- Open Access
- Total Downloads : 78
- Authors : Mohmmad Sediq , Mrs. Kambham Amani
- Paper ID : IJERTV8IS060389
- Volume & Issue : Volume 08, Issue 06 (June 2019)
- Published (First Online): 18-06-2019
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License: This work is licensed under a Creative Commons Attribution 4.0 International License
Comparative Analysis of T Shape 8 Storey Asymmetric RCC Srtucture with and Without Base Isolation
Mohmmad Sediq1, Mrs. Kambham Amani 2
1Post Graduate Student, School of Civil Engineering, REVA University, Bengaluru.
2Assistant Professor, School of Civil Engineering, REVA University, Bengaluru.
Abstract The need for design of structures to resist earthquake is to protect the human lives, infrastructures, economy and important buildings such as hospitals, military bases, etc. from the damaging effects of earthquake and reduce the hazards after seismic event. Normally seismic design of structures is based on the method of increasing the resistance capacity of structures against earthquake by using shear walls, braced frames and moment resistance frames. However, these methods are often results in high floor acceleration and inter storey drift for stiff and flexible buildings in order to minimize inter-storey drift and reduce floor acceleration the concept of base isolation is increasingly being adopted. In the present study, a T shape, G+8 storey RCC structures has been designed and analyzed for fixed and isolated base, base isolator used in this study is lead rubber bearing[LRB], analysis and design is performed in accordance with IS standards for seismic design and ETABS 2016 software using Response spectrum method of analysis. Results obtained from analysis of fixed and isolated base models clearly shows that, modal period increases for isolated base building, storey drift decreases for BI building and displacement is more for isolated base compare to fixed base model because of flexibility of base isolated building.
Key Words: Earthquake, fixed base, isolated base, lead rubber bearing(LRB), response spectrum, storey drift, modal period.
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INTRODUCTION
Earthquake generates lateral forces on the buildings and should be considered during the design of structures to resist earthquake. The purpose of the design of structures to resist earthquake is to protect human lives, economy
loses and important buildings form the damaging effect of earthquake. Base isolation is a technique
that helps in increasing the earthquake resistance capacity of the structure. BI separates super structure from substructure and decouple the structure from horizontal ground motion induced by earthquake and provides stiff vertical components to the base of superstructure in contact with the substructure (foundation). It results in providing more flexibility to the structure as it increases displacement and modal periods and decrease in the storey drift and base shear of the building. The building under study in this paper is a RCC beam-column framed, hospital building with number stories G+8 located in seismic zone 5. The basic wind speed is 47m/sec as per IS 875 part 3. Material used are M30 grade concrete and HYSD 500 grade steel.
ETABS 2016 is used for analysis and design of frame. A typical floor plan is shown in figure 1 with a 3D view of the building in figure 2.
Fig. 1 A typical floor plan
Fig. 2. 3D view
Floor to floor height
3m
Ground floor height
4.5m
Slab thickness
150mm
Beam size
450x350mm
Column size
500x500mm
Grade of steel
HYSD500
Seismic zone
5
Structure class
B
Terrain category
2
site type
2
Wind speed
47
Response R factor
5
Dead load
12
Live load
3 2
Partition wall
150mm
Wall loads
6.12
Seismic zone factor
0.36
Floor to floor height
3m
Ground floor height
4.5m
Slab thickness
150mm
Beam size
450x350mm
Column size
500x500mm
Grade of steel
HYSD500
Seismic zone
5
Structure class
B
Terrain category
2
site type
2
Wind speed
47
Response R factor
5
Dead load
12
Live load
3 2
Partition wall
150mm
Wall loads
6.12
Seismic zone factor
0.36
Table. 1 Data of structure.
II. DESIGN OF LEAD RUBBER BEARING(LRB)
Lead rubber bearing are made up of a standard elastomeric
laminated rubber bearing the rubber compound can be 80
natural or chloroprene rubber. The shape can be round or
rectangular. The calculations for the design of LRB is 60
outlined in table2, which has been performed as per the 40
provisions of UBC-97.
