Flexural Buckling of Steel Cold-Formed Hollow Profiles in the Framework of Eurocodes

The use of cold-formed hollow structural (CFHS) steel has been growing in the past decade due to several advantages such as superior behavior towards lateral-torsional buckling, aesthetic structures, and feasibility of using internal volume to increase load-carrying capacity. The cold-forming process can change drastically the shape of the stress-strain curve and strength parameters of the base material. There are several investigations on buckling tests of hollow section columns; however, studies on cold-formed hollow sections are still lacking. This shortcoming of experimental data becomes more pronounced when the corner behavior of CFHS is under consideration. Only a limited number of corner coupon tests can be found in international literature. This Ph.D. thesis is developed in line with the progress of the European project INNOvative 3D JOINTS for Robust and Economic Hybrid Tubular Construction (INNO3DJOINTS). The primary objective is to advance, through analytical and experimental research, knowledge on the flexural buckling behavior of CFHS columns. An extensive experimental program ( 21 flexural buckling tests) on SHS and RHS columns has been carried out varying the steel grade (i.e. S275 and S355) and the overall slenderness ratio. This database serves as the basis for the assessment and improvement of the flexural buckling curve for CFHS. The stress-strain behavior of the sections was investigated by performing tensile coupon tests (81 tests) from both flat and corner areas. special effort was made to obtain the static stress-strain data by pausing the test for 60 seconds during the test. We also employed an innovative method to perform the coupon test on the corner area of CFHS. This procedure will reduce the secondary effect results from methods such as welding a plate or flattening the end grip, and therefore reduce the bias in the results of the corner area. A comprehensive discussion on the definition of safety and the adopted safety levels in EN1990 and EN1993-1-1 has been presented. The results show some potential criticisms of the application of current rules for plastic design and analysis of such a column and a further investigation is required on the matter. In addition, it is shown that the current material test procedures in BSI and ATM standards may lead to misevaluate the static strengths of material which is required for the design of structures