SEISMIC ISOLATION OF RC FRAMED STRUCTURE WITH AND WITHOUT INFILLS

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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 2, February 2017, pp. 316–327 Article ID: IJCIET_08_02_034 Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=2 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed

SEISMIC ISOLATION OF RC FRAMED STRUCTURE WITH AND WITHOUT INFILLS

Santhosh H.P Research Scholar, Department of Civil Engineering,

Sahyadri College of Engineering and Management, Mangaluru, India.

Harsha M.S Student, Department of Civil Engineering,


Manohar K Assistant Professor, Department of Civil Engineering,


Pradeepa B.B Student, Department of Civil Engineering,


ABSTRACT Earthquakes are the nature’s greatest hazard which cannot be exactly predicted by human

beings. Earthquake is a sudden movement of earth’s surface with the release of massive energy in the form of seismic waves. There are so many methods to control the vibration of the buildings like passive, active and hybrid control systems.[1] Base isolation is a passive control system. In the present work, a (G+3) structure considering with and without infill’s subjected to seismic force along with static gravity loads are analyzed using Response Spectrum Method in ETABS-2015 package. From the study, the response of the structure obtained is time period, base shear, story displacement and inter-story drifts and also the comparison is drawn for the same parameters considering structure as bare and infill frames. The variation in percentage of steel is also presented. It is found that the time period and story displacement increases while base shear and story drifts gets reduces with the provision of effective base isolators.

Key words: Base isolation, Base shear, Story- drift, Acceleration, Infill. Cite this Article: Santhosh H.P, Harsha M.S, Manohar K and Pradeepa B.B, Seismic Isolation of RC Framed Structure with and without Infills. International Journal of Civil Engineering and Technology, 8(2), 2017, pp. 316–327. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=2

Santhosh H.P, Harsha M.S, Manohar K and Pradeepa B.B


1. INTRODUCTION Earthquake is a natural calamity that has taken the lives of millions of people through the ages in the recorded and unrecorded human history. A disturbance that causes shaking on the surface of the earth due to underground movement along the fault plane or through volcanic activity is called an earthquake. In the seismic design of low to medium rise buildings, the fundamental frequency of vibration is much lower than the earthquake force that means building acts as an amplifier and the acceleration experienced at each floor level increases to the top. Because of this, inter-story drift and stresses in the member’s increases and damage to the column occurs between the floors. These accelerations can cause severe damage to the occupants and the contents of the floors without causing much damage to the structure. Only way to achieve flexibility in low to medium rise buildings is by the use of base-isolation method. In recent years, this new technology has emerged as an alternative solution to the conventional seismic strengthening. This technology has attained professional and academic interest throughout the world and many hundreds of buildings were built in U.S.A, New Zealand, China and Japan using seismic isolation technology. In India, base-isolation technique is rarely used in some public and residential buildings like Bhuj hospital building and an experimental building at IIT, Guwahati.[2]

1.1. Mechanism of Base-Isolation During the strong ground vibrations a base isolated structure undergoes decoupling i.e, the superstructure is decoupled from the energy absorbing substructure. Period shift is the main mechanism in the base isolation system. A building is subjected to many frequencies during the seismic activity. The lowest first natural frequency of the structure is the most predominant one and it causes severe damage to the structure during vibration. The frequency of the isolated structure is less than the fixed base frequency of the structure and hence one can observe the reduced response of the superstructure. So, the fundamental frequency of the structure is reduced, i.e. time period of the structure is increased. [1]

Figure 1 Response of the structure with fixed base and base isolation.

1.2. Structural Action of Infill Frames Under very low lateral loads masonry infill’s remains in contact with the RC frame and hence one can accept there is a composite action between RC frame and infill’s. But when the lateral load increases at the interface between RC frame and masonry infill, masonry infill starts to crack. Further, when the lateral load increases there is a separation between masonry infill and RC frame. Masonry is very weak in tension because bond between them is weak and hence when subjected to lateral loading it does not behave elastically even for small deformations. Therefore, masonry always resists

Seismic Isolation of RC Framed Structure with and without Infills


compressive forces. However, by providing reinforcement between the two courses, tensile stresses can be eliminated.[3]

Figure a. Predominant frame action Figure b. Predominant truss action

Figure 2 Lateral load transfer mechanism of frame system having with and without infill.

