Design of lightweight web core sandwich panels and application to residential roofs.

Abstract

Steel sandwich panels with integrated structural and thermal insulating performance offer several advantages over conventional construction methods for residential roofs. These advantages include improved energy efficiency, reduced construction time, architectural benefits, and amenability to other energy-saving technologies such as integrated solar photovoltaic panels. Traditional foam core panels are unsuitable for roofs, however, because they rely on the core material (which has low stiffness and is subject to creep) for long-term structural performance. This work focuses on an alternative solution, web core panels, consisting of foam-filled panels with interior webs, with or without an additional insulating layer. The structural performance of web core panels is provided by the face sheets and webs. The core is used both as an insulating material and to strengthen the thin sheet metal components against local buckling failure. The webs, in particular, must be thin and widely-spaced to minimize their impact on thermal performance. As a result, they are subject to a variety of potential local failure modes that limit the range of feasible roof designs. The influence of the core material on local failure modes is examined in this work. Models are developed by treating the webs as plates on a Pasternak elastic foundation, with expressions for the foundation constants derived using energy methods. The models are validated through finite element analysis and prototype testing. The factors that limit the range of feasible web core designs are investigated. The limiting factors for design are thermal performance, panel deflection, face sheet buckling, and shear buckling of the webs. Thermal performance is particularly limiting due to the high thermal conductivity of the steel webs, and large panel depths are generally required. Panel depth can be reduced by using a web material with lower thermal conductivity, such as stainless steel, or by providing an external insulating layer. Design procedures are developed for panels with and without an external insulating layer and used to investigate the tradeoff between panel depth and weight. Based on minimum weight design, panels with stainless steel webs are recommended, especially as the applied load and R-value requirements increase.