Hot rolled sections such as I- and H-beams are commonly used as structural elements in offshore platforms. They exhibit a superior fatigue life compared to welded sections due to smooth corners and the absence of welds between the flange and web plates. Hence, when loaded in fatigue, the butt welds joining different sections are the hotspots in the design.

Due to the size of the rolling installations, the maximum height for such sections is limited. To overcome these limitations, a hybrid welded beam concept is studied. This concept consists of 2 hot rolled T-sections with a plate welded in between to form an H-beam with increased section height. The advantage of this approach is that it maintains the smooth corners of the hot rolled sections and places the longitudinal welds closer to the neutral axis of the beam where the stress levels in bending are reduced compared to the flange-web interface. Hence, the butt weld between different beam sections remains the hotspot instead of the longitudinal weld.

In the present work the fatigue design of such hybrid welded beams is studied. As part of this research, an efficient experimental assessment methodology is proposed. Fatigue tests are performed on a large-scale resonant bending test setup allowing a testing frequency of around 30 Hz. Tests are performed on HISTAR HE girders with a section height of 696 mm and 524 mm. As a reference, fatigue tests are performed on hot rolled HE beams joined by butt welding. Hybrid welded beams will be tested in the same setup at a later stage. During the tests, fatigue crack initiation is detected using an acoustic emission system. Further crack propagation is monitored through local strain measurements and by intermediate inspections during the tests. In addition, the beach marking method is applied, allowing to investigate the evolution of the crack front by post-mortem analysis.

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