Experimental study on T-stubs connected by thread-fixed one-side bolts under cyclic load

Highlights

• T-stubs connected using thread-fixed one-side bolts (TFOSBs) were tested under cyclic load.

• Tension yield strength and failure mode under difference load types were presented.

• Strength and stiffness were reduced under cyclic load compared to that under monotonic load.

• Failure mode could be accurately predicted by the current design methods.

• Tension yield strength predicted by current design methods were on safety side.

Abstract

Experimental studies on behavior of T-stubs connected using thread-fixed one-side bolts (TFOSBs) under monotonic and cyclic loads were presented. Three failure modes, which were the complete flange yielding failure, the bolt failure, and the flange yielding accompanied by hole thread failure, were observed in both tests. Effects of the cyclic load on the behavior of T-stubs were discussed from aspects of the deformation, the load transfer mechanism, the stiffness degradation and the tension yield strength. Compared to the monotonic tests, the cyclic load caused stiffness degradation and made threads on the bolt hole wall become more vulnerable to fail. In addition, comparisons showed that available design methods applied to predict the behavior of T-stubs under monotonic load also were applicable to predict that under cyclic load.

Introduction

Many types of one-side bolts for steel beam to tubular column bolted end-plate connections have been developed and tested. Korol et al. [1] conducted tests of extended end-plate connections for square hollow section columns using Hunk blind bolts. An effective analytical model for such connections was proposed to simulate the moment-rotation relationship and evaluate the initial stiffness. It was founded that this joint had perfect plastic rotation capacity. Lee et al. [[2], [3], [4]] studied the behaviour of Ajax Engineered Fasteners bolted T-stub connections to unfilled hollow section columns under static loading. It concluded that the connection bolted using such one-side bolts was classified as a semi-rigid joint. France et al. [5] investigated the joints connected by one-side bolts fabricated through the flow-drill technology. Both the good plastic capacity of the supported beam and the large rotational capacity of the joint were obtained. In addition, Hollo-bolts that fixed through deformed sleeve, as demonstrated in Fig. 1, were also widely applied in the steel beam to tubular column bolted end-plate connections [6]. Elghazouli et al. [7] carried out a series of tests on blind-bolted angle connections between open beams and tubular columns. Simplified approaches to estimate the initial stiffness and yield parameters were proposed and assessed. Test results showed that the extension induced within the blind bolts that is directly related to prying action and component separation, as well as the bending deformations within the column face, should be limited in design in order to satisfy serviceability requirements. These studies on different types of one-side bolts applied to the beam-to-column connections showed that the one-side bolts would have great prospect in practical engineering.

The thread-fixed one-side bolt (TFOSB) was fixed through threads on the hole wall, as shown in Fig. 2.

Simplified models greatly facilitated the study of bolted end-plate connections. Based on the component analytical approaches [8], the equivalent bolted T-stub recommended by Zoetemeijer [9] was a feasible method to describe the performance of the region of bolted connections under tension, which was also given in Eurocode 3 [10]. Agerskov [11] proposed an effective method that could be used to determine the bolt force in bolted T-stub and end-plate connections, although it would be affected by the prying action. The difference between test results and those by formulas given in the current AISC Manual of Steel Construction [12] was considerable. Yee et al. [13] and Shi et al. [14] proposed a mathematical model to predict the moment-rotation relationship of bolted extended end-plate connections. Bolted T-stubs model would be used here to study the behavior of endplate connections using TFOSBs.

Liu et al. [15] experimentally studied the tension yield resistance of T-stubs connected by TFOSBs under monotonic load and proposed a series of design methods. Through changing the flange thickness of the screwed T-stub plate, two new failure modes caused by the internal hole thread failure occurred in the tests. Zhu et al. [16] investigated the T-stubs fixed by threaded holes with backing plate and concluded that the stiffened plate could improve the tension resistance.

Wulan et al. [17] compared tension yield strengths of different failure modes of a bolted T-stubs under monotonic load. For the standard design of T-stubs using high strength bolts with nuts, three failure modes, which were complete flange yielding failure, flange yielding accompanied by bolt failure and bolt failure, would occur. For the bolted T-stubs using TFOSBs, two new failure modes related to threads failure, which were flange yielding accompanied by thread failure and thread failure, would occur. Tension yield strengths of different failure modes were shown in Fig. 3. It could be seen that the tension yield strengths of those two new failure modes, which related to thread failure, were not the minimum value. It indicated that there would be no reduction in tension yield strength of bolted T-stubs using TFOSBs compared with those bolted T-stubs using the traditional bolts. Whether there was strength reduction for the T-stubs bolted by TFOSBs under cyclic load should be studied further.

For the behavior of bolted connections under cyclic load, research results demonstrated that the failure of connections could be classified into two aspects in terms of the failure position, which were the plate failure and the bolt failure. Wang et al. [18] assessed the response of the connection mainly controlled by the hollow section column under cyclic load through tests. It was concluded that the cyclic load had influenced on all three failure modes occurred in the tests. Piluso et al. [19] analyzed the degradation law of the stiffness and strength and brought out a proposal to predict the cyclic behavior of T-stub connection bolted by traditional bolts through experiments and modeling. Test results indicated that the performance of bolted connections under cyclic load was mainly determined by the sensitivity of flange to cyclic load as the flange yielding failure occurred.

In addition, the bolted connections using one-side bolts under cyclic tests also were investigated. Pokharel et al. [20] carried out tests on the pull-out of single double-headed anchored blind bolts with various tube sizes, bolt sizes, and concrete types. The stiffness deterioration under cyclic load was small until the load applied on the bolt reached 60% of the bolt nominal ultimate strength. Liu et al. [21] compared the failure of connections bolted by traditional bolts and blind bolts through tests. Meanwhile, test results also proved that the blind bolt was influenced by the amplitude and peak of the cyclic load. Wang et al. [22] reported the mechanical behavior of the individual blind bolt under tension and the performance of beam-to-column connections bolted by the blind bolts under cyclic load through tests, which generally showed that a properly designed connection could exhibit very satisfactory cyclic performance and demonstrated a great potential of such connection for seismic resistance applications. Moreover, D'Aniello et al. [23] carried out monotonic and both variable and constant amplitude cyclic inelastic tests on bolted connection. Test and analytical results indicated that the rational selection of the most suitable bolt type may avoid premature reduction of joint strength.

This paper presented an experimental investigation on the bolted T-stubs using TFOSBs under monotonic and cyclic load. Both the load-displacement curves and failure modes of tested specimens under different loading regimes were presented. The load transfer mechanism and the failure principle of internal threads tapped on the wall of bolt holes were also investigated. In addition, the effect of cyclic load on the degradation of stiffness, the tension yield strength and the deformation were analyzed. The failure mode and the tension yield strength calculated by available design equations were compared with those obtained from tests.