Hat-Shaped Purlin-to-Rafter Screw Connections: Pull-Through Behavior and Design Implications

Abstract

This study investigates the pull-through failure behavior of cold-formed steel (CFS) hat-shaped purlin-to-rafter screw connections. A total of 27 tests were conducted to analyze the effects of cross-section parameters such as top flange width, bottom flange width, and section thickness, along with diameter of the screw head. The experimental study employed three small-scale test setups: two-span purlin (Test setup A), cantilever purlin (Test setup B), and short-span purlin (Test setup C) to simulate wind uplift forces. Three-dimensional digital image correlation (3D-DIC) was used to determine the strain contours and verify the failure mechanisms. Additionally, an imperfection study was conducted to assess the distribution of load pattern to the adjacent screws of the hat-shaped purlin. Three different types of failure were identified: tearing-type, rupture-type, and bearing-type pull-through failure modes, which varied based on the thickness and width of the bottom flange of the hat sections. The study found existing design guidelines to be unconservative and proposed a modified design equation to better estimate the pull-through capacity of these connections. A reliability analysis was also conducted to determine appropriate resistance and safety factors for the proposed design equations. Cold-formed steel (CFS) residential construction is gaining popularity around the globe due to its high strength-to-weight ratio, versatility, and ease of construction. However, one of the primary objectives during design of CFS building is to protect the roof connection which are particularly vulnerable to cyclonic events. There exists a gap in the current design standards and existing literature for moderate-strength thicker (thickness, t>1.15 mm) hat-shaped purlin-to-rafter screw connections. This study addresses this critical issue by experimental testing equipped with 3D-DIC. The study led to the development of a more accurate and reliable design equation that considers section thickness (t) and diameter of the screw head (dw). Implementing these findings can enable construction of more resilient roofing systems that can better withstand high winds, thereby reducing the risk of progressive roof collapse. Integrating this new equation into building codes can significantly enhance the safety and reliability of CFS roofing systems, particularly in cyclone-prone regions areas such as India, Japan, and the coastal regions of the United States.