Objective Modular steel construction is an innovative building approach that divides traditional buildings into three-dimensional units. Each modular unit is transported to the construction site for assembly after factory prefabrication is completed. This construction method not only ensures the scalability of building space but also improves the reusability of structural components. It provides advantages such as high construction quality, resource conservation, and environmental sustainability. Therefore, it is widely used in apartments, schools, hotels, and other low-rise buildings. The connections between modules directly affect the overall structural performance and therefore represent a key research focus in modular steel construction. Under large wind loads or strong seismic actions, these connections are subjected primarily to shear forces, and shear failure can become the governing mode of structural failure. Therefore, investigating the mechanical behavior of connections under pure shear loading is essential, as it directly determines the lateral bearing capacity, lateral stiffness, and overall stability of modular steel structures. Methods This study proposes a new type of socket connection composed of an upper-column plug, a lower-column bearing interface, and high-strength bolts. After assembly, the integrity of the connection is further enhanced through grouting. The proposed connection is simple to install, reliable in quality, and characterized by a clear force transmission path, satisfying the requirements of integrated modular building design. Static loading tests are conducted on two specimens to analyze the working mechanism and strain distribution to investigate the shear behavior of socketed column-to-column connections in modular steel buildings. A finite element model of the connection is established using ABAQUS, and its accuracy is validated through comparison to the experimental results. The effects of plug thickness, socket thickness, nesting depth, mortar strength, and steel yield strength on the shear performance of the connection are systematically analyzed. Results and Discussions The test results show that grouting ensures the overall integrity and good ductility of the connection, and the shear stiffness reaches 952 kN/mm, representing an increase of 121%. Grouting significantly reduces both the longitudinal and shear strains in the primary load-bearing regions. A strong synergistic shear resistance develops between the mortar and the plug, while interface friction and natural bonding effectively resist shear forces within the section, enhancing the shear performance of the connection. The maximum longitudinal strain and maximum shear strain of specimen ZJ‒2 are 54% and 50% lower than those of specimen ZJ‒1, respectively. The shear-displacement curves obtained from the finite element analysis and experimental tests show good agreement, with the elastic stages being nearly identical and the shear stiffness error remaining within 10%. At the ultimate bearing state, buckling failure occurs in the plug sections of both specimens ZJ‒1 and ZJ‒2. The top gap between the plug and the bearing interface in specimen ZJ‒1 increases, while the bottom surfaces come into contact. In specimen ZJ‒2, the mortar within the gap and the core region of the connection experiences significant crushing. The finite element analysis results indicate that increasing the socket thickness and nesting depth has little effect on the shear capacity and shear stiffness of the connection. Increasing the mortar strength improves the shear performance of the connection, although the effect remains limited. In contrast, increasing the plug thickness and steel yield strength significantly enhances the shear performance of the connection. When the plug thickness increases from 6 mm to 8, 10, and 12 mm, the shear stiffness increases by 8.7%, 15.5%, and 19.3%, respectively, while the shear capacity increases by 20.5%, 39.7%, and 59.1%, respectively. When the steel yield strength increases from 235 MPa to 345, 390, and 420 MPa, the shear stiffness increases by 22.2%, 34.6%, and 44.4%, respectively, while the shear capacity increases by 36.7%, 50.2%, and 60.7%, respectively. Based on the finite element parametric analysis, calculation formulas for shear capacity and shear stiffness are proposed by considering the contributions of both the plug web plate and mortar. The accuracy of these formulas is verified through comparison to the finite element analysis results. The shear capacity of the new socket column-to-column connection consists of the shear capacity Vs of the plug and the shear capacity Vc of the mortar within the core region of the connection, with the latter accounting for the confinement provided by the steel tube. The initial stiffness of the connection is calculated using the component method. The shear stiffness of the core region is primarily composed of the shear stiffness of the plug web and the mortar, while the contribution of the plug flange to the overall shear stiffness is neglected. The ratio of the finite element analysis results to the theoretical calculations for shear bearing capacity ranges from 1.01 to 1.09, with a mean value of 1.06, a standard deviation of 0.062, and a coefficient of variation of 0.058. Similarly, the ratio of the finite element analysis results to the theoretical calculations for shear stiffness ranges from 1.02 to 1.13, with a mean value of 1.08, a standard deviation of 0.082, and a coefficient of variation of 0.076. These results demonstrate that the proposed formulas for shear capacity and shear stiffness achieve high accuracy and can be used in the shear performance design of this type of socket column-to-column connection. Conclusions This study investigates the interlaminar shear performance of socket column-to-column joints in steel modular units under lateral loading through experimental testing, finite element analysis, and theoretical analysis. The experimental results reveal the deformation characteristics and stress development during the elastic stage under interstory shear forces. The finite element analysis evaluates the influence of parameters such as socket depth, plug thickness, bearing interface thickness, and material properties on shear performance. In addition, calculation methods for shear bearing capacity and shear stiffness are derived, providing valuable references for the engineering application of this new type of socket connection.
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