Study On The Tensile Behavior Of Thread-fixed One-side Bolted Connection At Fire And Post-fire Circumstances

Thread-fixed one-side bolts are the bolts that are anchored by the threads inside bolt hole instead of a nut.The Thread-fixed one-side bolt can be installed from one side only and offer a new way of installing connections with closed section component such as square hollow section columns.The simple structure,low cost and ease of installation make the thread-fixed one-side bolt ideal for the on-site installation of assembled steel structures.Fire is one of the main threats to the steel structures during their service life,but there is still lack of enough attention paid on the research of fire performance of thread-fixed one-sided bolted connections,nor on the residual mechanical behavior of such connection after fire.And there are also no relevant design codes for reference.In order to investigate the change of the load-bearing performance of thread-fixed one-side bolted connections under high temperature caused by fire and to reasonably assess the residual load-bearing capacity after high temperature,and to provide reference for the practical application of such bolted connections,the mechanical properties of thread-fixed one-side bolted connections under and after fire were investigated through various methods including experiments,finite element simulations and theoretical derivations.The main research contents and results are summarized as follows:(1)Tests containing 20 connections were carried out to investigate the performance of the thread-fixed one-side bolted connection at elevated temperature connection.The basic mechanical properties of the one-side bolted connections at room temperature,including failure modes and load carrying capacity,were firstly obtained through ambient temperature tests.On the basis of the ambient temperature tests,the high temperature test was carried out by both steady-state test method and transient-state test method,and the influence of the key parameters such as bolt diameter,column wall thickness,fire temperature and load ratio on the performance and failure mode of the connection was studied.(2)A novel strengthening method of installing H-shaped steel inside the column was proposed to reduce the out-of-plane deformation of the column wall and to enhance the anchorage provided by the threaded holes on the bolts.High-temperature tests containing 10 components were carried out on the one-side bolted connection reinforced by the internal Hshaped steel by both steady-state and transient-state high temperature test methods,and tests on standard bolted connection containing 10 components were also carried out for comparison.The effect of the internal H-shaped steel on the failure mode of the one-side bolted connections and the enhancement of the fire resistance are analyzed.The results of the standard bolted connections are used to calibrate the mechanical behavior of one-side bolted connections and H-shaped steel strengthened one-side bolted connections at elevated temperatures,and to analyze the advantages and disadvantages of the fire resistance of the two types of bolted connections and to verify the reliability of the one-side bolted connections at high temperature.(3)Finite element models of one-side bolt connections and strengthened one-side bolt connections are established,and the effects of the key parameters on the fire performance of the connection such as the bolt preload,the material properties of the steel,the geometric properties of the connection and the internal H-shaped steel,the fire temperature are analyzed.The degradation law of stiffness and load carrying capacity of one-side bolted connections and strengthened one-side bolted connections at high temperatures are analyzed and discussed in depth,and the damage mechanisms of connections with different failure modes are revealed.Based on the results of the tests and finite element analysis,the prediction method of the failure modes and load carrying capacity of the one-side bolted connections and one-side bolted connections strengthened with internal H-shaped steel were proposed,taking into account the material deterioration and large deformation of the connections at high temperatures.And the accuracy of the proposed prediction method was verified.(4)Tests containing 20 components on the residual mechanical behavior of one-side bolted connections after exposed to high temperature,and tests containing 10 components on the residual mechanical behavior of internal H-shaped steel strengthened one-side bolted connections,as well as the comparison test on standard bolted connections containing 10 members,were carried out.In the tests,the full process test method was used to consider the influence of the load-bearing state of the connections during the fire on their post-fire residual mechanical behavior.The influence of the parameters such as the fire temperature and the load ratio on the post-fire residual mechanical behavior of one-side bolt connections including the failure mode and residual resistance were analyzed.The residual resistance of the one-side bolted connections after fire is compared with the residual mechanical behavior of one-side bolted connections is also compared with those of the standard bolted connections to provide a reference for the reuse of such one-side bolt connections after fire.