Investigations on stainless steel T- and Y-joints in cold-rolled circular hollow sections

https://doi.org/10.1016/j.jcsr.2020.106462Get rights and content

Highlights

  • Nine stainless steel T- and Y- joints in CHSs were reported.

  • Totally, 192 finite element models were analyzed.

  • Modifications were made on the CIDECT equations to get better predictions.

Abstract

With the wide applications of stainless steel structures, stainless steel joints in cold-rolled tubes are increasingly used in building structures. However, the behaviors and the design methods of tubular joints were rarely studied. This study conducted experimental and numerical investigations on austenitic stainless steel T- and Y-joints in cold-rolled circular hollow sections with axial compression in brace. A total of six T-joints and three Y-joints were tested. The effects of the geometric configurations and the preload in the chord on the behaviors of the joint were considered. The material properties, failure modes, joint strengths and load-deformation curves were all reported. Test results showed that the failure mode of T-joints was the plastification failure of the chord on the chord face as well as the chord side wall. The failure mode of Y-joints was similar to that of T-joints while the deformation mainly occurred on the side of chord face with an obtuse angle to brace. Comparisons of the predictions using the CIDECT recommendations and the tests results indicated that the predictions were conservative. Finite element models were developed and validated using the test results. A total of 192 finite element models were analyzed to evaluate the effect of geometric properties and the chord preload on the joint strength. The functions of Qu and Qf in CIDECT for the chord plastification failure were modified based on the parametric analysis results. Comparisons of the predictions and the test results show that the proposed modified equations had a good accuracy.

Introduction

With the advantages of the high corrosion resistance, good ductility properties and low maintenance requirements, stainless steel structures have a wide application prospect in civil and industrial structures. Researches have been widely conducted on the material properties and the mechanical behaviors of the stainless steel structural members, while the researches on the tubular joints are still in a primary stage. Circular tubular joints have the advantages of simple form, convenient construction and low cost, which are suitable for spatial structures. Thus, it is necessary to carry out the investigations on the stainless steel tubular joints.

Researches on low-carbon steel tubular joints have been fully developed since 1980s. Investigations on T- and Y-joints in tubular sections were conducted to obtain the failure modes and the ultimate strengths under various types of loads [[1], [2], [3]]. The further numerical studies were performed and observed in good agreement with the test results [[4], [5], [6], [7]]. And the researches on the design formulas of carbon steel tubular joints were proposed and developed [[8], [9], [10], [11]]. However, the literature about the mechanical performances of stainless steel tubular joint were relatively few. Experimental investigations on the stainless steel tubular X- and K- joints in SHS and in CHS were carried out by Rasmussen and Young [12], and Rasmussen and Hasham [13], respectively. And the test results showed that the 0.2% proof stress could be used to replace the yield stress of carbon steel in calculating the joint strength according to the design equations in CIDECT [14]. Comprehensive investigations on cold-formed high strength and normal strength stainless steel tubular T- and X-joints fabricated from SHS and RHS were carried out by Feng and Young [[15], [16], [17], [18]]. The experimental and numerical results indicated that it was reasonable to use the 0.2% proof stress in the design formulas for low-carbon steel joints to predict the ultimate and serviceability strengths of stainless steel joints. Feng et al. [19,20] also conducted tests on the hybrid stainless steel tubular T-, Y- and X-joints including the CHS brace to SHS chord and SHS brace to CHS chord. The effect of geometric properties on the joint strength and failure mode was significant and the current design rules commonly produced conservative predictions. Some researches on grouted stainless steel SHS and CHS tubular X- and T-joints were also reported recently [21,22]. The grouting generally improved the joint strength except for the tubular X-joints with grouted chord member only. In addition, Lan et al. [23] and Feng et al. [24] performed numerical investigations on stainless steel tubular T-joints and X-joints in SHS, RHS and CHS at elevated temperatures and the related design equations of joint strength were proposed. Based on the above analysis, it is obvious that investigations on stainless steel tubular joints mainly focused on the rectangular and square hollow sections while there were few researches about the behaviors of stainless steel joints in circular hollow sections (CHSs). According to the author's knowledge, no test has been conducted on the T- and Y- joints in CHSs with the compressive preload in the chord.

The objective of this paper is to study the behaviors of the stainless steel tubular T- and Y-joints in CHS. A test program was carried out to obtain the material properties, failure modes, and joint strengths. Finite element models were built and validated based on the test results. Furthermore, parametric studies were carried out to study the effect of geometric function Qu and chord stress function Qf on the joint strength. By modifying the expressions of Qu and Qf, design formula were developed.

Section snippets

Test specimens

A total of nine specimens including six T-joints and three Y-joints in CHSs were prepared. The chord and brace members were cold-rolled from annealed austenitic stainless steel sheets. The brace members were welded to the center of the chord members at different included angles. The included angles θ between the brace and chord member were 90° for T-joint specimen and 45° for Y-joint specimen, respectively. The fillet welds with the nominal leg length of 3 mm were laid using the manual Tungsten

Failure modes

The failure mode of all the T- and Y-joint specimens was the chord plastification. At the initial stage of loading, the joint had a good overall stiffness and bent downward together under the brace axial compression. As the load increased gradually, the chord face indentation began to appear in the joint zone accompanied by the chord side wall buckling outward. And then, the overall deflection at the mid-length of the chord increased rapidly due to the bending moment and the local buckling.

Finite element modeling

ANSYS were used to develop the numerical models in this study. The geometric model was directly built in the software. And the measured dimensions were used to generate the geometric model. The chord length of T-joint and Y-joint was taken as 10d0 while the brace axis length was 5d1, which included the length between the end of member and the corresponding hinge. The material model proposed by Gardner et al. [27] was adopted to generate stress-strain data points inputted in the software. The

General

After validated the numerical models, a parametric study was carried out to investigate the effect of the geometric parameters and the chord preload on the strength of stainless steel T- and Y-joints. Based on the design equations for the chord plastification failure in the CIDECT shown in Eq. (1), the influence factors on joint strength were separated into two aspects, expressed by the Qu and Qf functions, respectively. The parameter Qu accounts for the influence of the geometric properties β

Proposed design equations

In this section, the current design equations of T- and Y-joint in CIDECT for the chord plastification failure would be modified based on the results of the parametric studies.

Conclusions

Experimental and numerical studies of austenitic stainless steel T- and Y-joints in CHSs with axial compression in brace were reported. The following conclusions can be drawn:

  • (1)

    Totally nine stainless steel T- and Y-joints were tested. The failure mode of stainless steel T- and Y-joints in CHSs with axial compression to brace members was the plastification of the chord face. No failure of weld occurred for all the test specimens.

  • (2)

    Joint strength was mainly governed by the deformation of the joint

Declaration of Competing Interest

The authors declared that they have no conflicts of interest to this work.

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

The research work described in this paper is supported by National Science Foundation of China through the projects No. 51808110, Jiangsu Science Foundation through the projects No. BK20180399 and National Key Technologies R&D Program through the projects No. 2018YFC0705502-4. Special thanks should be given to the Jiangsu Dongge Stainless Steel Ware Co., Ltd. for the supplying of the test specimens.

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