Test and design of stainless steel K-joints in cold-formed circular hollow sections

Highlights

• Nine stainless steel K-joints in cold-formed CHS were tested.

• Two key factors Q_u and Q_f in the design rules in CIDECT were modified.

• A new factor was proposed to consider the variations in chord material property.

Abstract

Stainless steel structures have obvious competitive advantages for buildings in the corrosive environment. In recent years, many studies have been conducted on the behaviors of stainless steel materials and members, but the behaviors of joint were rarely reported. This study carried out nine tests on stainless steel K-joints in cold-formed circular hollow sections. The material properties of circular hollow sections, the ultimate loads, the failure modes, and the load-deflection curves of joint were all reported. All the joints failed in the chord plastification. Finite element models were developed for K-joints, and were validated using the test data. Parametric studies were then conducted. Totally, 720 finite element models were built, and the effects of the geometric configuration, the chord preload, and the chord material properties on the joint capacity were explored. Based on the results of the parametric studies, the parameter Q_u accounting for the influence of geometric configurations (i.e. β, γ, and g) and the parameter Q_f accounting for the influence of chord preload were modified based on the corresponding formulae in the CIDECT design guide for low-carbon steel K-joints. To further improve the accuracy of prediction, a new parameter Q_y400 was proposed to consider the effect of the chord material property variations. The predictions of the proposed were compared with test data collected in literature. It is concluded that the proposed design formulae have good performances in predicting the capacity of stainless steel K-joint in circular hollow section.

Introduction

In the corrosive environment, steel structures may suffer from the corrosion issue which leads to continuous reduction in the durability and even causes collapse in the end. Compared with low-carbon steel, stainless steel has an excellent corrosion resistance, good mechanical properties, and architectural renderings etc. Thus, it has become an ideal construction material for structures built in the corrosive environment and structures with the long service life.

Tubular joint has several advantages, e.g. the attractive appearance and the convenience in fabrication etc., and is one of the best choices for spatial structure joints. For the low carbon steel and the high-strength steel structures, a lot of researches have been carried out on tubular joints in circular hollow sections (CHS) and square hollow sections (SHS) [[1], [2], [3], [4], [5]], and corresponding design methods [6,7] have been established and used for many years. Although stainless steel structures have drawn more and more interests in recent twenty years, very limited research has been reported on stainless steel joints in literature.

Rasmussen et al. [8,9] conducted tests on X- and K-joints in circular and rectangular hollow sections. Design formulae in CIDECT design guide (first edition) [6] were modified by replacing the yield stress of low-carbon steel with the nominal yield stress of stainless steel. Feng et al. [[10], [11], [12]] carried out in-depth studies on stainless steel T- and X-joints in cold-formed rectangular hollow sections (RHS). Test results were compared with the predictions of Eurocode [13], Australia/New Zealand codes [14] and CIDECT design guide [7], and modifications were proposed to adapt to the stainless steel joint characters. Feng et al. [15] developed yield line mechanisms for the SHS T- and X-joints subjected to axial compression. Zhou et al. [16] studied the flexural behaviour of circular concrete filled stainless steel tubular trusses, and indicated that the concrete filled in both the top and bottom chords could significantly improve the rigidity and the load capacity of truss. Feng and Chen et al. [[17], [18], [19]] conducted a series tests on the concrete-filled stainless steel SHS X- and T-joints, A full set of design formulae of ultimate strengths for empty and grouted stainless steel joints were developed considering both the contributions of the stainless steel tube and the concrete. Feng et al. [20,21] carried out tests on empty and grouted stainless steel tubular X-, T- and Y-joints with CHS chord under axial compression, and proposed design formulae for hybrid tubular grouted joints by introducing correction factors to modified the design rules of traditional CHS tubular joints. Feng et al. performed tests on stainless steel CHS-to-SHS hybrid tubular X-, T-, and Y-joints [22,23] and K-joints [24], and modified the design rules in CIDECT and EC3 by introducing several correction factors to consider the effect of geometric parameters. Besides studies on the stainless steel tubular joints, Elflah et al. [25,26], Hasen et al. [27,28], Bu et al. [29], Gao et al. [30], Tao et al. [31] carried out test on several other types of fabricated stainless steel beam-columns joints.

Over all, the researches on the stainless steel tubular joints in CHS were only reported in Refs [8,16,21]., and more test should be conducted and relevant design rules need to be improved to adapt to the characters of stainless steel. This study will focus on the behaviors of the stainless steel K-joints in cold-formed circular hollow sections. Tests were conducted to get the joint capacity and the failure mode. Then, finite element models were developed, validated, and used to expand the valid ranges of parameters. Finally, modifications were proposed and the predictions were compared with the test results available in literature.