Experimental Research on Thread-Fixed One-Side Bolted Connection Between T-stub and Steel Tube Subjected to Tension

The closed section of steel frame column has the advantages of high bearing efficiency and large torsional modulus. The problem that steel beam and Hollow Square Column(HSC) cannot be directly connected by ordinary high-strength bolts can be solved by using thread-fixed one-side bolt(TOB). TOB is to directly tighten the high-strength bolt on the threaded hole of the column wall by machining the threaded bolt hole on the column wall, instead of the traditional nut, so as to realize the installation and tightening on the outside of the steel tube column. Based on the study of the thread-fixed one-side bolted double T-joint model, the stress mechanism and failure mode of thread-fixed one-side bolted T-stub with steel tube are further studied. The tensile tests of 10 thread-fixed one-side bolted T-stub with steel tube joints were carried out. The failure modes, bearing capacity mechanisms, displacement-load curves, yield capacity and ultimate bearing capacity of different joints were analyzed and compared.
It is found that there are four kinds of failure modes, which are thread shear failure, tube wall local yield with thread shear failure, bolt rod tensile failure and tube wall yield failure. When the thickness of the steel tube wall is small, the thread shear failure in the bolt hole occurs. When the tube wall is thick and the bolt diameter is relatively small, the bolt rod is broken and the internal thread of the threaded hole remains intact, indicating that when the internal thread length of the bolt hole is sufficient, the internal thread of the bolt hole of the steel tube wall has sufficient bearing capacity and the thread anchoring method is feasible.
At the same time, the effects of bolt spacing, bolt diameter and tube wall thickness on the bearing capacity of the joint are compared and analyzed. The test results show that the bearing capacity of the joint can be improved by increasing the bolt diameter, tube wall thickness and bolt spacing. The influence of bolt spacing on the bearing capacity of the joint is related to the shape of the yield line of the steel tube wall under the bolt tension. When the bolt spacing is small, the yield lines of tensile bolts on the steel tube wall are superimposed. With the increase of bolt spacing, the bearing capacity of the joint is significantly improved. However, when the bolt spacing is large, the yield line of each tensile bolt to the steel tube wall is oblong. With the increase of bolt spacing, the bearing capacity of the joint will remain unchanged. With the increase of the bolt diameter, the failure mode of the joint gradually changes from the bolt rod tensile failure to the thread shear failure or tube wall yield failure, and the bearing capacity of the joint increases. The increase of tube wall thickness significantly increases the yield capacity of steel tube wall, and the growth rate is approximately square with the growth rate of wall thickness.
In order to avoid the problem of insufficient anchoring force in the bolt hole of the steel tube wall when the steel tube wall is thin, the influence of the steel tube with a backing plate on the mechanical performance of the thread-fixed one-side bolted T-stub with steel tube joint is further studied. The test results show that the anchor length of the thread is increased, the bearing capacity of the strengthened joint is significantly improved, and the shear failure of the thread is effectively avoided, but the initial stiffness changes little.