DYNAMIC RESPONSE OF RC FRAME BUILDING USING BASE ISOLATORS

GUIDED BY,

B S JAYASHANKAR BABU

ASSOCIATE PROFESSOR, DEPARTMENT OF CIVIL ENGINEERING.

PREMKUMAR M K4th sem, M.Tech,CAD structures,PESCE.

INTRODUCTION

Earthquakes cause inertia forces proportional to the product of the building mass and the ground accelerations.

Due to this mass asymmetry in building center of mass is shifted from center of stiffness causing eccentricity. As this eccentricity increases, torsion in building also increases.

As the ground accelerations increases, the strength of the building must be increased to avoid structural damage.

It is not practical to continue to increase the strength of the building indefinitely.

Base isolation is one of the most widely accepted techniques to protect structures and to mitigate the risk to life and property from strong earthquakes.

OBJECTIVES

To find out the response of 14 storey RC bare frame with and without base isolator (i.e., Mode period, displacement, acceleration, base shear, storey drift) by response spectrum analysis. Numerical modelling and analysis are carried out using finite element based software ETABS.

To study the response of base isolated building using lead rubber bearing and sliding bearing system for bare frame of symmetric model using time history analysis of El Centro earthquake data.

To compare the performance of RC bare frame having plan and elevation irregularities with and without base isolator.

Study of isolation system variations and isolation system hysteresis in case of lead rubber bearing (LRB) and PTFE system.

LITERATURE REVIEW

Bill Robinson et al. (1993), have written a book called seismic isolation for designers and structural engineers. This book gives total insight into the practical methods of construction of seismically isolated buildings. The different types of isolation devices and their properties are discussed here.

W h Robinson (1997), describes the principles of seismic isolation and discuss some of the isolation systems available before giving some examples of the application of seismic isolation to structure in new Zealand.

M Kikuchi and S Kamamato (2007), presents an analytical model for lead-rubber bearing to predict bearing force-displacement behavior under extremely large deformations.

LITERATURE REVIEW

Kaab mohamed zohair (2011), presented the use of the base isolation device LRB (lead rubber bearing) allows the control of the deformation which are localized on this last, and also allows carrying out a satisfactory compromise between the reduction of the seismic forces and the increase in the deformations of the base isolation.

Kelly et al., studied the effectiveness of seismic base isolation in controlling the deformations in prefabricated concrete structures. When utilized in prefabricated concrete structures seismic base isolation has the potential to reduce the ductility demand from these structures under seismic loading.

BASE ISOLATION: Base isolation is a passive vibration control system.

The goal of base isolation is to reduce the energy that is transferred from the ground motion to the structure.

(a) Conventional structure (b) base isolated structure.

THE PURPOSE OF BASE ISOLATION As for all the load cases encountered in the design process, such as

gravity and wind, should work to meet a single basic equation: CAPACITY > DEMAND.

This can be achieved by,

Ductility

Leads to higher floor accelerations.

Damage to structural components, which may not be repairable.

EFFECTS OF DUCTILITY

TYPES OF ISOLATOR

Lead rubber bearing (LRB)

Flat sliding bearing (PTFE)

COMPONENTS OF LRB

COMPONENTS OF PTFE

FLAT SLIDING ISOLATOR

DESIGN OF ISOLATOR 1. A displacement is assumed, using the total rubber thickness as a starting point.

2. The effective stiffness of the bearing at this displacement is calculated.

3. The effective period is calculated using the total seismic mass and the effective period.

4. The equivalent viscous damping is calculated from the area of the hysteresis loop. For HDR, the damping and shear modulus are interpolated from tabulated values of these quantities versus shear strain.

5. The damping factor, b, is calculated for the equivalent viscous damping.

6. The spectral displacement is calculated from the acceleration response spectrum at the effective period, modified by the damping factor b.