20
Table 2. Design results of LRB 0
storey displacement
Base
Base
storey 1
storey 1
storey 2
storey 2
storey 3
storey 3
storey 4
storey 4
storey 5
storey 5
storey 6
storey 6
storey 7
storey 7
storey 8
storey 8
Fixed base isolated base
Grond floor
Grond floor
Maximum vertical load w
2978.554kN
Shear modulus, G
0.7 Mpa
Design time period TD
2.5sec
Seismic zone factor
0.36
Effective damping
5%
Damping coefficient
1
Bearing stiffness, keff
1917.857kN/m
Post yield ratio
0.1
Distance from end,j
0.0044m
Yield strength
51.143kN
Stiffness for U2 &U3
17160kN/m
Maximum vertical load w
2978.554kN
Shear modulus, G
0.7 Mpa
Design time period TD
2.5sec
Seismic zone factor
0.36
Effective damping
5%
Damping coefficient
1
Bearing stiffnes, keff
1917.857kN/m
Post yield ratio
0.1
Distance from end,j
0.0044m
Yield strength
51.143kN
Stiffness for U2 &U3
17160kN/m
Chart.1. showing variation of storey displacement.
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Storey drift
Table. 4 storey drift
Storey Load case Direction
Fixed base
Isolated base
III. RESULTS AND DISCUSSIONS
(m) (m)
Base RS X 0 0
Fixed and isolated base T shape structures are analyzed and designed using ETABS 2016 software and the results [ displacement, modal periods, storey drift, storey shear and base shear] are computed by Response spectrum method of analysis and results are compared with fixed base structure.
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Displacement
Table. 3 Maximum storey displacement.
Storey
Load case
Direction
Fixed base
Isolated base
(mm)
(mm)
Base
RS
X
0
43.148
Ground floor
RS
X
1.056
45.662
Storey1
RS
X
7.102
51.099
Storey2
RS
X
14.287
56.095
Storey3
RS
X
21.182
60.502
Storey4
RS
X
27.33
64.245
Storey5
RS
X
32.487
67.275
Storey6
RS
X
36.473
69.553
Storey7
RS
X
39.341
71.156
Storey8
RS
X
41.026
72.097
Ground floor
Ground floor
storey1
storey1
storey 2
storey 2
Table 3 shows maximum displacement for fixed base(41.026mm) and (72.079mm) for isolated base, is showing an increase of 43% in case of base isolation which is an evidence that base isolator provides more flexibility to the structures.
Ground
RS X 0.000704 0.00236
floor
Storey1
RS
X
Storey2
RS
X
Storey3
RS
X
Storey4
RS
X
Storey5
RS
X
Storey6
RS
X
Storey7
RS
X
Storey8
RS
X
floor
Storey1
RS
X
Storey2
RS
X
Storey3
RS
X
Storey4
RS
X
Storey5
RS
X
Storey6
RS
X
Storey7
RS
X
Storey8
RS
X
0.002016 0.001818
0.002398 0.001687
0.002312 0.001495
0.002079 0.001285
0.001769 0.001054
0.0011394 0.000803
0.001033 0.000573
0.000621 0.000338
storey drift
0.003
0.002
0.001 Fixed base
0 isolated base
storey drift
0.003
0.002
0.001 Fixed base
0 isolated base
storey 3
storey 3
storey 4
storey 4
storey 5
storey 5
storey 6
storey 6
storey 7
storey 7
storey 8
storey 8
Chart.2. showing variation of storey drift.
