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

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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.

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