7. This displacement is compared with the displacement assumed in step 1. Above. If the difference exceeds a preset tolerance, the calculated displacement defines a new starting displacement and the procedure is repeated until convergence is achieved.

SYMMETRIC MODEL PLAN

DETAILS OF RC FRAME

Number of bays in x-direction = 6 bay

Number of bays in y-direction = 3 bay

Number of storeys = 14 storey (G +13)

Bottom storey = 3.0 m

Other storeys = 3.0 m

Link element = 0.5 m

Beam size = 0.7 m x 0.4 m

Column size = 0.5 m x 0.5 m

Slab thickness = 0.15 m

Live load on the slab = 3 kN /m2

NONLINEAR LINK TYPE: LRB ISOLATOR

U1 Linear effective stiffness = 1253000 kN/m

U2 and U3 Linear effective stiffness = 1174 kN/m

U2 and U3 Nonlinear stiffness = 8599 kN/m

U2 and U3 Yield strength = 117 kN

U2 and U3 Post yield stiffness ratio = 0.09

NONLINEAR LINK TYPE: PTFE ISOLATOR

U1 Linear effective stiffness = 5000000 kN/m

U1 Nonlinear effective stiffness = 5000000 kN/m

U2 and U3 Linear effective stiffness = 2038 kN/m

U2 and U3 Nonlinear stiffness = 2000000 kN/m

U2 and U3 Friction coefficient, slow = 0.04

U2 and U3 Friction coefficient, fast = 0.06

U2 and U3 Rate parameter = 40

U2 and U3 Radius of sliding surface = 0

FIXED BASE MODE PERIOD AND LRB BASE ISOLATED MODE PERIOD

Mode number

Fixed base mode period (Ts)

Base isolated mode period (Tb)

1 1.4801 3.0809

2 1.4316 3.0526

3 1.4284 2.9881

RESPONSE OF SYMMETRIC MODEL OF FIXED BASE AND LRB BASE ISOLATED BUILDING

Floor level vs. lateral displacements graph Floor level vs. storey drift graph

RESPONSE OF SYMMETRIC MODEL OF FIXED BASE AND LRB BASE ISOLATED BUILDING

Floor level vs. acceleration graph Floor level vs. storey shear graph

MODE PERIODS OF THE FIXED BASE AND PTFE BASE ISOLATED BUILDING

Mode number Fixed base mode period (Ts)

Base isolated mode period (Tb)

1 1.4801 2.5505

2 1.4316 2.5329

3 1.4284 2.4816

RESPONSE OF SYMMETRIC MODEL OF FIXED BASE AND PTFE BASE ISOLATED BUILDING

Floor level vs. Lateral displacements graph floor level vs. Storey drift graph

RESPONSE OF SYMMETRIC MODEL OF FIXED BASE AND PTFE BASE ISOLATED BUILDING

Floor level vs. Acceleration graph Floor level vs. Storey shear graph

PLAN ASYMMETRY

Plan asymmetric model P1 Plan asymmetric model P2

(2% Eccentricity) (5% Eccentricity)

MODE PERIODS OF THE FIXED BASE BUILDING FOR ASYMMETRIC MODELS (P1 AND P2)

Mode number

Fixed base FBP1 (Ts)

Fixed base FBP2 (Ts)

1 1.4830 1.4729

2 1.4230 1.3597

3 1.3894 1.2579

RESPONSE OF PLAN ASYMMETRIC MODEL OF FIXED BASE BUILDING

Floor level vs. Lateral displacement graph Floor level vs. Storey drift graph

RESPONSE OF PLAN ASYMMETRIC MODEL OF FIXED BASE BUILDING

Floor level vs. Acceleration graph Floor level vs. Storey shear graph

ELEVATION ASYMMETRY

Asymmetric model (E1) Asymmetric Model (E2) Asymmetric model (E3)

