Masonry failure and debris throw characteristics under dynamic blast loads

The hazard to persons and structures derived from secondary explosion effects, associated with blast loads on structural components resulting in e.g. debris throw, may exceed the hazard range from the blast wave itself. The debris throw hazard potential is related to the initial fragment throw parameters as launch velocities, angles and masses during the structural failure. These parameters strongly depend on the overpressure characteristics of the explosion, controlled by the explosive amount and category resulting in either detonations (typically high-order explosives, e.g. TNT or ANFO) or deflagrations (typically low-order explosives, as e.g. gas-air mixtures or propellants). Detonation shock fronts are typically characterized by very fast propagation velocities (supersonic), high peak overpressures and a small spatial extent. In contrast, the deflagration shock front propagates much slower (subsonic) and is characterized by lower peak overpressures but a larger spatial extent. Understanding the break-up process during the structural failure and the resultant initial fragment throw characteristics, related to the loading conditions (explosive amount and category), is thus essential to assess the hazard potential from explosions and provide the basis for proper consequence modelling in risk analyses. However, the data basis for hazard assessment and model development on debris throw from masonry structures is still very limited. This contribution describes recent shock-tube experiments with the goal to characterize the hazard from single span masonry walls subjected to dynamic blast loads with pressure-time characteristics of typical high-order explosions. Based on stereo high-speed video images and computer vision techniques, we analysed the masonry break-up process and derived debris fragment trajectories, velocities, launch angles and mass distributions. Based on the observed debris throw characteristics, approaches for hazard assessment of masonry failure are described in order to discuss the transferability of the results with respect to the different loading characteristics of low-order explosions and the structural component.