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Modal periods
STOREY SHEAR
STOREY SHEAR
ISOLATED BASE
ISOLATED BASE
Case
Mode
Fixed base
isolated base
sec
sec
Modal
1
0.851
1.575
Modal
2
0.835
1.547
Modal
3
0.743
1.424
Modal
4
0.278
0.42
Modal
5
0.276
0.416
Modal
6
0.248
0.37
Modal
7
0.162
0.217
Modal
8
0.16
0.217
Modal
9
0.146
0.194
Modal
10
0.108
0.142
Modal
11
0.106
0.14
Modal
12
0.098
0.127
Case
Mode
Fixed base
isolated base
sec
sec
Modal
1
0.851
1.575
Modal
2
0.835
1.547
Modal
3
0.743
1.424
Modal
4
0.278
0.42
Modal
5
0.276
0.416
Modal
6
0.248
0.37
Modal
7
0.162
0.217
Modal
8
0.16
0.217
Modal
9
0.146
0.194
Modal
10
0.108
0.142
Modal
11
0.106
0.14
Modal
12
0.098
0.127
Table.5 modal periods of base isolated & fixed base
5000
4000
3000
2000
1000
0
FIXED BASE
5000
4000
3000
2000
1000
0
FIXED BASE
From table 5 it is evident that time periods are increases for base isolated building by 45.96% for first modal (fundamental mode).
Modal periods
Modal periods
5000
4000
3000
2000
1000
2
1.5
1
2
1.5
1
0
Chart.4. showing variation of storey shear.
GROUND FLOOR
GROUND FLOOR
STOREY 1
STOREY 1
STOREY 2
STOREY 2
STOREY 3
STOREY 3
STOREY 4
STOREY 4
STOREY 5
STOREY 5
STOREY 6
STOREY 6
STOREY 7
STOREY 7
STOREY 8
STOREY 8
Base shear
fixed isolated
Fixed base
isolated base
Fixed base
isolated base
Chart.5. showing variation of base shear
0.5
0
0.5
0
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 6 7 8 9 10 11 12
Chart.3. showing variation of modal periods.
-
storey shear
Table.6. storey shear of the structure
Storey
Load case
Direction
Fixed base
Isolated base
kN
kN
Base
RS
X
0
0
Ground floor
RS
X
3856.1856
2986.42
Storey 1
RS
X
3810.136
2709.5
Storey 2
RS
X
3632.24
2425.3
Storey 3
RS
X
3336.69
2122.3
Storey 4
RS
X
2946.14
1797
Storey 5
RS
X
2472.43
1444.9
Storey 6
RS
X
1910.43
1062.26
Storey 7
RS
X
1255.23
650.2
Storey 8
RS
X
658.23
314.475
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CONCLUSION
-
-
Applying base isolation to a structure reduces the base shear of the building which results in reducing the earthquake effects on the structure.
-
The modal periods of the structure with LRB base isolation is more than the structure with fixed base.
-
Displacement of each storey with LRB base isolation is increased which results in high ductility and flexibility to a structure.
-
Storey drift in each storey decreases for base isolated compare to fixed building which results in an increase in storey drift.
-
Using base isolation systems increases the structural stability which results in reducing the earthquake effects on the structure.
REFERENCES
-
Charles K. Erdey Earthquake Engineering application to design vol.1 No.2,pp.25-26,2007.
-
Bungale S, Tranath Phd,S.E. wind and Earthquake resistant buildings Analysis and design vol.4 pp 104-105,2004.
-
Wai-Fah Chen, Charles Scwathorn Earthquake engineering vol 5 pp.826-860, 2003.
-
R.S. jangid optimum lead rubber Isolation bearing for buildings1995.
-
uniform building code, international Council of building officials, 1997
-
IS 456, Indian standard for plain And reinforced concrete-code of Of practice, bureau of Indian Standards, new delhi,2000.
-
IS 1893 Indian standard code of Practice for earthquake Resistance design of structures.
-
M celebi, design of seismic Isolated structures from Theory to practice vol.16 No,3 pp.709-710,2000.
-
V.kailar usage of simplified N2 method for analysis of Base isolated structure 14th World conference china 2008.
-
ETABS building design Software. California USA.
AUTHORS PHOTO