6H

   H

   H

   H

   0.6H

   r

   2.

   1.7.

   2.

   2/

   (3 + 3)/(2 + ).

   2.

   0 1

   = (.)

   g

   = 2(36000)

   = 3.5

   = = 3.4

   = (1)

   = 2(1)

   = 3.7

   = (. )

   = 1/( + ))

   = 2 (.) + 1.2

   = 1()/( + )

   = (2 + 2)/( + )

   B [66( )2 + 642 ]0.5 1 1

   1 2 914

   . 1

   (1 + )

   + 0.63

   362 + 642 1

   2

   1 + 0.63( )

   3 2

   1.3 1/3

   {1 + 5.1( ) ( ) }

   )

   1 + 0 (1 + 457)(1 + 122)(1 +

   1

   42

   1

   (1 + 712)56

   1

   3(1 + 2)43

   1

   1

   (2 + 2)56

   1

   1

   1 + 0.9( + 0.63

   E

   (1 + 702)56

   S 1

   40(1 + )

   9.51

   (1 + 10.32)3

   0.61

   22

   [1 + 3.51] [1 + 41]

   1

   [1 + 3.51] [1 + 41]

   1

   [1 + 3.51] [1 + 41]

   1

   (0.53 + 0.47)

   6.81

   (1 + 10.21)53

   400(1 +

   [1 + ] [1 +

   Lh

   0.25

   100(10)

   (10)

   0.25

   1000 (10)

   0.5

   100 (30)

   0.25

   100 (10)

   300 (1300)

   IJERTV6IS110127

   www.ijert.org

   (This work is licensed under a Creative Commons Attribution 4.0 International License.)

   257

3. COMPARISON OF WIND VELOCITY PROFILE: The mean wind velocity profile is affected by averaging

   time, ground roughness, fetch length. As different codes and standards included within the scope of this work have different averaging time hence different wind velocity profiles are observed. Broadly the wind velocity profile can be classified as either logarithmic or power law, ASCE 7 (1998) and Eurocode utilize logarithmic wind velocity profile whereas the remaining codes and standards including IS 875 Part-III (2015) uses power law type wind velocity profiles. However all the wind

   velocity profiles can be represented as a general power law of form:

   Vz = V0.b.(z/10)a

   In the above general power law a, b are constants which depend upon the terrain category. For an open terrain case exposure 2(C) at 10 m height, b is equal to unity since the basic wind velocity is defined for this exposure. Table 3 presents values of a, b for respective codes and standards for category 2(C) terrain.

   TABLE III. MEAN WIND VELOCITY PROFILE IN CODES AND STANDARDS[1]

   Category

   IS 875 part- III(2015)

   ASCE 7(1998)

   AS1107.2(1989)

   NBC(1995)

   RLB-AIJ(1993)

   Eurocode(1993)

   a

   b

   a

   b

   a

   b

   a

   b

   a

   b

   a

   b

   2(C)

   0.092

   1.00

   0.11

   1.00

   0. 7

   1.00

   0.14

   1.00

   0.15

   1.00

   0.16

   1.00

4. COMPARISON OF TURBULENCE INTENSITY PROFILE

   The turbulence intensity profile can also be expressed in terms of a power law as follows:

   I(z)=c.(z/10)-d

   where c and d are constants depending on the terrain category and z denoted height in 'm'. These coefficients are given in codes and standards or can be found by using simple mathematical techniques.

   In order to determine the value of c, the z is chosen as 10 m and corresponding value of Iz is determined. In order to determine the value of d following mathematical manipulations were carried out.

   Taking natural logarithm both sides in the equation(i), we

   get

   () = () . (10)

   ( )

   The average of various values of d obtained from above equation is taken as approximate value for coefficient d. In order to determine the coefficient the above process is repeated on a spread sheet as shown in Table 4.

   For the purpose of comparison between turbulence intensity values with respect to height and for various exposure terrains of various codes the coefficients c, d have been shown below in Table 5.

5. QUANTITATIVE COMPARISON OF CODES AND STANDARDS :

   In order to compare the codes and standards in a quantitative manner a numerical was solved and final results are compared quantitatively for better understanding

   An example is solved for better understanding and comparison of various codes and standards. The building considered has following parameters H = 200 m, B = D = 33 m, f1 = 0.2 Hz, and linear mode shape in two translation

   = (

   ).

   ( )

   10

   directions, z = 0.01, Cd = 1.3; and building density = 180 kg/m3. The building is located so that it has exposure 4(A) on one side and exposure 2(C) on the other, and the basic 3 s wind velocity = 40 m/s. The table below shows the values for the numerical from different codes and standards.

   TABLE IV. CALCULATION OF COEFFICIENT C, D USING SPREADSHEET FOR IS 875 PART-III (2015)

   Height (z) mtrs	category 1	category 2	category 3	category 4	d1	d2	d3	d4
   10	0.157	0.183	0.239	0.342
   15	0.152	0.176	0.225	0.342	0.07982	0.09619	0.14887	0
   20	0.147	0.171	0.215	0.342	0.09494	0.09784	0.15267	0
   30	0.14	0.162	0.203	0.305	0.10431	0.11094	0.1486	0.10422
   40	0.133	0.156	0.195	0.285	0.11966	0.11514	0.14676	0.13151
   50	0.128	0.151	0.188	0.27	0.12688	0.11942	0.14913	0.14687
   75	0.118	0.14	0.176	0.248	0.14172	0.13293	0.15185	0.1595
   100	0.108	0.131	0.166	0.233	0.16247	0.14518	0.15828	0.16667
   150	0.095	0.117	0.15	0.21	0.1855	0.16517	0.17201	0.18009
   200	0.085	0.107	0.139	0.196	0.20482	0.17914	0.18092	0.18583
   250	0.08	0.098	0.129	0.183	0.20945	0.19401	0.19157	0.19426
   300	0.074	0.92	0.121	0.173	0.22115	0.20219	0.20012	0.20037
   400	0.068	0.082	0.108	0.155	0.22682	0.21761	0.21533	0.21453
   500	0.58	0.074	0.098	0.141	0.25455	0.23144	0.22788	0.22649
   Average					0.164007	0.1544	0.172615	0.146949
   coeff. c	0.157	0.183	0.239	0.342

