Fatigue Tests and Design of Welded T Connections in Thin Cold-Formed Square Hollow Sections Under In-Plane Bending

There is an increased use of thin-walled $\left(t<4\mathrm{mm}\right)$ hollow sections in the manufacture of lighting poles, traffic sign supports, swing ploughs, linkage graders, trailers, and haymakers. These structures are subjected to fatigue loading. A review of current fatigue design guidelines showed that there is a lack of design rules for nodal joints made up of thin-walled $\left(t<4\mathrm{mm}\right)$ hollow sections. This paper describes the tests carried out on welded thin-walled $\left(t<4\mathrm{mm}\right)$ tube-to-tube T connections made up of square hollow sections under cyclic in-plane bending. Different failure modes were obtained during fatigue testing. The experimental stress concentration factors determined in this investigation were found to be significantly lower than the stress concentration factors from the existing parametric equations from the International Institute of Welding. The design $S_{r\text{.}\mathrm{hs}}-N$ curves have been determined for the hot spot stress method from $S_{r\text{.}\mathrm{hs}}-N$ data based on both experimental and parametric equation stress concentration factors. Design $S_{r\text{.}\mathrm{hs}}-N$ curves have been derived from the fatigue data using the least-squares method. The design $S_{r\text{.}\mathrm{hs}}-N$ curves derived from the experimental fatigue data are lower than the expected design curve resulting from the current trend for design $S_{r\text{.}\mathrm{hs}}-N$ curves for different thicknesses from the International Institute of Welding and the International Committee for the Development and Study of Tubular Construction. Three design approaches are proposed for welded T connections in thin cold-formed square hollow sections under cyclic in-plane bending.