1.3. Objectives of the Present Study The main analysis of the study is to compare the results for the parameters Base shear, Time period, Story displacement, Inter – Story drifts and variation of percentage of steel for ,

Fixed base and isolated base structures having bare frame.

Fixed base and isolated base structures having infill frame.

2. DESCRIPTION OF THE BUILDING The description of the building used in the analysis is as shown in the fig 3.3. A (G+3) storey building located in Zone V on medium soil is taken and analysis of all the structural members including seismic evaluation is done as per IS 456: 2000 and IS 1893 (part 1): 2002, using ETABS 2015 software. The typical floor plan of the building is as shown. All dimensions are in m.

Figure 3 Plan of the building.



Table 1 Details of the RC frame structure.

2.1. Design of Lead Rubber Bearing For designing of isolators the loads at the base of the column obtained from the ETABS is considered. These loads are grouped into three categories for the purpose of isolator design.

Isolator 1 loads considered 1233.47 kN, 1319.29 kN and 1400.14 kN. Isolator 2 loads considered 1566.54 kN, 1608.89 kN, 1641.49 kN and 1635.66 kN. Isolator 3 loads considered 1719.09 kN and 1783.5 kN. Three isolators were designed and maximum load from each group is considered. In the present

work the design procedure for isolator 3 for a load of 1783.5 kN is discussed. The following data’s were assumed for the design of LRB.[2]

1) The design target period, TD of the isolated structure should be greater than 3times the fixed base period. For fixed base structure, for mode 1 time period is 0.826 sec. Therefore, for isolated structure, TD = 0.826 X 3 = 2.478 sec = 2.5 sec.

Type of frame Ordinary moment resisting frame. No of bays along X direction 5 No of bays along Y direction 5 No of storey’s G + 3 Storey height 3.5 m Seismic Zone V Zone factor 0.36 Soil type Type ɪɪ (Medium) Importance factor

1.0

Response reduction factor

3.0

Damping of the structure

5%

Cross- sectional properties Flexural member Compression member Depth of slab

0.2 X 0.6 m 0.3X 0.75 m 0.15 m.

Concrete Density of concrete ( self- weight) Modulus of Elasticity of concrete. Poisson’s ratio of concrete

25 kN/m3(As per IS:875(Part-ɪ) – 1987) =27386.128N/mm2. 0.2

Masonry Masonry (Infill) thickness Density of Masonry (self-weight) Compressive strength of Masonry Elastic modulus of infill Poisson’s ration of masonry

0.2 m. 20 kN/m3(As per IS:875(Part-ɪ) – 1987) 10 N/mm2 (taken). =5500 N/mm2. 0.16

Unit Weight of rebar 78.5 kN/m3 Imposed load 2 kN/m2 (As per IS:875(Part-ɪɪ) –1987). Floor finish 1.25kN/m2(As perIS:875(Part-ɪ)– 1987) Earthquake analysis

IS: 1893 (Part-ɪ) – 2002.

Design Philosophy

Limit State Method. (IS: 456 – 2000).



2) Max. Shear strain, γmax = 50 %. 3) Effective damping ratio, ζeff= 10 %. 4) Damping ratio, BD = 1.2 (Table 1623.2.2.1 of IBC 2000). 5) Seismic coefficient, SD = 0.4(Table 1615.1.2.2 of IBC 2000).

2.1.1. Analysis 1) Effective horizontal stiffness of the isolator,

Keff= (W/g) (2π/ TD) 2 = (1783/9.81) (2 π/ 2.5) 2

= 1148 kN/m = 1.148 kN/m. 2) Design displacement,

DD= (g/4π2) (SDx TD/BD) (Equation 16.79 of IBC 2000). = (9.81/4 π2) x (0.4 x 2.5) /1.2 = 0.207 m ˂ 0.3 m.

