Comparative Analysis of T Shape 8 Storey Asymmetric RCC Srtucture with and Without Base Isolation

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.

1. 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.

2. 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.

1. 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.

   1. 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.

   2. 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

   3. CONCLUSION

1. Applying base isolation to a structure reduces the base shear of the building which results in reducing the earthquake effects on the structure.

2. The modal periods of the structure with LRB base isolation is more than the structure with fixed base.

3. Displacement of each storey with LRB base isolation is increased which results in high ductility and flexibility to a structure.

4. Storey drift in each storey decreases for base isolated compare to fixed building which results in an increase in storey drift.

5. Using base isolation systems increases the structural stability which results in reducing the earthquake effects on the structure.

REFERENCES

1. Charles K. Erdey Earthquake Engineering application to design vol.1 No.2,pp.25-26,2007.

2. Bungale S, Tranath Phd,S.E. wind and Earthquake resistant buildings Analysis and design vol.4 pp 104-105,2004.

3. Wai-Fah Chen, Charles Scwathorn Earthquake engineering vol 5 pp.826-860, 2003.

4. R.S. jangid optimum lead rubber Isolation bearing for buildings1995.

5. uniform building code, international Council of building officials, 1997

6. IS 456, Indian standard for plain And reinforced concrete-code of Of practice, bureau of Indian Standards, new delhi,2000.

7. IS 1893 Indian standard code of Practice for earthquake Resistance design of structures.

8. M celebi, design of seismic Isolated structures from Theory to practice vol.16 No,3 pp.709-710,2000.

9. V.kailar usage of simplified N2 method for analysis of Base isolated structure 14th World conference china 2008.

10. ETABS building design Software. California USA.

AUTHORS PHOTO