 Open Access
 Total Downloads : 1039
 Authors : S. Farrukh Anwar, A. K. Asthana
 Paper ID : IJERTV2IS60302
 Volume & Issue : Volume 02, Issue 06 (June 2013)
 Published (First Online): 17062013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Evaluation Of Seismic Design Forces Of Indian Building Code
S. Farrukh Anwar+, A. K. Asthana++
Abstract
To keep abreast with the rapid development and extensive research carried out in the field of earthquake engineering, the recent fifth revision of Indian Seismic Code, IS:1893 has been split into five separate parts for different types of structures. The new code, IS:1893 (Part1) 2002 contains provisions specific to buildings only, along with general provisions applicable to all structures. This paper deals with the comparison of seismic design forces for multistoreyed buildings, obtained by using the new code, with those obtained by the previous 1984 version. From the results of seismic analysis of buildings it is concluded that the new code is more conservative for buildings resting on soft and medium soils.
INTRODUCTION
India was hit by many great earthquakes having magnitudes exceeding 8 on Richter Scale. Some of the greatest earthquakes of the world have occurred in the NorthEastern region of India, Himalayan belt, IndoGangetic plains, Western India, Kutch and Kathiawar regions. A major part of Peninsular India have also witnessed strong earthquakes, relatively few in number and having lesser intensity. Even moderate earthquakes ( M = 6 – 7 ) have caused considerable damage and loss of life in India. Over 50% area in the country is considered prone to earthquakes (Jain, 1998).
In view of the heavy construction programme launched all over the country after independence, IS:18931962 was published and subsequently revised five times. A number of important modifications have been made in the new 2002 version of the code. The objective of this paper is to compare the seismic design forces obtained by the latest 2002 version with those obtained by the previous 1984 version in different cases of buildings.
+ RESEARCH SCHOLAR, ++ PROFESSOR OF CIVIL ENGINEERING JNTUH, HYDERABAD PRINCIPAL,
HEAD DEPT. OF CIVIL ENGINEERING, KESHAV MEMORIAL INST. OF TECH. NSAKCET, MALAKPET, HYDERABAD NARAYANGUDA, HYDERABAD
METHODS OF SEISMIC ANALYSIS OF BUILDINGS
Provisions of Previous Code ( IS: 1893 1984 )
Two methods are prescribed by the code.
Seismic Coefficient Method This is a Pseudo static or equivalent lateral force procedure. For the entire building, the design baseshear, VB , is worked out by the codal formula
VB = K C h w ( 1 )
where K is the performance factor , W is the total dead load + appropriate amount of liveload, C is the flexibility coefficient of the structure, depending upon the fundamental time period T, estimated for moment resisting frames without bracing or shear walls using
T = 0.1 n = n / 10 ( 2 )
where n is the number of storeys including basement storeys. For all other buildings
T = 0.09 H / d ( 3 )
where H is the total height of the building in metres and d is the maximum base dimension in metres in the direction parallel to the applied seismic force. h is the design value of horizontal seismic coefficient computed using
h = I o ( 4 )
where o is the basic horizontal seismic coefficient , is the soil foundation coefficient and I is the Importance factor .The code suggests a parabolic distribution of forces such that the Seismic shears are higher near top storeys for the same baseshear. The lateral seismic force Qi (at floor i) is given by
Qi = VB Wi hi2 / Wj hj2 ( j = 1 to n ) ( 5 )
where hi is the height of floor i above the base of the building; Wi is the lumped weight at floor i , VB is the base shear and n is the number of storeys.
The shear in j th storey Vj , is obtained by summing up the lateral forces above that level
Vj = Qi (i = j to n) ( 6 )
Response Spectrum Method The lateral seismic load Qir acting at any floor level i , due to r th mode of vibration is given by
Qir = K Wi ir Cr hr ( 7 )
where ir is the mode shape coefficient at floor i in the rth mode of vibration, obtained from free vibration analysis; hr is the design horizontal seismic coefficient corresponding to appropriate period and damping in rth mode of vibration, computed using
hr = I F0 ( Sa / g ) ( 8 )
in which F0 is the seismic zone factor and ( Sa / g ) is the average spectral acceleration coefficient for appropriate natural period and damaging of the structure.
The mode participation factor Cr is found using Eq.(9) ( for i = 1 to n )
Cr = Wi ir / Wi ir2 ( 9 )
The maximum seismic shear Vi, acting in rth storey may be obtained by the superposition of first 3 modes using Vi = ( 1 ) Vir + [Vir ]2 ( r = 1 to 3 ) ( 10 )
where Vir is the absolute value of maximum seismic shear in the rth storey in rth mode,
value depends upon the height ( All coefficients values are given in code ).
The total earthquake lateral load acting at roof level n and floor level i may be computed using
Qn = Vn
Qi = Vi Vi + 1 ( 11 )
Provisions of New Code ( IS : 1893 – 2002)
Seismic Coefficient Method The design seismic base shear VB (or total design lateral force) along any principal direction is computed using
VB = Ah Wi ( 12 )
where Wi is the Seismic Weight of the entire building and A is the design horizontal acceleration coefficient for the structure, computed using