3) Short term yield force, QD = WD/4DD = π/2 x Keffxζeffx DD

= (π/2 x 1148 x 10 % x 0.207) = 37.33 kN.

4) Stiffness of the lead core, = QD / DD = 37.33 / 0.207 = 180.34 kN/m.

5) Post yield horizontal stiffness , Kd= Keff- QD / DD

= (1148 – 180.34) Kd = 967.66 kN/m.

6) Pre yield stiffness , Ku = 10 Kd = (10 X 967.66)

Ku= 9676 kN/m.

2.1.2. Design 1) Design of lead core.

Assuming the yield strength of the lead core to be fpy = 8.82 x 103kN/m2. The required lead area, Ap = QD / fpy Ap = (37.33 / 8.82 x 103) = 4.232 x10 -3 m2.

Therefore, use diameter dP = 7 cm. 2) Design the area and dimensions of rubber layer.

a) Total height of rubber layer , tγ= DD / γmax = (0.207 / 50 %) tγ = 0.414 m.



b) Select the rubber properties , assuming the hardness of the rubber to be IRHD – 60. Hardness = IRHD – 60. Elongation at break, €b = 500 %. Young’s modulus, E = 4.45 x 103kN/m2. Shear modulus, G = 1.06 x 103kN/m2. Modified factor, K = 0.57.

c) Shape factor , S E (1+ 2KS2) / G ≥ 400 = (445 (1+ 2 x 0.57 x S2) / 106 ≥ 400) S = 9.09 Use S = 10. Ec = E (1+ 2KS2) = (4.45 x 103(1+ 2 x 0.57 x 102)) Ec = 511750 kN/m2.

d) The effective area Ao of the bearing based on the allowable normal stress Ϭcfor the vertical load case,

Ϭc= P / Ao≤ 7.84 x 103kN/m2 = 1783/ Ao= 7.84 x 103 Ao = 0.227 m2.

e) The effective area, At of the bearing for the shear strain condition for the vertical load case,

6S (P/ Ecx At) ≤ €b / 3 = (6 x 10 x (1783 / 511750 x At) ≤ 500% /3)

At = 0.12 m2. f) Elastic stiffness Kr of the bearing,

Kd= Kr (1+ 12(Ap / Ao )) 967.66 = Kv(1+ 12(4.232 x10 -3/ 0.227)) Kr = 793 kN/m.

g) Based on the shear failure condition , the effective area A of the individual rubber layers, G = Kr x tγ/ Aeff

(1.06 x 103) = 793 x 0.5 / Aeff Aeff= 0.374 m2.

For a circular bearing, the diameter corresponding to the area 0.374 m2 is d = 0.69m. Effective area can be calculated as A2, d2/4 (β – sinβ) Where, β = 2 cos-1( DD/d)

= 2 cos-1(0.207/0.69) β = 2.532.

A2 = (0.692/4 (2.532 – sin 2.532))

A2 = 0.296 m2.

A = max. (Ao, At, A2) = (0.227, 0.12, and 0.296) = 0.296 m2. h) The size and dimensions of rubber layer for a circular bearing ,

Are = d2/4 (β – sinβ)



Where, β = 2 cos-1( DD/d) = 2 cos-1(0.207/0.7) β = 2.54.

Are = (0.72/4 (2.54 – sin 2.54)) A re = 0.24 m2.

i) For a circular bearing, single layer thickness t, and no. of layers N. S = d / 4t 10 = 0.7 / 4t t = 0.0175m = 2 cm = 20 mm No. rubber layers, N = tγ/ t = 0.414 / 0.02 N = 21 nos.

j) Steel plate thickness , ts ts≥ 2 (ti + ti+1) P / Area x Fs Fs= 0.6 Fy = 0.6 x 274.4 = 164.64 MN/m2

ts = 2( 0.02 + 0.02) x 1.783 / (0.224 x 164.6)ts= 2mm. K) Total height of the bearing,

Assuming the thickness of the top and bottom cover plates to be 2.5 cm. Total height is, h = tγ+ 21 tsx + 2 x 0.025 h = 0.592 m = 0.6 m = 600 mm = 60 cm.

