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Air-blast effects on structural shapes
Following the recent increase in bombings over the last 10 years, blast loading has become a research topic of renewed interest. Many different approaches exist in the design of blast resistant structures, ranging from simplified hand procedures to fully coupled nonlinear numerical analysis. However, many designs begin initially from the simplified hand procedures. Hopkinson and Cranz generated experimental data for key loading parameters such as peak overpressure, arrival time, impulse, and load duration. These parameters are then used to determine a simplified loading history to design the member or structure. A possible oversight of this approach is that the experimental data from which the loading parameters are determined is based on a reflective surface of considerable size, effectively infinite, such as a bomb shelter. Individual structural members however have much smaller widths and could be considered finite surfaces in most scenarios, potentially lowering the loading. A study was performed to investigate the effect of a W-shape section with finite dimensions on the loading parameters. Two main mechanisms were hypothesized for reducing the loading; 'clearing' and 'wrap-around'. Firstly, the ability of the blast wave to clear around the section potentially could lead to the pressure decaying faster than that of an infinite surface, a process known as 'clearing'. The other mechanism of wrap- around occurs when the pressure wave engulfs the section, causing the load on the rear face of the flanges to oppose the load on the front faces, which lowers the impulse imparted to the section. Using the hydrocode Air3d, design of experiment procedures and linear regression techniques, a series of analyses were performed. The results of the study, suggest that peak pressures remain unchanged regardless of the 'finiteness' of the section and impulse appears to be substantially influenced by both 'clearing' and 'wrap- around'. For a given charge mass, held constant for a range of stand-off distances, R, impulse is proportional to 1/R when considering an infinite surface. The impulse when clearing is considered is still proportional to l/R ,however, it is 40-60% lower than the experimental value of impulse, which represents a substantial reduction. The impulse when 'wrap-around' can occur is no longer proportional to 1/R but rather 1/R2. The implications of such a relationship are dramatic, as for a 10 fold increase in R the impulse is 1% of the previous value rather than 10% as the Hopkinson-Cranz data suggests.