Single-Degree-of-Freedom response of finite targets subjected to blast loading – The influence of clearing

When evaluating the dynamic response of a structure subjected to a high explosive detonation, it is common to simplify both the target properties and the form of the blast pressure load – a standard approach is to model the target as an equivalent Single-Degree-of-Freedom (SDOF) system with the blast load idealised as a pulse which decays linearly with time. Whilst this method is suitable for cases where the reflecting surface is large, it is well known that for smaller targets, the propagation of a rarefaction ‘clearing’ wave from the edges of the target may cause a premature reduction in the magnitude of the blast pressure and hence reduce the total impulse acting on the structure. In this article, a simple method for calculating clearing relief, based on an acoustic approximation of the rarefaction wave, is coupled with an SDOF model to investigate the influence of clearing on the dynamic response of elastic targets. Response spectra are developed for a range of target sizes and blast events that may be of interest to the engineer, enabling the effects of blast wave clearing to be evaluated and situations where blast wave clearing may increase the peak displacement of the target to be determined. When the natural period of the target is large compared to the duration of loading, the reduction in positive phase impulse leads to significantly lower values of peak displacement when compared to an identical system subjected to a triangular blast load. For systems where the natural period is comparable to the duration of the loading, the early onset of negative pressure (attributed to blast wave clearing) can coincide with the rebound of the target and result in greater peak displacements. It is concluded that blast wave clearing should be evaluated and its influence quantified in order to ensure that blast resistant designs are efficient and safe.