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Angela Mejorin
Climate change effects are causing an increase in extreme wind events’ frequency and severity, worldwide. Regions that previously were not prone to these extreme weather conditions, are increasingly searching for ways to strengthen urban resilience. In windstorm events, wind-borne debris is one of the major causes of building envelope damage. Wind-borne debris causes 40% of property damage in extreme wind events, being the major cause of façade damage during windstorms. Façades serve as the main defense for people and property from the external environment.
In the last 50 years, code and standard requirements have been created to provide wind-borne debris impact protection for the building envelope. International standards use standardized projectiles that are not always representative of local environments and objects that are recorded to fail and fly in windstorms.
An alternative performance-based design framework is presented, to identify case-specific impact test requirements, to mitigate the effects of wind-borne debris in extreme wind events. This performance-based approach aims to be used to verify the wind-borne debris resistance of façades. Building aerodynamics, the trajectory, and the velocity of specific debris elements are considered to deploy performance-based façade solutions.
A case study is presented to discuss how to identify case-specific façade impact test requirements. Typical roofing elements that are typically recorded to fail and fly during windstorms are analyzed through a flight trajectory analysis. Roof shingles are the debris elements considered in the case-study. The reference target building, the object of the design for this case study, is an essential facility, to avoid disruption of essential services, especially in the post-event scenario.
If new buildings and façade retrofit projects can improve their resilience to wind-borne debris impacts, there can be a notable mitigation of the overall consequences of extreme wind events. Adopting performance-based design impact tests, building envelope solutions can sustainably address local needs to improve urban resilience.
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Nicholas Oberts • Mark K. Weaver • Phillip Benshoof
Explosions can result in very high loads of extremely short durations. Protective design to mitigate the effects of blast loading requires façade hardening, which includes special considerations for glazing. Hardened systems may require specialized design skills or the use of advanced design software to provide solutions that will be effective in protecting building occupants by ensuring catastrophic structural or component failures do not occur. Explosions are an extreme load, typically requiring glazing to be designed to accommodate permanent damage. The focus of design is to control costs while protecting building occupants. During an explosion, glass can shatter and send high velocity shards into occupied space. To mitigate this hazard, it is common to utilize laminated glazing panels, which consist of flexible interlayer(s) sandwiched between multiple layers of glass. The interlayer exhibits ductile response to blast loads, allowing displacement while retaining fragments. Polyvinyl butyral (PVB) is the most common film interlayer utilized for blast resistance. However, ethylene-vinyl acetate (EVA) is widely available and is a potential alternative to PVB.
This paper summarizes the basic protective design principles utilized for the selection of glazing layups and highlights a recently completed multiyear effort to assess the relative efficacy of EVA-laminated glazing panels to resist blast loads. This effort involved interlayer material characterization, shock tube testing and open-air blast testing of laminated glazing panels with EVA and PVB interlayers. Results indicated that the EVA films tested exhibited similar qualitative and quantitative responses under dynamic loading conditions. While EVA holds promise for inclusion in hardened facades, care must be taken in implementation to effectively harness its protective capabilities. Findings from this effort were used to derive recommendations for the inclusion of EVA in facades designed to resist blast loading. These recommendations, as well as possible extensions of this effort, are highlighted.
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Jon Kimberlain • Jie Feng • Will Wholey • Dan Aggromito • Valerie Hayez
Designing for blast performance for glazing units can be very complex. The test methods for evaluating performance include subjecting units to actual or simulated blast conditions, which occur rapidly on a very short time scale. Connecting laminated glass to a metal frame using structural silicone sealant creates a unique composite based on the use of a brittle plate with an elastic soft rubber to a ductile rigid metal.
Glazed units were tested with a shock tube charged with various levels of explosive to record the damage development with various modes of failure of the material. Two test results were modeled to compare to the actual observations. Based on the outcome, techniques for effective modeling are discussed as well as future needs.
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