Study on Tensile Performance of Square Tubular Column-Column Joints of Modular Steel Structure with Bidirectional Bolt Connection

Modular steel structure building is a new type of prefabricated building, which has a high level of industrialization and has been vigorously developed in China. The expansion project of Jiaguang Middle School in Rongcheng County of Xiong’an New Area adopts modular steel structure laminated box system. In order to facilitate the connection and installation between the modules of the upper and lower unit rooms, a new type of unit room socket connection column-column joint is proposed. The joint realizes the connection between the upper and lower columns through bolts and sleeves. The vertical connection between the units should not be separated vertically under wind load and frequent earthquakes. Tensile tests were carried out on three full-scale joint specimens to analyze their failure modes, bolt shear distribution and tensile bearing capacity. The influence of grouting and the number of bolts on the bearing capacity of the joints was discussed. Based on the calculation method of the design value of the shear capacity of high-strength bolts in the existing literature, the design formula of the tensile strength of the new joint is proposed. A reliable numerical model was established, and the number of bolts and the diameter of bolts were analyzed. The parametric analysis results were compared with the formula calculation results.
The results show that under the axial load, the shear failure of high-strength bolt group occurs in all specimens. At the same time, the inner sleeve hole wall buckling occurs in the non-slurry joint specimens, and the local crushing phenomenon of grouting material occurs in the grouting joint specimens, accompanied by the bond failure between steel and grouting material interface. In the early stage of test loading, the shearing force of high-strength bolt group is large at the end and small at the center. In the later stage of loading, the shear force difference of high-strength bolt group decreases gradually. When the ultimate bearing capacity is reached, the shearing force of high-strength bolt group tends to be evenly distributed. The new joint relies on the shear resistance of high-strength bolts to bear the axial tensile load. The average shear bearing capacity of a single bolt is 76.9% higher than the calculated value of GB 50017-2017 Steel Structure Design Standard; After the grouting material is injected into the joint, the grouting material and the high-strength bolt work together, and the tensile bearing capacity is 14.1% higher than that of the non-slurry joint. The number of bolts increased from 3 to 5, and the tensile bearing capacity of the joint increased by 80.9% due to the increase of friction force and the decrease of shearing force of single bolt. With the increase of bolt diameter, the bolt pre-tightening force in the elastic stage increases, the contact area between the screw and the hole wall in the plastic stage increases, and the tensile bearing capacity of the joint increases. The error between the finite element parametric analysis results and the theoretical calculation results is controlled within 10%. The proposed joint tensile strength design formula accurately predicts the tensile bearing capacity of the new joint under axial tension, which provides a theoretical reference for practical engineering applications.