EVALUATION ON EFFECTIVE DIMENSIONS OF BASE ISOLATORS APPLIED AT EXISTING RC BUILDING DUE TO EARTHQUAKES

This study investigates the structural performance of High Damper Rubber Bearings (HDRB) as base isolators in reinforced concrete (RC) buildings subjected to seismic loads. Given the potential for damage or collapse in RC buildings during earthquakes, the utilization of base isolators offers a promising solution to mitigate seismic risks. Finite element analysis was employed to assess the displacement efficiency of different HDRB configurations, focusing on the SMP 5 Muhammadiyah building in Surabaya as a case study. Results indicate that HDRB type HH090X6R exhibits superior performance, achieving displacement efficiencies of 90.56% in the x-direction and 95.87% in the y-direction. These findings underscore the importance of selecting optimal base isolator configurations to enhance the seismic resilience of RC structures. The study's implications extend to seismic retrofitting and structural design practices, offering valuable insights for improving the seismic performance of existing buildings and bolstering community resilience to earthquakes.


Introduction
Indonesia is a country prone to earthquakes.The cause is the meeting of several tectonic plates that stretch almost throughout the territory of Indonesia.Several earthquakes that have occurred in Indonesia have caused many casualties and major damage to buildings.
The construction of reinforced concrete buildings although designed to be earthquake-resistant, but if exposed to earthquake loads, the building still has the potential to experience damage.Building damage caused by horizontal earthquake loads is more dominant than vertical earthquake loads, so a mechanical device is needed that can dissipate energy from the earthquake.Building damage that occurs due to earthquake force in Indonesia because planning and implementation have not been in accordance with the rules and standards of planning earthquake-resistant buildings (Simanjuntak, 2020).With base isolator can reduce the displacement of reinforced concrete structures by 26% (Wijaya et al., 2020).Using base isolator on reinforced concrete structures, achieving faster performance levels of Immediate Occupancy structures (Wijaya et al., 2019).Using a base insulator on reinforced concrete structure can reduce the base shear force by 12.56% (Syahnandito et al., 2020).Using a base insulator on reinforced concrete structure can reduce displacement by 66% (Cahyani et al., 2021).Flexibility of the base of the building increases by using base isolators (Ravi et al., 2021).Base isolator will Evaluation on Effective Dimensions of Base Isolators Applied at Existing RC Building due to Earthquakes make the displacement of the structure will be lower (Gunawan et al., 2020).The level of structural performance in controlling damage due to earthquake loads has arranged in FEMA 440 and ATC-40 (Masrilayanti et al., 2023), and also considered by ASCE 14-13 (Enggartiasto et al., 2023).
To assess the horizontal displacement capacity of several types of base isolators based on the results of analysis of existing building structures due to earthquake loads, the appropriate type of base isolator can be applied in existing buildings.This study investigates the structural performance of High Damper Rubber Bearings (HDRB) as base isolators in reinforced concrete (RC) buildings subjected to seismic loads.

Research Method
Before conducting a study of the base isolator to be used in the existing building, it must first check the structural model, and whether the reliability of the existing building structure has been met.At the stage of checking the existing structure model, namely checking the natural vibration time of the structure, checking shear forces, checking mass participation, checking the deviation limit between levels, checking the influence of P-Delta, checking horizontal irregularities and vertical irregularities, checking the Strong Coloumn-Weak Beam (SC-WB) concept and conducting pushover analysis to determine the level of performance of the structure.
Data from the analysis due to earthquake load was obtained from the results of the running program ETABS V.18.0.2.The regulations used in this analysis refer to the applicable standards in Indonesia, refer to (SNI 1726(SNI , 2019)), (SNI 1727(SNI , 2020) ) and (SNI 2847(SNI , 2019)).

Results and Discussion
Before conducting a study of the base isolator to be used, firstly it was evaluated the reliability of structural model of the SMP Muhammadiyah 5 building that located in Surabaya.The building was constructed by a reinforced concrete that having 15 floors, as shown in Fig. 1. (Source: google.com,2024)

Structural Models
By using ETABS V.18.0.2, structural analysis was conducted, that involve the fixed base portals for x and y directions without the base isolator, as shown in Fig. 2 and Fig. 3.Meanwhile, structural analysis on the fixed base portals for x and y directions with the base isolator, can be seen in Fig. 4 and Fig. 5.