Ah = ( Z / 2 ) ( I / R ) ( Sa / g ) ( 13 )
where Z is the seismic Zone factor for the Maximum Considered Earthquake (MCE); I is the Importance factor; R is the Response reduction factor, and Sa /g is the average spectral acceleration coefficient for the approximate fundamental natural period of vibration (Ta ) in seconds, given by
Ta = 0.075H0.75 ( 14 )
for RC momentresisting frame building without brick infil panels. For all other buildings, including moment resisting RC frame buildings with brick infil panels, Ta may be estimated using Eq. (3) of old code (1984) Similarly, the Eq.(5) of old code is used to find the lateral seismic forces Qi at various floor levels, by distributing the computed design base shear VB along the height of the building.
Dynamic Analysis This may be performed either by the Time History Method or the Response Spectrum
Method. However, in either method, the design base shear ( VB ) shall be compared with the baseshear ( VB ) calculated using fundamental period Ta . If VB < VB, then all the response quantities (e.g. member forces, displacements, storey forces, shear and base reactions) shall be increased by multiplying by the ratio ( VB / VB
).
Response Spectrum Method First the natural frequencies of vibration (or periods T) and mode shapes {} of the building are obtained by performing the freevibration analysis. The number of modes to be considered
(r) in the analysis should be such that the sum total of modal masses considered is atleast 90% of the total seismic mass (W / g ).
The peak response is obtained by combining the modal responses using complete Quadratic Combination (CQC) method
= i j ij ( for i = j = 1 to r ) ( 15 )
where i , j are the responses in modes i and j ; ij is the cross modal coefficient and r is the number of modes considered. If the building does not have closelyspaced modes then even squarerootofsumofsquares (SRSS) method can be used
2
2
= i ( i = 1 to r ) ( 16 )
The Modal mass ( Mk ) of mode k is given by
2
2
Mk = [ Wi ik ]2 / g Wi ik ( i = 1 to n ) ( 17 )
where ik is the mode shape coefficient at floor i in mode k and g the acceleration due to gravity. The modal participation factor ( Pk ) of mode k is given by
2
2
Pk = Wi ik / Wi ik ( i = 1 to n ) ( 18 ) The peak lateral force ( Qik ) at floor i in mode k is given by
Qik = Ak Pk ik Wi ( 19 )
where Ak is the design horizontal spectral acceleration coefficient for mode k using Eq.(13) for the period of vibration Tk. The peak seismic storey shear ( Vik ) acting in storey j in mode k is obtained using
Vik = Qik ( i = j to n ) ( 20 )
The peak seismic storey shear Vik in storey i due to all modes considered is obtained by combining the modal values. The design lateral forces (due to all modes considered) Froof and Fi at roof and at floor i are given by
Froof = Vroof
Fi = Vi – Vi + 1 ( 21 )
EXAMPLE BUILDING
To have a check on the results, the 15 storeyed building (shown in Fig.1) of SP:221982 is analysed by the two methods of old and new codes described above. The building is located in Zone V in hard soil. The live load is 2KN/ sqm. The sizes in mm are beams (400×500), columns (600×600), slab (150) and wall alround 120 mm thick, floor height is 3 m. Earthquake force is applied in the Y direction.
DISCUSSION OF RESULTS
Table 1 shows that the first three modes of the building are well separated. The lateral forces Qi and the seismic shears Vi obtained using Seismic Coefficient Method of old and new codes are compared in Table 2. For this
R.C. ductile building having Special Moment Resisting Frames (SMRF), located on hard soil in the highest seismic zone V, it is observed that old code gives higher responses. However using Response Spectrum Method
, it is observed from Table 3 that for lower storeys old code gives higher responses while the steppedup responses of new code are more for higher storeys. Table 4 shows the comparison of design base shear VB (or the total design laterial force) for the 15 storeyed building located in different seismic zones. It is observed that for both ordinary and ductile buildings located on soft and medium soils the new code gives higher responses. However for hard soil, old code gives higher responses for all zones.
CONCLUSIONS
The seismic forces of 15 storeyed building (T=0.85) obtained by the seismic coefficient and Response Spectrum methods of old and new codes are compared. On the basis of this study it is concluded that for buildings resting on soft and medium soils the new code gives higher seismic forces while for those resting on hard soils the old code gives higher forces.
REFERENCES
IS : 1893 1962 Recommendations for Earthquake Resistant Design of Structures, ISI, New Delhi. IS : 1893 1984, Criteria for Earthquake Resistant Design of Structures, 4th Rev., ISI, New Delhi.
IS : 1893 (Part 1) 2002, Criteria for Earthquake Resistant Design of Structures, Part 1 General Provisions of Buildings, 5th Rev., BIS, New Delhi.
Jain, S. K., (1998), Indian Earthquakes : An Overview, the Indian Concrete Journal, Vol. 72.
Table 1 Periods and Mode Share Coefficients at various levels for first three Modes
Mode (r) 
1 
2 
3 
Period in Seconds 
1.042 
0.348 
0.210 
Mode share coefficients at various floor levels 

