Fatigue Behavior of Integral Built-Up Box Y-Joints between Concrete-Filled Chords with Perfobond Ribs and Hollow Braces

A novel integral built-up box Y-joint welded to rectangular hollow section braces and concrete-filled square hollow section chord stiffened by perfobond ribs was proposed and studied in fatigue. Eight specimens, including one control with a hollow chord, were tested under axial loadings in the brace. Static tests initially were conducted to estimate the hot-spot stresses in the brace–chord weld intersection and at the arc transition point of the gusset plate. Stress concentration factors (SCFs) were compared with those of the hollow chord specimen to assess the influence of concrete fill and perfobond ribs. Fatigue tests then were carried out to failure under cyclic tension force in the brace. Crack initiation locations, crack propagations, failure modes, and the corresponding fatigue data were recorded. Using test results, the design Sr,hot-Nf curve for the proposed joint based on hot-spot stress method was established, adopting the least-squares method considering a 95th percentile. The Sr,hot-Nf curves of the proposed joint and conventional welded joints without gusset plate were compared for both square and circular braces connected to square chords. Additionally, the Sr,hot-Nf curves were compared with those in available design guides. It was shown for the proposed joint that SCFs are significantly lower than for joints with hollow chords and its fatigue strength is much higher than other conventional joints. One design guide Sr,hot-Nf curve was shown to overestimate the fatigue strength of the proposed joint; however, curves from other design guides underestimated its fatigue strength.