Structural Silicone Effect on Window Frames Subjected to Blast Loading

Abstract

Blast-resistant windows are typically made of laminated glass. Laminated glass consists of two or more layers of glass bonded to a relatively low strength but ductile interlayer, such as polyvinyl butyral (pvb). The glass layers fracture under applied blast loads, but the system deforms in a ductile manner as a tension membrane develops to resist the load and restrain the shattered glass. This tension membrane capability represents a significant portion of the blast resistance or capacity of the glazing system and imparts in-plane loads to the window frame. Structural sealant is typically used to glaze the glass to the frame. This sealant transfers the tensile membrane forces to the surrounding frame and/or mullions and ultimately to the supporting wall through anchors. Commonly used analytical models provide edge reactions for the glazing based on the resistance function for a Single Degree of Freedom (SDOF) system of lite assuming rigid supports. This assumption, that in-plane displacements are zero, can cause under-prediction of out-of-plane displacements because in reality the supports exhibit in-plane flexibility. This paper discusses the effect of the structural silicone, applied along the glazing edges, on the tensile forces transferred into the window frame and then to the supporting structure. Structural silicone is modeled as a flexible support with non-linear properties for the in-plane forces acting along the glass edge. Two models are developed, assuming one-way response, for: 1) rigid supports, and 2) flexible supports. The effect of the restraining conditions on the dynamic response of the glazing system was investigated using simplified dynamic models and then compared to advanced analytical dynamic methods using finite element analysis (FEA). The results showed good agreement with the simplified models indicating that the proposed simplified procedures can be used to include the effects of the glazing restraints in the overall design of the window frame and the supporting structural system. Finally, a MultiDegree-of-Freedom (MDOF) tool was developed which included the proposed simplified formulation and then compared against SDOF response with rigid supports. Recommendations are provided regarding the inclusion of in-plane reaction load for the window and anchorage design.