Comparative Study on Seismic Response of Regular and Plan Irregular Base isolated structures

Earthquake is one of most destructing threat which cause severe damage to life and property. Recently, due to increasing demand and need for the present generations has made the designers and engineers to plan for structures having irregularities. It is impossible to avoid designing uneven buildings in reality. It is important to research how this irregular structure behaves in earthquake-prone areas, especially for structures with base isolation. When compared to regular structures with base isolation, structures with plan irregularity react differently. When a reinforced concrete building with varied plan configurations, such as an H, U, or L shape, is subjected to seismic loading, the impact of plan irregularity is explored. In the current study, a regular structure is also considered to compare the responses of the isolated irregular base structure. Building response spectrum analysis for seismic analysis was done in seismic zone III using IS 1893-2002 code [5]. The structural analysis software tool ETABS 18 was used to conduct the analysis. Among different types of isolators present, in the present study LRB isolator and friction isolators was used as a Nonlinear Support. The dynamic response of the structure such as base shear, time period and story displacement are used to compare the regular and irregular structures. The effectiveness of the base isolators in the structures in earthquake prone area is evaluated in the present study.


INTRODUCTION:
One of nature's biggest dangers, earthquakes have historically resulted in a substantial number of fatalities and significant property damage, particularly in areas that are more vulnerable to them.Because of these earthquakes, engineers and architects developed a variety of crucial design criteria to get beyond the standard design procedure.The seismic base isolation can be utilised to effectively address a variety of seismic design issues thanks to well-established methodologies that have been examined by several researchers.1282 (2023) 012015 IOP Publishing doi:10.1088/1757-899X/1282/1/012015 2 One of the most successful strategies and one that has gained relevance around the world is the employment of base isolation techniques to safeguard buildings against damage brought on by seismic strikes.This is due to base isolation techniques, flexible bases that largely decouple the structure from ground motion, and the fact that structural reaction accelerations are often lower than ground accelerations.In general, the structure's displacements and acceleration reactions may be reduced as extra damping is added.It was observed from the literature reviews that the researchers had done a lot of work regarding the use of base isolation technique.They have modelled different story RC framed structures and are analyzed using different base isolators, using different seismic analysis methods and found the effectiveness of the different isolators by comparing the base shear, displacements, time period, story drifts etc (Ashok et al., (2012) [1], Reddy and Ramesh (2015) [2], Darshale and Shelke (2016) [3], Babu and Jose, (2018) [4]).This study's primary goal is to assess how plan irregularity affects a base isolated structural system's response.The current study investigates if a regular base isolated structure performs better during an earthquake than an irregular base isolated structure.It also investigates whether a low rise building performs better than a medium rise structure with base isolator and imperfections.In the current study, G+5 and G+15 story reinforced concrete moment-resisting frame buildings with and without base isolation are taken into consideration in order to observe the impact of base isolation.

MODELLING AND ANALYSIS OF THE STRUCTURE
This study includes the modeling and analysis of the structures using ETABS 18 structural analysis software.The following design steps are performed: ▪ Modelling of the G+5 & G+15 storey RC framed building for both regular & plan irregular structure and assigning all the material and section properties for all the members of the building such as beams, columns, slabs etc.
▪ Assigning the joint restraints properties as fixed at the bottom of the column.
▪ Assigning the loads on the members and also the lateral loads.
▪ Analyzing the fixed model for given loads for response spectrum for zone III and type II soil.
▪ Assigning properties of base isolators as LRB isolator and Friction isolator using N-link spring at base of the structure.
▪ Analyzing the base isolated structure for response spectrum for zone III.
▪ Comparing the results of earthquake zone III for fixed base and base isolated RC buildings.