L) Shear strain and stability condition. a) Verticals load,

γsc = 6S x P / (Ec x A ) = (6 x 10 x 1783) / (511750 x 0.384) = 0.544 ≤ €b/3 γsc = 0.544 ≤ 1.67. Hence ok.

a) Stability check, Ϭc = P/ A = 1783/ 0.384 = 4643 kN/m2 ≤ G x S x L / (2.5 tγ) ≤ (1.06 x103 x 10 x 0.7 (2.5 x 0.414)) 4643 ≤ 7169 kN/m2.

b) Vertical stiffness , Kv = ( 6 x G x s2 x A) / tγ = (6 x 1.06 x103 x 102 x 0.296)/ 0.414 Kv= 454724 kN/m.

And the Width of diagonal strut is calculated as per the Main- stone’s equation suggested in various literatures. The values of equivalent strut width for different bay sizes were shown in the table 2 below.

Table 2 Values of equivalent strut width.

Bay size (m)

Dimension of the infill Width of infill(m) Depth of infill(m)

3.5 x 3 0.2 0.545 3.5 x 4 0.2 0.628 3.5 x 5 0.2 0.724



2.2. Modeling of LRB in ETABS 2015 The values of the rubber isolator are designed as a two joint link property and assigned at the base of each column to act as a base isolator. The calculated stiffness values of LRB are entered in ETABS 2015 as shown below.[2]

For Isolator 1, U1 Linear effective stiffness = 484760 kN/m.

U2 & U3 Linear effective stiffness = 901.44 kN/m.

U2 & U3 Non Linear effective stiffness = 7598.4 kN/m.

U2 & U3 yield strength = 29.31 kN.

U2 & U3 post yield stiffness ratio = 0.1

Effective damping = 0.05

2.3. ETABS Models

Figure 4 3-D View of Bare frame (Fixed base). Fig.3.5 3-D View of infill frame (Fixed base).

Figure 5 3-D View of bare frame model (Isolated base).



3. RESULTS

Table 3 Comparative study of Time period for fixed and isolated base for Mode 1.

Framing Type

Time Period (Sec.) Percentage Difference

Fixed base Base isolated

Bare Frame 0.826 2.083 60.35 % Infill Frame 0.386 1.982 80.52 %

Graph 1 Comparison of Time Period of fixed and base isolated models for Mode 1.

Table 4 Comparative study of Base shear for fixed base and base isolated structures having bare and infill frames.

Graph 2 Comparative study of Base shear (kN) for fixed base and isolated base.

Fixed base Base isolatedBare Frame 0.539 1.809Infill Frame 0.237 1.733

00.20.40.60.8

11.21.41.61.8

2

Tim

e Pe

riod

(Sec

.)

Bare Frame

Fixed base Base isolatedBare Frame 3925 1155With Infill 4236 1260

0500

10001500200025003000350040004500

Bas

e She

ar (k

N)

Bare …

Framing Type

Base Shear (kN) Percentage Difference

Fixed base Base isolated

Bare Frame 3925 1155 70.57 % Infill Frame 4236 1260 70.25 %



Graph 3 Variation of Story displacements for bare frame in X- direction.

Graph 4 Variation of Story displacements for infill frames in X- direction.

Graph 5 Variation of Inter – Story drifts for bare frame in X- direction.

0

1

2

3

4

5

0 10 20 30 40 50

No.

of st

orey

s

Displacement (mm)

Fixed Base

0

1

2

3

4

5

0 10 20 30 40 50

No.

of st

orey

s

Displacement (mm)

Fixed Base

0

1

2

3

4

5

0 2 4 6 8 10

No.

of st

orey

s

Storey Drift (mm)

Fixed BaseBase isolation



Graph 6 Variation of Inter – Story drifts for infill frames in X- direction.

3.1. Percentage of Steel Variation in Columns The percentage of steel in case of isolated building is 0.8 % in all the floors except in GF. And also the percentage of steel in the models like fixed with infill and isolation with infill is 0.8%. Therefore only the percentage of steel in GF in both fixed base and base isolated building with bare frame is shown below.

Table 5 Percentage of Steel variation.