Floor No. 

1 
0.037 
0.108 
0.175 
2 
0.073 
0.206 
0.305 
3 
0.108 
0.285 
0.356 
4 
0.143 
0.336 
0.315 
5 
0.175 
0.356 
0.192 
6 
0.206 
0.342 
0.019 
7 
0.235 
0.296 
0.158 
8 
0.261 
0.222 
0.296 
9 
0.285 
0.127 
0.355 
10 
0.305 
0.019 
0.324 
11 
0.323 
0.089 
0.208 
12 
0.336 
0.190 
0.039 
13 
0.347 
0.273 
0.140 
14 
0.353 
0.330 
0.283 
15 
0.356 
0.355 
0.353 
Table 2 Comparison of Lateral Forces and Seismic Shears using Seismic Coefficient Method
Floor No. 
Wi (KN) 
hi (m) 
Old Code IS:18931984 
New Code IS:18932002 

Qi (KN) 
Vi (KN) 
Qi (KN) 
Vi (KN) 

1 
5143 
3 
2.9 
3463 
2.7 
3198 
2 
5143 
6 
11.7 
3460 
10.8 
3196 
3 
5143 
9 
26.3 
3448 
24.3 
3185 
4 
5143 
12 
46.7 
3422 
43.1 
3160 
5 
5143 
15 
73 
3375 
67.4 
3117 
6 
5143 
18 
105 
3302 
97.0 
3050 
7 
5143 
21 
143 
3197 
132.1 
2953 
8 
5143 
24 
187 
3054 
172.5 
2821 
9 
5143 
27 
236.3 
2867 
218.3 
2648 
10 
5143 
30 
291.8 
2631 
269.5 
2430 
11 
5143 
33 
353.1 
2339 
326.1 
2161 
12 
5143 
36 
420.2 
1986 
388.1 
1834 
13 
5143 
39 
493.1 
1566 
455.5 
1446 
14 
5143 
42 
571.9 
1073 
528.3 
991 
15 
3924 
45 
500.9 
501 
462.6 
463 
Table 3 Comparison of Lateral Forces and Seismic Shears using Response Spectrum
Floor No. 
Wi (KN) 
hi (m) 
Old Code IS:18931984 
New Code IS:18932002 

Vi (KN) 
Qi (KN) 
Vi (KN) 
Qi (KN) 
Stepp ed up Vi (KN) 
Result ing Qi (KN) 
Stepp ed up Qi (KN) 

1 
5143 
3 
3913 
84.4 
258 
3198 
56.2 

2 
5143 
6 
3829 
157.4 
2219 
3142 
98.7 

3 
5143 
9 
3671 
210.9 
2149 
3043 
121.5 

4 
5143 
12 
3461 
241.5 
2063 
2922 
129.3 

5 
5143 
15 
3219 
250.0 
1972 
2793 
130 

6 
5143 
18 
2970 
241.4 
1880 
2663 
134.5 

7 
5143 
21 
2728 
268.5 
1785 
2528 
144.7 

8 
5143 
24 
2460 
288.2 
1683 
2383 
158.9 

9 
5143 
27 
2171 
289.7 
1571 
2224 
274.9 

10 
5143 
30 
1882 
270.9 
1377 
1950 
221.3 

11 
5143 
33 
1611 
283.7 
1220 
1728 
281.9 

12 
5143 
36 
1327 
296.8 
1035 
1446 
296.6 

13 
5143 
39 
1030 
340.5 
811 
1150 
378.4 

14 
5143 
42 
690 
382.5 
545 
771 
426.9 

15 
3924 
45 
307 
307.3 
243 
344 
344.5 
Table 4 Comparison of Base Shears for the same Building located in different Seismic Zones
Zone No. 
Ductile R.C. Building (SMRF) Base Shear (VB) KN 
Ordinary R.C. Building (OMRF) Base Shear (VB) KN 

Old Code IS:18931984 
New Code IS:18932002 
Old Code IS:18931984 
New Code IS:18932002 

(a) Soft Soil: 

II 
865.6 
1485.2 
1385 
2475.4 
III 
1731.2 
2376.7 
2672.8 
3960.6 
IV 
2164.1 
3561.2 
3462.5 
5937.9 
V 
3462.5 
5345.6 
5540 
8914.4 
(b) Medium Soil: 

II 
865.6 
1207.3 
1385 
2012.2 
III 
1731.2 
1928.7 
2672.8 
3227.1 
IV 
2164.1 
2900.6 
3462.5 
4836.8 
V 
3462.5 
4350.9 
5540 
7259.1 
(c) Hard Soil: 

II 
865.6 
648.5 
1385 
1080.5 
III 
1731.2 
1421.4 
2672.8 
2369.1 
IV 
2164.1 
2132.2 
3462.5 
3553.6 
V 
3462.5 
3198.2 
5540 
5330.4 
Fig. 1 PLAN OF THE BUILDING