Two-side fastened trapezoidal sheet diaphragms: Investigation of the diaphragm-to-rafter connection

In current Eastern European construction practice, trapezoidal sheet roof cladding systems are predominantly fastened to the primary structure on two sides only. This paper presents a comprehensive experimental investigation of two-side fastened trapezoidal sheet roof diaphragms, with particular attention to the diaphragm-to-rafter connection. A total of 20 full-scale panel assemblies were tested under monotonic in-plane loading, and their behaviour was evaluated primarily from a serviceability limit state perspective, considering the diaphragm and its components as integral parts of the supporting structure. The results show that two-side fastened trapezoidal sheet diaphragms exhibit early non-linear behaviour, characterised by progressive degradation of panel stiffness. At the load level corresponding to 60% of the diaphragm shear capacity, the panel flexibility may increase by more than 230% compared with its initial value. Despite this progressive flexibility increase, the diaphragm still provides a substantial stiffening effect, reducing the flexibility by approximately 68–94% compared with the bare frame-purlin arrangement. Based on the experimental observations, a more restrictive serviceability displacement limit in the range of frame spacing/250 to frame spacing/300 is proposed for two-side fastened roof diaphragms with profiled metal sheets, intended to maintain elastic behaviour of the cladding components. Comparison of the diaphragm tests with the corresponding lap-joint component tests reveals that the loading conditions in the sheet-to-purlin fasteners within the diaphragm deviate from the pure shear conditions assumed in lap-joint testing. This behaviour results from the combined action of shear and tension arising from the deformation of the purlin web and outer flange.