Bidirectional Seismic Behavior Of Bottom Story Beam-to-column Joint Subassemblies Of RC Moment Resisting Frames

The structure seismic design has come through several developmental stages whenpeople get known more about the structure earthquake response or the increase ofpeople demand for higher living quality and production quality. The core of thecapacity design is the ductile failure mode, which is one of the most popular method ofperformance-based seismic design. The design principle ‘Strong column-weak beam’and ‘strong joint-weak member’ principles are required in capability design for thebeam-to-column joints. These principles are presented in seismic provisions in China.In the Wenchuan earthquake and Lushan earthquake, the response of the structure,which were not the first level frame, showed that the ‘Strong column-weak beam’ willnot always be satisfied, the layer collapse may also happened.In order to study the seismic anti-collapse behavior of RC moment resisting frames, a3rdlever RC frame which designed according to the GB50010-2001was designed.Two-way beam-to-column joints which include the bottom end of the column in thebottom story were selected from the frame. These specimens were subjected tobidirectional cyclic loading to investigate the impact of bidirectional loading and slabcontribution on the seismic performance, including their strength, ductility and failuremodes. FEM analysis was used to study the failure mechanism of the specimens.The main research works and results are as follows:(1) Six RC3D beam-to-column joints were designed, which consist of3interiorjoints and3exterior joints. The test force and displacement was carried out on the topof the column. The test results show that all specimens failed to exhibit the desired‘strong column-weak beam’ mechanism under bidirectional cyclic loading regardlessof the presence of the slabs, although they are designed in accordance with the seismicprovisions of code for seismic design of building in China, which is published beforeWenchuan Earthquake. Instead, the interior joint subassemblies sustained failure at thebottom end of the column and the exterior ones experienced joint failure, which islikely the consequence of the reduced beam confinement for the joint zone and leads tomuch lower strength as compared to the interior counterparts. Under such failuremodes, the presence of the slabs has little impact on the strength of the specimens;while on the other hand, the confinement for the exterior joint zones by the slabs ishelpful to increase the deformability.(2) FEM models were built based on the experiment result in the FEM softwareABAQUS, which use beam element to simulate the beam and column, shell element tothe slab. The panel zone behavior is simplified by softened strut-and-tie model. Twoparts of analysis work was carried out: fitting the results of the FEM model and test result, parameter analysis based on the verified models. The process of the experimentswas well reproduced, while the interior joints failure on the bottom end of the columnand the exterior joints show large deformation on the panel zone which exceeded thedamage deformation of the panel zone. The minimum value of the strong column andweak beam coefficient was calculated by means of changing the coefficient and thecase with or without the slabs.