The axial compressive experiments were carried out on 21 welded composite T-shaped concrete-filled steel tubular columns, and 395 finite element models were established for parameter calculation. The calculating formula of axial compressive bearing capacity of welded composite T-shaped concrete-filled steel tubular columns is established. The results show that three typical failure modes were found: middle buckling, end local buckling, and integral bending. When the slenderness ratio λ exceeds the elastic instability limit λ_p, the axial stress of steel is lower than yield strength f_y, and the axial stress of core concrete is lower than axial compressive strength f_c. Increasing the thickness of steel has a more obvious effect on increasing the axial compressive bearing capacity of specimen. The theoretical calculating formula can effectively predict the axial compressive bearing capacity, and the theoretical calculation is partial to safety. The average ratio of axial compressive bearing capacity of the theoretical calculation to the experimental is 0.909, and the standard deviation is 0.075. The average ratio of axial compressive bearing capacity of the finite element calculation to the experimental is 0.957, and the standard deviation is 0.045. The average ratio of axial compressive bearing capacity of the theoretical calculation to the finite element calculation is 0.951, and the standard deviation is 0.039.

In order to study the compression bearing capacity of concrete-filled rectangular stainless steel tubular columns, the influence of the stainless steel tube thickness, relative eccentricity, and slenderness ratio on the compression bearing capacity is analyzed, and then the calculation formula of compression bearing capacity is proposed. The results show that the finite element model can effectively simulate the compression bearing capacity, the mean of finite element calculations N_ufem to the test N_uexp is 0.985, and the variance is 0.000621. The slenderness ratio and relative eccentricity have a great influence on the load–displacement curves. The thickness of the stainless steel tube has little influence on the load–displacement curves. With the increase in slenderness ratio and relative eccentricity, the compression bearing capacity decreases. With the increase in the slenderness ratio, the failure model of the specimen gradually changes from plastic failure to elastoplastic failure and then elastic failure. When the slenderness ratio is the same, if the relative eccentricity is larger, increasing the thickness of the stainless steel tube will be more effective in improving the compression bearing capacity. When the relative eccentricity is the same, if the slenderness ratio is smaller, increasing the thickness of the stainless steel tube will be more effective for improving the compression bearing capacity. The slenderness ratio and relative eccentricity have a great influence on the longitudinal stress distribution in the cross-section. When the slenderness ratio and relative eccentricity are greater, the longitudinal compressive stress in parts of the cross-section gradually becomes longitudinal tensile stress. The proposed formula can effectively predict the compression bearing capacity of concrete-filled rectangular stainless steel tubular columns. The mean of theoretical calculations to the test and the finite element is 1.054, and the variance is 0.0247.

To study stress field distributions during the construction of a group of tunnels using the pile–beam–arch method, the excavation for an underground station of Beijing subway line 7 was taken as an engineering problem, and a numerical structural model of a tunnel group was built with the aid of software. In this model, four stress-monitoring planes were set up to extract the data obtained from monitoring stress. The construction of the pilot tunnels was simulated according to six established construction schemes, and images were generated for the stress field distribution on each monitoring plane using different schemes. Finally, a verification of physical model tests was carried out. The results show that the effect of excavation on vertical stress in pilot tunnels has a range that is essentially equal to twice the width of the station. After the excavation of pilot tunnels, peaks appeared both at the tops of the pilot tunnels and between the bottoms of the tunnels; moreover, valleys appeared both at the bottoms of the pilot tunnels and between the tops of the tunnels. Pilot tunnels located at the top of the group were less affected during excavation than the tunnels located at the bottom of the group both in stress concentration factor and in the point of extreme stress. Finally, the model verified the numerical simulation results of the stress variation law.

A new finite element (FE) model was constructed with ABAQUS, and the applicability of the model was verified by the coincidence with typical damage modes and load-compression curves in the tests, and the axial compression performance of the precast circular semi-continuous concrete-filled steel tube (PCSCFST) columns was investigated. The effects of diameter-thickness ratio, slenderness ratio, yield strength, etc. on the axial compression performance of the PCSCFST columns were investigated by parametric analysis. The changes in slenderness ratio, yield strength and diameter-thickness ratio of the upper and lower steel tubes have obvious effects on the bearing capacity of the specimen, while the changes in bolt diameter and diameter-thickness ratio of the outer steel tube have little effects on the bearing capacity. In particular, the diameter ratio of bolt to steel tube (d/D) increases to 1/10, the bearing capacity increases slightly, the ratio (d/D) continues to increase until the bearing capacity decreases slightly, and the bearing capacity appears to increase significantly after the ratio (d/D) reaches 1/7; the yield strength ratio of bolt to upper and lower steel tube (f_yb/f_y1) increases from 1 to 2, the bearing capacity decreases slightly, and the bearing capacity increases significantly when the ratio (f_yb/f_y1) reaches about 2. After that, the change is minimal. In addition, a theoretical model was developed to predict the ultimate bearing capacity of the PCSCFST columns, and a close correlation was found between the FE simulation results and the theoretical model. The mean ratio of the FE ultimate load N_u,FE to the predicted ultimate load N_u,pre was 1.006 with a standard deviation of 0.0389.

To study the axial compression performance of precast circular semi-continuous concrete-filled steel tube (PCSCFST) column, a total of 17 specimens were prepared and tested, including 13 PCSCFST columns and 4 continuous CFST columns. This PCSCFST column is made up of upper and lower steel tubes and outer steel tubes connected by bolts, which has the advantages of easy factory production and convenient transportation. The results showed that shear failure occurred in continuous CFST columns, but not in PCSCFST columns, bolts had obvious influence on failure mode. Maintaining the consistency of the thickness of each steel tube on the specimen can not only improve the local buckling, but also avoid the short plate effect to give full play to the material performance. Moreover, with the increase of steel tube thickness, concrete strength, bolt diameter, reduction of specimen length and pure bolt connection instead of bolt and weld combination connection, the deformation resistance of bolts and axial compression performance of specimens had been enhanced. However, the increase of bolt diameter from 12 to 18 mm resulted in a better strength degradation rate, but worse bearing capacity and deformation resistance. Finally, the predictions of the ultimate bearing capacity of PCSCFST columns under axial compression agree well with the test results, the average value of N_exp/ N_pre and the variance are 0.998 and 0.00111.

To study the mechanical performance of bolted connections with different structural forms of reinforced rings, based on the results of monotonic loading tests on two bolted connections between a concrete-filled steel tubular column and a steel beam with an outer reinforcing ring, this article uses ABAQUS v.2020 software to establish a three-dimensional refined finite element analysis model of such connections using appropriate constitutive models for concrete and steel. Subsequently, the effect of the dimensions of the steel beam, reinforcing ring, and cover plate on the load-bearing properties and the failure mechanism of the connections is investigated, and the numerical model is consistent with the verification test results. Then, the numerical simulations comparing bolted exterior reinforced rings under seven different construction measures (i.e., number of bolts, stiffeners) based on a conventional welded exterior reinforced rings with rigid connections (i.e., CGJ) are standardized. The research results indicate that when four rows of bolts are introduced on exterior reinforced rings, the web of steel beam is welded with stiffeners, and the top and bottom reinforced rings are also added with stiffeners; this bolted connection with an external reinforcing ring (i.e., GZ-7) can achieve the rigidity and load-bearing capacity of a fully welded external reinforcing ring rigid connection. At the same time, the reinforcing ring plate is bolted to the flange of the steel beam, and the force transmission path at the connection is changed to avoid the brittle fracture easily caused by the welded flange joints. It is also in line with the development trend of sustainable construction of “assembly” and “disassembly”.