RESULTS AND DISCUSSION
The dynamic response of a regular and irregular structures was evaluated for isolated and fixed case to show the effect of isolation for both G+5 and G+15 story structures.The fixed and isolated structures' base shear, t time period and maximum story displacement values characterize the seismic performance of the structures.The maximum values at each story were evaluated and compared for the analysis.From the above results it is clear that the maximum decrease in the base shear is 23.21 % and 23.07%for LRB isolator and Friction isolator respectively in case of G+15 R model when compared with other models.Base isolation shifts the main period of the earthquake and the fundamental period of the structure.When compared to other models, the fundamental period of the fixed structure for the G+5 R model is raised by 60.21% for the LRB isolator and 61.18% for the friction isolator, respectively.This lessens the accelerations of the floor and prevents the building from experiencing severe shaking.The fundamental period of the fixed structure is increased maximum for G+15 R model by 44.82% and 45.81 % for LRB isolator and Friction isolator respectively when compared with other models, which helps the structure to avoid severe shaking due to period shift provided by isolators.

Time Period:
Story Displacement:

CONCLUSION:
The results from the analysis of the G+5 story and G+15 story structure shows how the main effect of base isolation is to greatly reduce the damage due to base shear and the effects of natural period.
It is observed that the maximum decrease in base shear is 53.60 % and 54.72% with LRB isolator and Friction isolator respectively in case of G+5R model when compared with other models in case of low story structures.From the results it is clear that the maximum decrease in the base shear is 23.21 % and 23.07 % for LRB isolator and Friction isolator respectively in case of G+15 R model when compared with other models in case of medium story structures.It is observed that fixed base buildings have higher base shear and isolated buildings has low base shear for both low story and medium story buildings.Therefore, it is clear that base isolation reduces the earthquake forces acting on the structure and thereby makes the structure stable.It is observed that the reduction in base shear is more in case of low story structure when compared with medium story structures.Therefore, it is best suitable for low story structures.It is also observed that the both isolators that is LRB isolator and Friction isolator shows same amount of reduction in base shear.Therefore, both the isolators are effective to use in the structure vulnerable to earthquake.The fundamental period of the fixed base structure is increased maximum for G+5R model by 60.21 % and 61.18 % for LRB isolator and Friction isolator respectively when compared with other models in case of low story structure.The fundamental period of the fixed structure is increased maximum for G+15 R model by 44.82 % and 45.81 % for LRB isolator and Friction isolator respectively when compared with other models in case of medium rise structure.It is observed that the period of the base isolated structure increased when compared with fixed base.Therefore, with increase in the fundamental time period of the structure there is reduction in the maximum acceleration and earthquake forces on the structure.From the maximum story displacement results, it is observed that displacement is zero at fixed base but in case of isolated structure there is displacement at the base of the structure.Thus, makes the structure flexible and has less damage due to earthquake.From the maximum story displacement results, it is observed that irregular structures have maximum displacement when compared to regular structures.Therefore, irregular structures are more vulnerable to earthquake when compared with regular structures.

Figure 3 . 5 :
Figure 3.5: Time period for G+5 regular and plan irregular models

Figure 3 . 6 :
Figure 3.6: Time period for G+15 regular and plan irregular models

Table : 3
.3 Time period for G+5 regular and plan irregular models S.no Models

Table : 3
.4 Time period for G+15 regular and plan irregular models:

Table :
From the above Tables, it is observed that the displacement is zero at fixed base and maximum displacement at top story but in case of base isolated structure there is displacement at base of the structure that is due flexibility of the structure due to isolator.From the Displacement tables, it is observed that the top story displacement is maximum for G+5 L model with a value of 11.03 mm, for G+5 U model with a value of 10.39 mm, for G+5 H model with a value of 9.20 mm and for G+5 R model with a value of 9.04 mm in case of fixed base for low story structure.It is observed that the top story displacement is maximum for G+15 L model with a value of 32.04 mm, for G+15 U model with a value of 31.31mm, for G+15 H model with a value of 24.26 mm and for G+15 R model with a value of 22.77 mm in case of fixed base for medium story structure.From top story displacement values, it is observed that as irregularity increases the top story displacement values are high.So irregular structures are more prone to damages due earthquake when compared with regular structure.
3.5 Maximum Story Displacement in X-direction for Fixed and isolated G+5 R model IOP Publishing doi:10.1088/1757-899X/1282/1/01201511Figure3.7: Displacement variation for G+5 regular and plan irregular models Table:3.9Storey Displacement in X-direction for Fixed and isolated G+15 R