Column no. Percentage of Steel Fixed Base Base Isolation

C1, C31 ,C11, C36 4.33 1.28 C4, C25, C12, C30 4.30 0.90 C13, C19, C18, C24 3.99 0.86 C2, C32, C7, C35 4.21 1.32 C3, C26, C10, C29 4.19 0.94 C14, C20, C17, C23 3.91 0.86 C5, C33, C6, C34 4.16 1.25 C8, C27, C9, C28 4.18 1.07 C15, C21, C16, C22 3.97 0.88

4. CONCLUSIONS From the results it is clear that with base isolation the natural period is shifted towards the target time

period of T = 2.5 sec, distancing from the predominant period of earthquake. This increases the response of the structure by preventing the structure resonating with frequencies of earthquake. From the results it is clear that the time period increases by 60.35% for the isolated structure.

Isolators induce a large flexibility to the structure at the isolator level, thus it is evident that a large reduction in base shear in case of bare and infill frame structures as compared with fixed base structure. The reduction in base shear is 70.57% in case of isolated structures.

The increase in the flexibility of the bare and infill frame structures with isolation has increased the total displacement of the structure. However this displacement is concentrated only at the isolator level and hence the displacement between the base and top story of the structure is well within the limits as per IS 1893 (Part -ɪ): 2002, clause 7.11.1. The increase in the displacement for the isolated structure for the bare frame is 53.57%.

0

1

2

3

4

5

0 2 4 6 8 10

No.

of st

orey

s

Storey Drift (mm)

Fixed BaseBase isolation



With the base isolation system, inter story drifts are reduced or almost negligible. This reduced story drifts enables both bare and infill frame structures to behave ideally stiff resulting in less damage to the structural and non structural components. The story drifts follows a non-liner pattern which can be observed in the graphs, however this non-linearity decreases in case of structures with infill compared to structures with bare frame. This is because infill’s increases the rigidity of the structure and hence one can observe the comparative reduction in the drift. The story-drifts obtained for various models are well within the limit as per IS 1893 (Part -ɪ): 2002, clause 7.11.1. The reduction in story-drift for the isolated structure is 65.47%.

Introduction of isolation system resulted in the reduction of base shear as a result one can observe a large reduction in the percentage of steel in case of isolated structures compared with fixed base structures. The reduction in percentage of steel for the isolated structure is 74.88%.

4.1. Scope for Further Study The same structure may be analyzed by considering the shear wall to know the cost differences and the

behavior for isolated structure and structure with shear wall.

By considering infill’s with different percentage of openings the structure may be analyzed.

The structure may be analyzed by considering the soil- structure interaction system.

REFERENCES [1] Tanmay Ramani (2015), “Smart Base Isolators for Seismic Control of Structures”, Vellore Institute

of Technology, Chennai.

[2] Ganga Warrier A and Dr. Sathish Kumar K.(2015), “Response Control of Structures Using Base Isolation”.

[3] Mr. V. P. Jamnekar1 and Dr. P. V. Durge (2013), “Seismic Evaluation of Brick

[4] Masonry Infill”, (IJETET) Vol. 02, No. 01, 2013 ISSN No. 2248-9592.

[5] IS 456:2000, “Plain and Reinforced Concrete – Code of Practice”, BIS, New Delhi, India.

[6] IS: 1893 (part 1): 2002, “Criteria for Earthquake Resistant Design of Structures”, BIS, New Delhi, India.

[7] UBC-97, “Uniform Building Code”, Dynamic methods of design complex.

[8] ASCE 7: “Minimum Design Loads for Buildings and other structures”.

[9] Prerna Nautiyal, Saurabh Singh and Geeta Batham, A Comparative Study of the Effect of Infill Walls on Seismic Performance of Reinforced Concrete Buildings. International Journal of Civil Engineering and Technology, 4(4), 2013, pp.208–218.

[10] Dr. D. V. Prasada Rao and G. Sulochana, Modelling of an Infill Wall for the Analysis of a Building Frame Subjected to Lateral Force, International Journal of Civil Engineering and Technology, 7 (1), 2016, pp. 180-187.