Making Sense of Blast Specifications

Peak and Impulse values are the critical numbers needed to assess the suitability of product to the specification. A blast specification that does not quote the required peak and impulse values, or does not provide enough information to facilitate the calculation of required peak and impulse values….is an incomplete specification. Blast testing is all about how windows perform under the particular Peak and Impulse values imposed on any given test sample. At each blast test, Peak and Impulse values are recorded at the face of the test cubicle.

Indirect references to Peak and Impulse values
Some specifications may not quote Peak and Impulse values but might provide the information for us to calculate it. For example, a specification may state that the window is to resist ‘an explosive charge of 500Lbs TNT at a ‘stand-off distance’ of 200 Feet from the building’. With this information it is possible to calculate the Peak and Impulse values using computer software. It is often the case that there will be security bollards installed to prevent vehicles getting close to the building hence the definitive distance quoted.

‘Reflected’ pressures
It is critical that the specification clarifies whether the peak and impulse values quoted are ‘reflected’. First, it is important to understand why this matters:

Imagine the blast wave not as one singular wave, but comprised of a multitude of energy waves tightly packed together moving towards the target one after the other. Once these pressure waves hit the target, they basically ‘pile up’ on top of each other as they hit the target and get reflected back, producing a larger pressure than had the static object not been there in the first place. This is what reflected pressure is.

Reflected pressure is not as extreme on a test cubicle because the shock waves are reflected around rather than back off the test cubicle. However, when that window is installed into a large elevation, there will not be possible for the shock waves to escape around the window, thus the reflected pressure will be much greater in both Peak and Impulse than on the actual elevation of the building than on the test cubicle.

Software exists that can factor in reflected pressures for the actual building elevation conditions using the non reflected pressures and elevation width and height. The specification however, may only quote and require non-reflected pressures to be met, hence the need for clarification.