การเสริมกำลังต้านทานแรงแผ่นดินไหวของจุดต่อคาน-เสาคอนกรีตเสริมเหล็กภายนอกด้วยค้ำยันเหล็กกล่องสี่เหลี่ยม

Abstract

Thailand is located in an earthquake-prone area, posing a severe threat to people's lives and property. Additionally, there are old building structures that have not been designed to withstand earthquakes. This research focuses on studying the behavior of the lateral force resistance of the exterior beam-column joint of reinforced concrete frame, externally strengthened with 50x50 millimeter square hollow steel braces (HSS). The design specifies that the steel bracing should fail before the concrete frame is damaged and fails. The testing is divided into 2 parts: (1) testing for cyclic load with specimens until failure, and (2) testing for cyclic load after repairing the damaged specimens from the first part. There are a total of 4 test specimens with the same details of reinforcement steel and strengthened positions. Specimen SC1 is the reference specimen without any strengthening, while the strengthened specimens are divided into 3 specimens (SC2, SC3, and SC4). Specimen SC2 was strengthened with 1.50 millimeter thick HSS; Specimen SC3 was strengthened with 1.50 millimeter HSS filled with mortar ; and Specimen SC4, which is similar to Specimen SC3 but with an increased thickness of the HSS steel to 2.30 millimeters. From the results of the first part of the testing, it was found that the specimen without any strengthening (SC1) experienced damage at the beam-column joint, while the strengthened specimens (SC2, SC3, and SC4) exhibited buckling of the square hollow steel or weld failure, leading to cracking at the joint until failure occurred. The strengthening enhanced the strength and stiffness in resisting lateral forces, as the HSS bracing helped transfer the lateral loads from the column to the beam, reducing the forces at the beam-column joint. Regarding to the energy dissipation, the reinforced specimens demonstrated lower energy dissipation compared to the un-strengthened specimen, and the energy dissipation increased as the drift ratio increased due to higher accumulated damage in the specimens. Additionally, the design choice of pre-failure at the bracing resulted in lower stiffness of the strengthened specimens compared to the un-strengthened specimen. The additional mortar filled into the HSS bracing was found to prevent localized buckling and enhance the stiffness of the braces, making them more efficient in load transfer. For the second part of the testing, following the damage that occurred in the first part, it was observed that specimens SC2 and SC3, although their resistance had decreased due to the damage at the HSS bracing while the concrete joint still exhibited minimal damage. Therefore, repairs were made to the HSS bracing (re-welded), and the specimens were retested to simulate post-earthquake aftershock scenarios. It was found that these specimens remained capable of withstanding lateral forces. As for specimen SC4, due to the high strength of the braces, significant damage occurred to the frame, and thus no repairs were attempted. In summary, reinforcing with square hollow steel braces enhances the performance of buildings not originally designed to withstand earthquakes, enabling them to better withstand forces and preventing collapse due to minimal damage to the concrete frame. This allows for the possibility of promptly repairing the braces to ensure safety in the event of subsequent earthquakes.