Fatigue performance and S-N classification of innovative joints in bracket-crane truss structures

Modern steel bridges and industrial buildings increasingly contain welded structural details that are not explicitly covered by existing fatigue design provisions; consequently, their S-N classification remains uncertain. This study experimentally investigates the high-cycle fatigue behavior and fatigue classification of newly developed web-member-to-lower-chord welded node details in bracket-crane truss structures. Four web-member-to-node transition configurations were considered, namely a circular-arc transition, 20 ° and 60 ° tapered transitions, and a direct connection without a transition segment. To improve testing efficiency, series-type specimens were adopted, in which two nominally identical details were tested under a single actuator. 32 uniaxial strain gauges were installed on each specimen to characterize local stress gradients during static preloading and to monitor strain evolution during fatigue loading. The fatigue tests showed that all failed valid specimens exhibited the same governing failure mode: cracks initiated at the fillet-weld toe adjacent to the weld-access hole and subsequently propagated transversely through the flange plate. Static strain measurements indicated that the transition geometry affected the local stress concentration at the outer-flange transition region; however, it did not alter the weld-access-hole-controlled fatigue failure mechanism. Fixed-slope S-N fitting with m = 3 showed that Detail 1 and Detail 2 exhibited similar fatigue resistance. The characteristic fatigue strength corresponding to the 97.5% survival-probability S-N curve was 79.65 MPa at 2 × 10 6 cycles, supporting classification as FAT 80. Detail 3 and Detail 4 successfully survived 2 × 10 6 cycles at 60 MPa, and their equivalent fatigue lives under two-stage loading generally fell within the scatter band defined by Detail 1 and Detail 2. Based on the common failure mechanism, fixed-slope statistical fitting results, and engineering applicability, FAT 80, corresponding to Chinese fatigue class Z7, is recommended for the investigated family of web-member-to-lower-chord welded node details under the tested material, geometry, welding procedure, and loading conditions.