RESPONSE OF ELEVATION ASYMMETRIC MODELS OF FIXED BASE BUILDING

RESPONSE OF ELEVATION ASYMMETRIC MODELS OF FIXED BASE BUILDING

The north-south component of the ground motion at El Centro, California

0 5 10 15 20 25 30 35

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

El CENTRO GRAPH

TIME (sec)

AC

CE

LE

RA

TIO

N (

m/s

2)

TIME HISTORY RESPONSE

Fixed base response

LRB base response

PTFE base response

TIME HISTORY RESPONSE

Fixed base response

LRB base response

PTFE base response

TIME HISTORY RESPONSE

Fixed base response

LRB base response

PTFE base response

The maximum positive and negative values of time history analysis

Types Time history

Fixed base

LRB Isolator

Reduction (%)

PTFEIsolator

Reduction (%)

 Acceleration

(m/s2)

 El

Centro

Max:0.3644

Max:0.3499

03.97 Max:0.2716

25.46

Min:-0.4183

Min:-0.2901

30.64 Min:-0.2570

38.56

 Displaceme

nt(m)

 El

Centro

Max:0.00986

Max:0.00506

48.68 Max:0.00147

85.09

Min: -0.00966

Min: -0.00489

49.37 Min: -0.00094

90.26

 Storey Shear(kN)

 El

Centro

Max:814.65

Max:454.42

44.21 Max:44.85

94.49

Min:-834.73

Min: -487.20

41.63 Min:-68.70

91.76

Isolation system variation for LRB and PTFE bearings

Bi-linear force displacement graph

Fo

rce Elastic

stiffness

Yield stiffness

Deformation

ISOLATION SYSTEM HYSTERESIS

ISOLATION SYSTEM HYSTERESIS

FUTURE SCOPE OF STUDY

The study on optimal base isolation system of multistory RC frame buildings in a probabilistic sense.

Study on laminated rubber bearing constructed with scrap tires filled crushed rock as bearing to reduce manufacturing costs.

With recent advancement in material technology, more study can be focused on material qualities used in isolators like their strength, durability, high vertical stiffness, low horizontal stiffness and high energy dissipating capacity.

CONCLUSIONS

Natural period of the structure increases in case of base isolated building in comparison with fixed base building

Base isolation reduces the seismic response of plan and vertical irregular models

Base shear, acceleration, storey drift decreases whereas generally lateral displacement increases in base isolated building.

Fixed base buildings have zero storey acceleration at base of building whereas, in case of base isolated building appreciable amount of storey acceleration will been found out at base.

In case of PTFE base isolated building, reduction in response is more for =0.06 rather than =0.15.

In case of PTFE base isolated building, reduction in response is more for =0.06 rather than =0.15.

PTFE isolator alone cannot be used in a building because it has no restoring force in the system and results in large lateral displacement

Lead rubber bearing effectively reduces 70% of the base shear in the building.

In case of LRB isolation system variations, with increase in time period displacement and damping of the system increases.

In case of PTFE isolation system variations, with increase in time period displacement increases. Damping remains constant for all values of time period.

REFERENCES

Ivan skinner, R., Trevor E. Kelly and bill robinson, W. H. (1993), A text book on seismic isolation for designers and structural engineers, robinson seismic limited, holmes consulting group.

W. H. Robinson (1998), “passive control of structures, new zealand experience”, ISET, journal of earthquake technology.

Kelly et al. Uasge of seismic base isolation to reduce the ductility demand from prefabricated concrete structure, holmes consulting group limited.

Trevor e kelly, s. E. (2001), design guidelines on base isolation of structures, holmes consulting group, new zealand.

Pankaj agarwal and manish shirkhande. (2010), A text book on earthquake resistant design of structures, PHI learning private limited, new delhi.

Anil k. Chopra, dynamic of structures, theory and application to earthquake engineering, third edition.

Wai-fah chen charles scawthorn, “earthquake engineering handbook”.

ISOLATOR INSTALLATION IN BUILDING

Thank you