Façade failure effects on blast propagation along city streets

Abstract

In assessing the potential hazard to buildings and people in a city landscape, it is usual to assume that building façades are perfectly rigid reflecting surfaces. This is an attractive assumption because it results in conservative estimations of impulse loads at locations remote from the explosion. Similarly, it enables both small-scale physical modelling and numerical simulation to be performed without the need for structural prototypes or response calculations. However, close to the source of an explosion, damage to building façades can be devastating. It is not unusual for all the glazing elements, light cladding and architectural features to be completely removed, leaving only the steel or reinforced concrete skeleton of the building. Because this region of devastation allows blast to enter the building, pressures and impulses experienced at locations further down the street in which the building is located will be reduced. The extent and significance, or otherwise, of any reduction is difficult to quantify as is the intensity of the blast that enters the building by way of the failed façade. The investigation presented in this paper has two complementary components. In the first part it is assumed that light, non-structural elements (such as windows) close to the source of an explosion, break instantly on detonation, and they do not impede the passage of the blast into the affected buildings. This approach allows the blast to propagate both through the buildings nearby, as well as along the street, or streets, in the vicinity of the building. In the second part, a complete building (forming part of a street of buildings) is studied. This building has frangible façade elements, separate floors, side and rear walls and a roof. The blast that enters the building is measured at locations on the central ‘core’ of the building. From the first part of the study, reduction in blast impulse resulting from the ingress into adjacent buildings has been quantified by both small-scale experiments and numerical simulations in which the proportion of façade removed from the buildings (termed ‘building porosity’) was varied systematically. A similar approach is used in the second part, where blast has entered the building, to assess the effects on blast resultants both when the walls of the building remain in place and when they are removed (corresponding to a low level of wall ‘robustness’). For the first part, results are presented in the form of impulse against scaled distance for various degrees of porosity and are compared with the performance of rigid façades. For the second part comparisons of the blast environment are made for different façade configurations and different levels of behind-façade confinement.