Static strength of cold-formed high strength steel tubular T- and X-joints at ambient and post-fire conditions

Abstract

Recent advancements in cold-forming technology have allowed the production of steels with various shapes and sizes. Cold-formed tubular members are becoming increasingly popular as structural elements in a variety of domains like in building constructions, transportation industry, highway equipment, agricultural equipment, recreational structures and many other applications. The design of tubular joints was mostly investigated under the directions of International Institute of Welding (IIW) and Comité International pour le Développement et l´Etude de la Construction Tubulaire (CIDECT). The recommendations proposed by these bodies are adopted by many international standards around the world. The design rules for tubular joints in the first editions of guidelines published by these organisations were originally developed for steels with yield strength up to 355 MPa. However, in their latest editions, the design rules are now extended up to the yield strength of 460 MPa. The rapid popularity and associated benefits of high strength steel have slanted the interests of construction sector towards the use of high strength steel materials with nominal yield strength greater than 460 MPa. However, the use of high strength steel in the construction field is very scant due to lack of investigation and adequate design provisions. Therefore, through experimental investigation, an attempt has been made in this study to understand the static strength of cold-formed high strength steel tubular T- and X-joints at ambient and post-fire conditions.

In this study, square, rectangular and circular hollow sections were used in the fabrication of tubular joints. These tubular sections were obtained by cold-forming of high strength carbon steel flat strips with nominal yield strengths of 900 and 960 MPa with subsequent high-frequency induction welding. The first part of the test programme consists of 36 and 80 material property investigation at ambient and different post-fire conditions, respectively. While, the second part comprised testing of 22 floor joist joints, 24 T-joints, 34 X-joints, 26 Non-90˚ X-Joints, 16 post-fire T-joints and 16 post-fire X-joints under brace axial compression. Thus, 116 material property and 138 tubular joint tests are covered in this study. Two configurations of tubular joints were fabricated, firstly, where both brace and chord members were made up of square and rectangular hollow sections, and secondly, where circular hollow section brace(s) were attached to square and rectangular hollow section chord member. The brace and chord members were joined together by semi-automatic and automatic gas metal arc welding processes.

The key findings including the static joint capacities, failure modes and load-deformation behaviour, were reported and discussed. The appropriateness of the existing design provisions of European and CIDECT specifications for tubular T- and X-joints were assessed using the test results. The comparison showed that the existing design provisions are not capable to provide accurate and reliable predictions for cold-formed high strength steel tubular joints made up of S900 and S960 steel grades. Therefore, modified design rules are proposed in this study. In order to evaluate the reliability of existing and proposed design rules, a reliability analysis was performed.