Structural Behavior of Wood I-Joist/OSB Roof Panel Assemblies

The wood construction industry is constantly evolving to incorporate innovative technologies. In the early 20th century, light-frame construction surpassed post-and-beam construction in popularity because the installation process for light-frame construction was less complex which made possible the rapid production of structures. Today, panelized systems are challenging traditional “stick-built” methods of wood construction. Panel sections for walls, floors and roofs are prefabricated and shipped to the construction site. This prefabrication and modular construction is often preferred by designers and contractors, because it reduces on-site labor costs and facilitates better quality control. Structural panels (e.g. stressed-skin panels, structural insulated panels) go one step further by allowing designers to recognize the composite nature of the system and treat the entire panel section as a single structural member, thus increasing strength and stiffness design capacities, and removing the need to field-install installation. On the other hand, these products have several drawbacks when used in roof applications, such as reduced span lengths and complex structural connections. In order for structural panels to be competitive in the light-frame roof construction market, an advanced panelized system must first be developed. Wood I-joist roof panels were developed and tested at The University of Maine AEWC Advanced Structures & Composites Center. The prefabricated panels incorporate framing, sheathing, insulation and ventilation into a single product to be used in light-frame construction applications. Panels are manufactured in a controlled environment and shipped to the job site, reducing site assembly time and labor costs. Once on-site, the four-foot wide panels are placed by small crane and interlock on the roof by an overlap of sheathing and a single row of nails or screws along the panel length. The panels consist of wood I-joist framing members and oriented strand board (OSB) sheathing on top and bottom flanges. Short-term bend tests were conducted on twenty panels with lengths of 16-ft and 24-ft. Results were positive, showing increases up to 125% in strength and 95% in stiffness when compared to test results of individual framing members. In 90% of bare I-joist test specimens, failure occurred at manufactured finger-joints in the bottom, tension flange. This failure mode only occurred in 25% of full-scale roof panel specimens; 75% failed in shear in the OSB web, demonstrating significant composite action. Creep-flexure tests were conducted to assess the long-term structural performance of panels. Results indicated that creep-rupture may be critical in panel design. In conjunction with panel tests, material-level tests of the constituent OSB and I-joists provided data for a strength- and stiffness-prediction model. Transformed section analyses were used as a simplified method to derive panel load-span tables which illustrate the potential for future roof designs