Effect of the air blast on glazing systems safety: structural analysis of window panel responses to air blast for mitigating the injuries from flying glass

2015-04-26Glass fragments are a prime source of injury to occupants of buildings subjected to an air blast event. Health and safety considerations and the principles of a reasonable degree of protection impose strict requirements on design engineers as well as on the building owners and managers. Since windows serve an important role in modern buildings design, balancing the safety requirements with physical appearance and cost-effectiveness is essential for the successful design and implementation of special security window systems. If the building might be exposed to air blast loadings, it is essential to enhance the safety and security; reduce the injuries and protect the occupants against the impacts caused by the generated glass shards or fragments. Therefore, it is necessary to have a better understanding of the mechanics of glass panel responses to different parameters of the glazing systems and the air blast characteristics. ❧ In this dissertation, the response of glazing systems subjected to air blast loadings is studied by application of an explicit finite element analysis in conjunction with fracture micromechanics principles. The window panel failure patterns and the glass crack propagation paths have been investigated and analyzed through the use of advanced finite element modeling techniques. The results of this research indicate that the response of window panels to blast loading cases is dependent upon the glazing material properties, window panel sizes, window framing and boundary conditions and the blast load intensities. ❧ These findings will eventually be helpful in developing more comprehensive flying glass injury models based on glass fragments (or shards) sizes, flying velocities and flying distances that enable the decision makers to use the appropriate structural, architectural and building perimeter choices to better address the threats facing the safety of the building occupants during an air blast event. ❧ In the first Chapter of this research, the resistance of glass panels to air blast loadings is studied and the finite element analysis results have been compared with those obtained using SDOF techniques. This comparison is mainly focused on the results for stresses, maximum reaction forces along the window panel edges and the mid-surface displacements. The scope of this comparison is also extended to modal analysis and the natural frequencies of oscillation for the first modes as well as determining the higher mode frequencies and the participation factors of each mode in different directions of the window glass motion. In the initial Chapter, glass panels are assumed to have elastic-perfectly plastic behavior. Therefore, the results of the analysis enable a better understanding of the nature of the window panel responses with respect to the studied parameters of the glazing system as well as the blast load characteristics. ❧ The second Chapter of this thesis is focused on developing a glass fracture model based on Griffith's brittle material failure model which is more compatible and more integrated with constructed finite element models. The proposed failure criterion is based on the concepts of the released strain energy and the stored surface energy at the time of crack formation or propagation. This leads to the definition of the ""Damage Index"" concept that addresses the failure time histories in the window panels. This model sets the foundations in Chapter 7 for finding the number of generated glass fragments and their flying velocities as well as the traveled distances of the shards in the air. This is crucial for developing a more comprehensive blast injury model. ❧ The existing retrofit techniques for blast resistant glazing that are discussed in Chapter 3, will help broaden the structural engineer's perspective with regards to the choice of the most efficient and cost-effective retrofit techniques, given the glazing system characteristics and the blast load intensities, in order to enhance the safety of the building occupants during an air blast. ❧ Introduction of the brittle failure principles and the yield line analysis method in Chapter 4 set the stage for predicting the crack propagation paths across the glass panels and the general failure paths of the glazing systems which include the glass panel material (annealed glass, fully tempered or laminated panels), window panel framing conditions (simply-supported or fixed-in-place) and the blast load intensities. ❧ In Chapter 5, the application of more advanced finite element techniques and the element removal method in modeling the brittle failure of the glazing systems make it possible to study and investigate the effect of glass panel sizes and aspect ratios on crack propagation paths and glazing system failure patterns for small, medium and large windows. Annealed, fully tempered and laminated glass panels for both conventional and fixed-in-place framing conditions subjected to small, medium and high-intensity air blasts are considered. Brittle failure fracture mechanics criteria are defined into the material properties. ❧ As the window panel response to blast loading cases is also dependent upon the glass imperfections, the type and location of glass defects across the window panel play a major role in determining the failure pattern of the glazing system. For this purpose, in Chapter 6, the failure of defective annealed and fully tempered conventional glass panels are modeled and investigated using explicit finite element analysis in conjunction with fracture micromechanics' principles. ❧ Finally, in the last Chapter, conventional simplified and empirical methods for evaluating glass fragmentation hazards are discussed and the findings are compared with those of the analytical calculations and the numerical simulation for the glass fragmentation problem: particularly the glass fragments (or shards) ejection velocities, flying distances and fragment sizes. Furthermore, the existing blast hazard mitigation standards and regulations are introduced and discussed in order to assess the hazard level of the proposed glazing systems used in this dissertation when submitted to different air blast loadings. The outcome of this Chapter will be used towards the development of a more comprehensive blast-resistant glazing injury model and design guidelines for security enhancement of glazing systems and safety improvement of building occupants during an air blast