Comparative Study of Seismic Response of Structure with Different Base Isolators

IJSRD - International Journal for Scientific Research & Development| Vol. 4, Issue 03, 2016 | ISSN (online): 2321-0613

Comparative Study of Seismic Response of Structure with Different Base Isolators Yogesh P. Patel1 Prof. P. G. Patel2 1 P.G. Student 2Associate Professor 1,2 Department of Applied Mechanics 1,2 L.D. College of Engineering, Ahmedabad Abstract— Conventional seismic design attempts to make building that do not collapse under strong earthquake shaking, but may sustain severe damage to non-structural elements as well as some structural members. This may render the building non-functional after the earthquake, which is not acceptable for important buildings, like hospitals. Special techniques are required to design buildings such that they will not suffer damage even in a severe earthquake. One of the technologies used to protect buildings from damaging earthquake effect is “Base Isolation”. The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building, or at least greatly reduced. This paper presents the seismic behaviour of R.C.C frame structure with different base isolators. Two types of base isolators are considered namely Lead Rubber Bearing (LRB) and Friction Pendulum System (FPS). Isolators are designed for different story height (12 m, 24 m, 36 m, 48 m). Response quantities like base shear, time period and story displacement will be extracted for building with fixed base and building with isolated base by performing Response Spectrum Analysis (RSA) to establish their effectiveness and final conclusion will be made on the bases of study. Key words: Seismic Isolation, Comparative Study, Elastomeric Bearing, Sliding System I. INTRODUCTION A high proportion of the earth is subjected to earthquakes and society expects that structural engineers will design our buildings so that they can survive the effects of these earthquakes. As for all the load cases generally encountered in the design process, such as gravity and lateral loads, for that work to meet a single basic equation CAPACITY > DEMAND Generally, we know that earthquakes happen and are uncontrollable. So, in that sense, we have to accept the demand and make sure that the capacity exceeds it. The earthquake causes inertia forces proportional to the product of the building mass and the earthquake ground accelerations. As the ground accelerations increases, the strength of the building, the capacity, must be increased to avoid structural damage. Base isolation takes the opposite approach; it attempts to reduce the demand rather than increase the capacity. We cannot control the earthquake itself but we can modify the demand it makes on the structure by preventing the motions being transmitted from the foundation into the structure above.

Fig. 1: Family of Isolation Devices II. LITERATURE REVIEW A. R.S. Jangid (2002) Increase the period of isolation, increases the bearing displacement but decreases the superstructure acceleration. The increase in the yield strength of lead-rubber bearing decreases the bearing displacement but increases the superstructure acceleration. The increase in the friction coefficient of sliding systems decreases the sliding displacement but increases the superstructure acceleration. In general, the lead-rubber bearings and sliding systems with restoring force such as FPS perform well in comparison to other isolation systems. B. M.K. Sharbatdar, S.R. Hoseini Vaez, G. Ghodrati Amiri (2011) In the vicinity of causative earthquake faults, ground motions at a particular site are significantly influenced by the rupture mechanism and slip direction relative to the site and by the permanent ground displacement at the site resulting from tectonic movement. Maximum base displacement can be differed up to 66% in a zone within a distance of about 4km from the ruptured fault. Maximum top floor acceleration can be differed up to 35% for the records of Imperial Valley. C. Sajal Kanti Deb (2004) Base isolation systems can be used at soft-soil sites where load on the isolation system and sizes of the isolation systems are sufficiently large. The resultant maximum bearing displacement is mainly due to the normal component of the near-fault. The effect of soil–structure interaction is small when the isolators are much more flexible, than the soil, if not the soil may contribute to the building behaviour.

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Comparative Study of Seismic Response of Structure with Different Base Isolators (IJSRD/Vol. 4/Issue 03/2016/214)

D. Chandak N. R. (2013) [4] The increase in period for structure with isolated base makes sure that the structure being completely removed from the resonance range of the earthquake. Friction pendulum isolators (FI) has reduces further the response of isolated building when compared to that of the response obtained with rubber bearings (RB). IS code depicting the higher values of base shear for similar ground types defined in EC-8 code which may lead to overestimate the overturning moments and could results in heavier structural members in the building. III. MODEL DEVELOPMENT This section presents the information for model development of R.C. frame building with LRB & FPS at GF in ETABS. The response of R.C frame building in the form of Story Displacement, Base shear and time period were calculated. The method of analysis used was Response Spectrum Analysis given in IS-1893 (part-1):2002. Various results such as storey displacement, base shear and time period are calculated for different building height (12 m, 24 m, 36 m and 48 m).