Checking of The Existing Structures
Design of spectrum response, graph of the response spectrum at the related location is shown in Fig. 6.Seismic design category, with SDS = 0.5852 g and SD1 = 0.3206 g, according to (SNI 1726,2019), the location includes KDS-D.Seismic force-bearing factors, for the category of buildings with special moment-bearing reinforced concrete frames, the value of the response modification coefficient (R) is 8, the stronger factor over the system (W0) is 3 and the deflection magnification factor (Cd) is 5.5 according to (SNI 1726(SNI , 2019)).Structure period, the period of the x-direction structure is 2.246 seconds, while for the y-direction it is 1.604 seconds.Total capital participation ratio, the sum of the mass ratios of capital participation of the x direction and y direction is 100% in mode 45.Base shear earthquake force, the base shear earthquake force in the x direction was 2544.44 kN and the y direction was 2686.28 kN.Earthquake force scale, the magnitude of the xdirection and y-direction earthquake force is 1.2775.Horizontal and vertical irregularities scale, the building does not have irregularities of torque, inner angles, soft stiffness, and also weight (mass).Interlevel deviation, the maximum deviation between levels in elastic condition (Inelastic drift) that occurs on the top floor is 4.17 mm (x direction) and 3.29 mm (y direction), this value is still smaller than the required 27 mm.Effect of P-Delta, the maximum stability coefficient that occurs on the 4rd floor is in the x direction of 0.0775 and the y direction of 0.0600, this is still smaller than the structure stability limit of 0.0909.Check strong column-weak beam (SC-WB), After checking the SC-WB at three points of the beam-column joint, the nominal moment of the column ≥ 1.2 from the nominal moment of the beam.

Pushover analysis
The results of pushover in the x-direction show that the occurrence of plastic hinges begin at step 3 and ends at step 26 are shown in Fig. 7 and Fig. 8.The results of pushover in the y-direction show that the occurrence of plastic hinges begin at step 2 and ends at step 20, as shown in Fig. 9 and Fig. 10.Based on the results of superposition performance points on the performance of structures (ASCE 14-13, 2013), it is found that the performance level of the structure is included in the Immediate Occupancy (IO) category level, as shown in Fig. 13 and Fig. 14.Here are the performance points based on FEMA 440, as shown in Fig. 15 and Fig. 16.Based on the results of (FEMA 440, 2005), the ratio of maximum total deviation of the x direction is 0.0078 and the y direction is 0.0080, where this figure shows the level of performance of the structure.So according to (ATC 40, 1996) the structure is included in the Immediate Occupancy (IO) category level.

Sway mechanism behavior
To determine the type of base isolator used, an analysis will be carried out on the displacement that occurs in existing buildings by providing horizontal forces due to earthquake loads.In this study will use the type of base isolator High Damper Rubber Bearing (HDRB) HH Series with a total rubber thickness of 20 cm produced by (Bridgestone, 2017).Based on the results of the analysis, displacement in the x direction and y direction occurred using the HDRB base isolator HH160X6R, as shown in Fig. 17 and Fig. 18.After obtaining the displacement that occurs, a check will be made against the displacement limit with 0.7 of the thickness of the base isolator.The following are the results of checking displacement using HDRB base isolator HH160X6R, as shown in Table 1.The following are the results of checking displacement using HDRB base isolator HH160X6R, as shown in Table 2. Based on the results of the analysis, it was found that the type of base isolator providing better performance was the HDRB base isolator type HH090X6R with an efficiency displacement in the x-direction of 90.56% and in the y-direction of 95.87%.

Conclussion
Based on the results of the analysis, it was found that the type of base isolator providing better performance was the HDRB base isolator type HH090X6R with an efficiency displacement in the x-direction of 90.56% and in the y-direction of 95.87%.
The results of the analysis showed that the HDRB base isolator type HH090X6R provides better performance, where the displacement efficiency reached 90.56% for the x-direction and 95.87% for the y-direction.

Figure 4 .
Figure 4. Portal in x-direction with a base isolator (Source: personal data, 2024)

Figure 7 .Figure 8 .
Figure 7. Formation of plastic hinges in the x-direction in step 3 (Source: personal data, 2024)

Figure 9 .
Figure 9. Formation of plastic hinges in the y-direction in step 2 (Source: personal data, 2024)