   TABLE V. TABLE 5: COMPARISON OF COEFFICIENTS C, D OF DIFFERENT CODES AND STANDARD[1]

   Category	IS 873 Part-III (2015)		ASCE 7 (1998)		AS1107.2 (1989)		NBC (1995)		RLB-AIJ (1993)		Eurocode (1993)
   	c	d	c	d	c	d	c	d	c	d	c	d
   4(A)	0.342	0.147	0.45	0.167	0.453	0.3	0.621	0.36	0.402	0.4	0.434	0.45
   3 (B)	0.239	0.173	0.3	0.167	0.323	0.3	0.335	0.25	0.361	0.32	0.285	0.3
   2(C)	0.183	0.154	0.2	0.167	0.259	0.3	0.2	0.14	0.259	0.25	0.189	0.2
   1 (D)	0.157	0.164	0.15	0.167	0.194	0.3			0.204	0.2	0.145	0.15
   E									0.162	0.15

6. QUANTITATIVE COMPARISON OF CODES AND STANDARDS :

   In order to compare the codes and standards in a quantitative manner a numerical was solved and final results are compared quantitatively for better understanding. An example is solved for better understanding and comparison of various codes and standards. The building considered has

   following parameters H = 200 m, B = D = 33 m, f1 = 0.2 Hz, and linear mode shape in two translation directions, z = 0.01, Cd = 1.3; and building density = 180 kg/m3. The building is located so that it has exposure 4(A) on one side and exposure 2(C) on the other, and the basic 3 s wind velocity = 40 m/s. The table below shows the values for the numerical from different codes and standards.

   TABLE VI. QUANTITATIVE COMPARISON OF CODES AND STANDARDS[1]

   	IS 875 Part-III (2015)		ASCE 7 (1998)		AS1107.2(1989)		NBC(1995)		RLB-AIJ(1993)		Eurocode (1993)
   	4(A)	2(C)	4(A)	2(C)	4(A)	2(C)	4(A)	2(C)	4(A)	2(C)	4(A)	2(C)
   V0	26	26	40	40	40	40	26	26	27	27	27	27
   z	200	200	120	120	200	200	200	200	200	200	120	120
   Vz	33.0	33.8	27.5	38.1	26.7	37.3	32.6	39.5	30.4	42.3	30.7	39.3
   Lz	211	211	190	250	2115	2115	1220	1220	1220	258	258	197
   B	0.616	0.616	0.583	0.624	0.633	0.633	0.300	0.300	0.582	0.582	0.500	0.529
   E	0.054	0.063	0.140	0.144	0.094	0.117	0.170	0.191	0.080	0.100	0.106	0.109
   S	0.046	0.064	0.048	0.079	0.080	0.123	0.077	0.101	0.154	0.212	0.087	0.121
   R	0.248	0.403	0.525	0.889	0.596	1.138	1.031	1.524	0.967	1.655	0.726	1.039
   gv gR	3.5 3.63	3.5 3.63	3.40 3.79	3.40 3.79	3.70 3.63	3.70 3.63	3.759 3.768	3.759 3.768	3.209 3.325	3.209 3.325	3.208 3.225	3.208 3.225
   G	0.965	1.01	2.691	1.854	2.495	2.021	2.833	2.544	2.103	1.868	2.500	2.026

7. RESULTS AND DISCUSSION :

The results obtained are quite different in-spite of same basic gust wind velocity with averaging time of 3 seconds, this is due to widely varied definitions of wind characteristics in different codes and standards. The dynamic response factor obtained is quite different for different codes because each code uses different mean wind velocity profile.

The GLF (Gust Loading Factor) values estimated by ASCE 7 and Eurocode 1-4 (1993) are also distinct due their varied definition of wind characteristic parameters [1]. The Indian code IS 875 Part III (2015) has strikingly predicted the lowest values among all the codes the lower value may be due to wrong estimation of design wind speed as the wind speed was given for 3 s averaging time but IS 875 Part-III uses 1 h averaging time due to which this speed has to be reduced for

1 h averaging time, error may have crept in during this reduction. Moreover, after discussion it can be concluded that the final values of dynamic response factor vary greatly from

code to code so the values of 0.965 and 1.01 are deemed to be rightly representing the prediction of dynamic response factor as per IS 875 Part-III (2015).

It can be concluded that the scatter in values obtained from different codes is mainly due to variation in definition of wind characteristics parameters and different averaging times involved. To achieve unification in codes and standards it is necessary to arrive at unified and common definition of all the wind characteristic parameters such as size reduction factor, peak factor, background factor etc, including the averaging times.

REFERENCES

1. Y. Zhou, T. Kijewski and A. Kareem, Along-Wind Load Effects on Tall Buildings: Comparative Study of Major International Codes and Standard, Journal of Structural Engineering vol III June 2002 pp 231- 237

2. M.M. Majed, P. Srinivas Rao, A Comparative Study of Wind Forces on Tall Building by Static Method and Dynamic Method Per IS 875-Part-III