IV. DESIGN OF ISOLATORS A complete design for base isolation should ensure that the isolators can support the maximum gravity service loads of the structure throughout its life, and the isolators can provide the dual function of period shift and energy dissipation to the isolated structure during earthquakes. Description Effective Horizontal Stiffness, Keff (kN/m) Short term Yield force, Qd (kN) Post Yield Horizontal Stiffness, kd (kN/m)

4 Story (12 m)

8 Story (24 m)

12 Story (36 m)

16 Story (48 m)

473.12

965.12

1457.52

1944.07

15.61

31.84

48.08

64.13

398.80

813.52

1228.57

1638.70

Table 2: Properties of LRB Description Effective Horizontal Stiffness, Keff (kN/m) Radius of Curvature, RFPS (m)

4 Story (12 m)

8 Story (24 m)

12 Story (36 m)

16 Story (48 m)

601.27

1262.07

1922.84

2583.63

1.5

1.5

1.55

1.55

Table 3: Properties of Fps V. ANALYSIS RESULTS A. Time Period

Fig. 2: Plan of Building in ETABS Geometry of building No of bays in X-direction (m) 4 No of bays in Y-direction (m) 4 Height of building (m) 15 Typical story height (m) 3 Length of building (m) 20 Width of building (m) 20 Column Size (m x m) 0.5 x 0.5 Beam Size (m x m) 0.23 x 0.45 Length and width of one bay (m) 5 No of column in one story 25 Material Data 3 25 Density of concrete (KN/m ) 2 25 Grade of concrete (N/mm ) 3 20 Density of brick (KN/m ) 2 415 Yield strength of steel (N/mm ) Loading Data 2 1.5 Terrace Live Load (KN/m ) 2 3 Floor Live Load (KN/m ) Wall Load (KN/m) 11.73

Table 1: Structure Configuration

Fig. 3: Period (Sec) B. Base Shear

Fig. 4: Base Shear (KN)


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C. Story Displacement

Fig. 7:

Fig. 5: Story

Fig. 8:

Fig. 6: Story Displacement

VI. CONCLUSION 

Max. Base shear reduction are 75.16 % and 77.33% in 12 m structure with FPS and LRB with respect to fixed base structure.


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For each & every case base shear are always decreased in both isolated structures compared to fixed base. But, as the story height increases rate of decrease of base shear are decreased. Max. Increment in time period, 3.55 times & 3.93 times are shown in 12 m structure with FPS and LRB with respect to fixed base structure. For each & every case time period are always increased in both isolated structures compared to fixed base. But, as the story height increases rate of increases of time period are decreased. Structure with FPS gives lower response compared to structure with LRB in all cases. In base isolated structure, max. displacement is concentrated at the isolation level, that’s why we get max. drift at ground floor. With increase in height, rigid body movement of superstructure are not achieved in base isolated structure REFERENCES

[1] R.S. Jangid, “Parametric Study of Base-Isolated Structures”, Advances in Structural Engineering Vol. 5 No. 2, 2002, pp. 113-122. [2] M.K. Sharbatdar, S.R. Hoseini Vaez, G. Ghodrati Amiri, “Seismic Response of Base-Isolated Structures with LRB and FPS under near Fault Ground Motions”, The 12th East Asia-Pacific Conference on Structural Engineering and Construction, ELSEVIER, pp. 32453251. [3] Sajal Kanti Deb, “Seismic Base Isolation- An Overview”, Special Section: Geotechnics and Earthquake Hazards, Current Science, Vol. 87, No. 10, 25 November 2004, pp. 1426-1430. [4] Chandak N. R., “Effect of Base Isolation on the Response of reinforced Concrete Building”, Journal of Civil Engineering Research 2013, Vol. 3(4), pp. 135142. [5] Yeong-Bin Yang, Kuo-Chun Chang, Jong-Dar Yau (2000), “Base Isolation”, Earthquake Engineering Handbook, CRC Press, Chapter 17, pp. 17.1-17.31.


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Comparative Study of Seismic Response of Structure with Different Base Isolators

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