SEISMIC EVALUATION AND REHABILITATION OF AN EXISTING OFFICE BUILDING DESIGNED BASED ON SNI 1726:2002 USING LEAD RUBBER BEARING (LRB) AND CARBON FIBER REINFORCED POLYMER (CFRP)

• Main Author: Nabila Sugiarto, Najmi
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/87989
• Institution: Institut Teknologi Bandung

Buildings designed using earlier standards often exhibit differences in design parameters compared to those specified in the latest regulations, including material requirements and seismic resistance criteria. One of the most significant changes in SNI (Indonesian National Standard) for earthquake-resistant building design pertains to seismic demand and the minimum required concrete compressive strength. In SNI 1726:2002, the seismic demand corresponds to an earthquake with a return period of 500 years, meaning a 10% probability of occurrence within 50 years. In contrast, SNI 1726:2019 adopts a return period of 2500 years, implying a 2% probability of occurrence within 50 years. Additionally, the minimum concrete compressive strength required by SNI 1726:2002 is 25 MPa, which is significantly lower than the 28 MPa requirement in SNI 1726:2019. These regulatory differences highlight the necessity for a performance evaluation of existing buildings to determine whether they still comply with the updated standards. For existing structures, evaluation cannot simply follow the latest design regulations but must instead refer to standards specifically developed for the assessment and strengthening of built structures. ASCE 41-17 is an internationally recognized guideline for evaluating the seismic performance of existing buildings based on a performance-based design approach. This standard allows for the assessment of whether a structure remains safe for use or requires strengthening interventions. This study aims to evaluate the seismic performance of an existing building designed according to SNI 1726:2002 using an appropriate evaluation method for existing structures, namely ASCE 41-17. Additionally, it seeks to identify effective seismic rehabilitation strategies to ensure compliance with ASCE 41-17 performance criteria. The proposed rehabilitation methods include Lead Rubber Bearings (LRB) and Carbon Fiber Reinforced Polymer (CFRP). LRB is employed to enhance the global seismic resistance of the building by reducing base shear and inter-story drift, while CFRP is applied to specific structural elements to improve their flexural, shear, and axial capacities. The structural modeling was conducted using ETABS software, considering gravitational loads such as self-weight, additional dead loads, and live loads. The structural analysis involved two primary methods: response spectrum analysis and pushover analysis. The response spectrum analysis served as a preliminary check to evaluate the Demand-Capacity Ratio (DCR) of the existing structure. Meanwhile, the pushover analysis was performed to assess structural performance under gradually increasing lateral loads until inelastic behavior was reached. The pushover analysis resulted in a base shear vs. displacement curve, which was then converted into the Acceleration Displacement Response Spectrum (ADRS) format. The capacity curve in ADRS format was then compared with the seismic demand curve, with their intersection defining the performance point. At this point, an evaluation of the plastic hinges that formed was carried out to determine whether the existing structure met the acceptance criteria specified in ASCE 41-17. This study utilized two seismic hazard level, BSE-1E (225-year return period) and BSE-2E (975-year return period), representing moderate and extreme earthquake scenarios for performance evaluation. In addition to assessing DCR and plastic hinge formation, an analysis was conducted to verify compliance with the Strong Column-Weak Beam (SCWB) requirement, ensuring that the failure mechanism prioritizes yielding in beams rather than columns. Through this approach, the study aimed to identify structural deficiencies and determine the most effective strengthening strategies to improve the building’s seismic resistance. The research results indicate that before rehabilitaion was applied, most structural elements were not strong enough to withstand seismic loads, posing a significant risk of structural damage. This is evidenced by the DCR values that do not meet the requirements and the formation of plastic hinges exceeding the acceptance criteria of ASCE 41-17. The implementation of Lead Rubber Bearing (LRB) significantly improved structural capacity, as indicated by the reduction in the number of plastic hinges and a more even distribution of deformation. However, some elements still did not meet the Strong Column-Weak Beam (SCWB) criteria and the plastic hinge limitations required by ASCE 41-17, necessitating additional reinforcement using Carbon Fiber Reinforced Polymer (CFRP). A comparison between structures reinforced only with LRB and those reinforced with a combination of LRB and CFRP shows that the overall capacity improvement is not highly significant. This is due to the nature of CFRP reinforcement, which primarily focuses on enhancing the local capacity of specific structural elements. Nevertheless, this combined reinforcement strategy has effectively improved structural resilience, ensuring compliance with ASCE 41-17 performance requirements. The results of the response spectrum analysis for the existing building indicate that in the existing structure, there are 276 beam elements and 123 column elements with a Demand-to-Capacity Ratio (DCR) exceeding 1. Subsequently, the pushover analysis results for the BSE-1E earthquake show the formation of 5 plastic hinges with the IO-LS performance level in the X direction and 1 plastic hinge (IO-LS) in the Y direction. For the BSE-2E earthquake, 25 plastic hinges exceeding the Collapse Prevention (CP) performance level formed in the X direction, and 40 plastic hinges (>CP) in the Y direction, indicating structural failure. After rehabilitation using Lead Rubber Bearings (LRB), the response spectrum analysis results show a reduction in the number of elements that do not meet the DCR requirements to 116 beam elements and 3 column elements. The pushover analysis results indicate that no plastic hinges formed under the BSE-1E earthquake, meaning the structure remains at the fully operational performance level. Meanwhile, for the BSE-2E earthquake, the structural performance level still exceeds CP, but the number of plastic hinges has been reduced to 8 in the X direction and 3 in the Y direction. Further strengthening was carried out using Carbon Fiber Reinforced Polymer (CFRP) on 18 columns that did not meet the Strong Column-Weak Beam (SCWB) requirement, as well as 116 beams and 46 columns that did not meet the DCR requirement and/or formed plastic hinges exceeding the acceptance criteria. In the structure strengthened with the combination of LRB and CFRP, the response spectrum analysis results indicate that all structural elements now meet the DCR requirements. Furthermore, the pushover analysis results show that the performance level for the BSE-1E earthquake demand remains at the fully operational level. For the BSE-2E earthquake, plastic hinges exceeding CP were successfully eliminated, leaving only 8 plastic hinges (IO-LS) in the X direction and 7 plastic hinges (IO-LS) in the Y direction. This demonstrates that the structure has met the acceptance criteria, as no plastic hinges exceed the CP category. With the applied strengthening strategy, the existing building achieved an adequate level of safety without necessitating a comprehensive increase in concrete compressive strength. This study highlights the importance of adopting specialized standards tailored to existing structures, rather than relying solely on modern design codes intended for new buildings when assessing and rehabilitating aging infrastructure.