Are Window Mullion Response Limits Too Conservative?

Abstract

Design of blast resistant windows is relatively new compared to other facade components such as steel, concrete, and reinforced masonry, but is slowly coming into its own in terms of design procedures and response criteria. Initial response criteria from the mid 1980s – when design procedures for blast resistant windows were initially formulated – limited window framing to L/264 but no more than 1/8-in. There were also stringent wall response criteria when windows attached directly to walls. These criteria were used because monolithic glazing, limited to low probabilities of fracture, was used. In the 1990s and early 2000s, when laminated glass started to become the glazing for blast resistant windows, software using SDOF approaches allowed for the fracture and even flyout of the glazing – depending on the level of protection to be used. However, frame response was still rather stringent and vacillated between L/60 and L/160 of the frame member. With the publication of the ASCE document on “Blast Protection of Buildings” and the PDC TR 10-02, “Blast Resistant Design Methodology for Window Systems Designed Statically and Dynamically”, frame response criteria for windows, storefront systems, and curtain walls became less stringent and varied depending on the level of protection to be used. Recent testing has shown that even the latest response criteria may be somewhat conservative for different levels of protection. The test information is somewhat limited since most window systems are tested to show compliance with different glazing hazard levels/performance conditions, but limited available data where deflection of mullions was tracked during dynamic loading, showed low glazing hazards even though frame response was greater than would be allowed for design. This paper presents a historical perspective of how response limits and glazing design have progressed over the years and then presents available data showing that low glazing hazards against blast loads can be achieved even though the frame response is much greater than the current allowable limits.