# A Review of Blast Loading in the Urban Environment
## 2 Current Knowledge
### Channelling and Shielding  
[PERSON] et al. [37] conducted experimental and numerical modelling to study the effects of areal density (defined as the percentage of the footprint area that consists of obstacles) and street layout on blast wave parameters. A singular wall was used as the point of interest for measuring pressure data, and the experimental work was shown to have good test-to-test repeatability. The computational software Air3D [38] was used to carry out the simulations, whereby a direct comparison was made to the experimental results for a particular setup. A good correlation was observed between the experimental and simulated results; thus, it was deemed suitable to continue the study via numerical methods only. Two particular behaviours captured in this study were channelling and shielding. A control scenario was initially undertaken where a blast was simulated without any objects present. The distance variable relates to the distance along a 4 m wall perpendicular to the shock propagation (not the stand-off distance); hence, the middle of the wall has the shortest stand-off distance to the blast. The maximum peak overpressure was approximately 225 kPa in the control scenario. The initial test scenarios had the same areal density, the critical difference being that the centre of the wall was shielded in one layout and was directly \"down the street\" in the other. These layouts were specifically chosen in an attempt to maximise both shielding and channelling behaviours, respectively. The layouts are illustrated in Figure 2. In the shielding-type layout, the central peak overpressure was 195 kPa, a 13.3% reduction compared to a no-obstacle scenario. In the channelling-type layout, a central peak overpressure of 305 kPa was recorded, a 35.5% increase compared to a no-obstacle scenario. Considering this pressure data, it is clear that obstacles between a blast and its target can significantly affect peak overpressure. Particular focus should be drawn to the conclusion that, in this instance, obstacles can have both positive and negative impacts on the peak overpressure, depending on the point of interest's location relative to the blast and surrounding obstacles. These findings may be beneficial in grid-type-layout cities, such as those commonly found in the USA [39]. However, many other non-grid-layouts need consideration. [PERSON] et al. [37] also conducted simulations using random arrays of buildings with varying areal densities, either featuring direct or non-direct paths from the explosion origin to the wall. The sum of the impulse measurements from a given configuration were compared to an equivalent \"no obstacle\" scenario, termed the shielded and unshielded value, respectively. This process was repeated for several layouts, with the shielded impulse loading relative to the unshielded loading for a given areal density summarised in Table 1.  
The results in Table 1 suggest that an increased areal density lessens the protective shielding elements of the various building arrays when there is a direct line of sight from the blast origin to the target wall. This is described as being _\"perhaps rather counter-intuitive\"_ as it was initially expected that an increased areal density would reduce the loading experienced on the wall. It is stated that a _\"combination of shielding and channelling\"_ produces these results [37]. It was posited that the increased areal density worsens the effects of channelling more than the protective effects of shielding, so an increase in loading is seen. An optimal areal density may exist where the shielding element is maximised and the channelling element minimised. Additionally, it could be argued that the \"no direct line\" results do not appear to follow any relationship from the data presented. The characterisation of an urban layout in the form of areal density may be too simple and too great a reduction of the underlying physics to develop meaningful loading predictions from.