Local multiscale method for beam-column joint and its application in large-span column-free underground spatial structures

For large-span underground structures with characteristics of various components, multiple joints, and complex construction processes, beam-column joints are the key to ensuring smooth force transmission in structures. For the precise mechanical performance of beam-column joints, the traditional local refined finite element method (LRFEM) has some problems, such as difficulty in determining boundary conditions, heavy modeling workload, and low computational efficiency. Therefore, the Local Multiscale Finite Element Method (LMFEM) based on the Multipoint Constraint Equation (MPC) was proposed in this paper. Examples demonstrated the advantages of LMFEM in less modeling workload, being easier to satisfy Saint Venant's principle on boundary conditions and higher efficiency with the same accuracy. With the proposed LMFEM, taking Gangxia North large-span columnfree underground structure as engineering background, the influences of the through modes on the mechanical performance of beam-column joints were further investigated. Results showed that compared with the beamthrough type, the column-through joint performs lower stress level, smooth force transmission, and insignificant stress concentration, which is appropriate to be applied in the Gangxia North Project. Based on in-site monitoring, the measured values of the stress state of the typical beam-column joint of the Gangxia North Project were compared with the calculated values obtained through different methods. Results showed that, for the same modeling scale and mesh size, the maximum errors between the measured values and the calculated values obtained by LRFEM and LMFEM are 11.8% and 12.5%, respectively. However, the nodes quantity of the model of the latter is only 51.2% of that of the former, and the computation time is only 32.8% of that of the former. Besides, the computational accuracy of the former is more susceptible to being affected by the boundary condition and mesh size than the latter. Thus, it is fully demonstrated that the accuracy and efficiency of the LMFEM, and can be effectively applied to the local stress analysis of complex large-scale structures. Compared to traditional simulation methods, LMFEM can more easily satisfy boundary conditions and achieve a balance between computational accuracy and efficiency.