The anatomy of a blast: How explosions harm buildings and people
Friday, 18 September, 2015
As physical protection experts, one of our main concerns is explosive attacks. Bombs and explosive devices have proven to be particularly destructive and dangerous to both people and structures. As we pointed out in our blog about attacks on embassies, explosive devices have become the go-to tool for those who would do harm to a building and its inhabitants. But why is that the case? Why are explosions so impactful, and what threat do they represent?
In this blog, we’ll first take a look at blasts, starting with the chemical reactions and physical processes that determine their strength. We’ll then examine the threats they represent, including methods of attack, and their damage and injury mechanisms for buildings and people.
Explosive events: An in-depth look at blasts
Let’s start by defining some basic, but important terms: explosion and blast.
Simply put, an explosion is a swift release of energy, taking the form of sound, heat, light, and shock waves.
While many people use both terms interchangeably, a blast generally means something more specific: It’s the initial shock wave (or movement of air) created by the explosion.
To understand the impact of blasts on buildings, it is important to break down an explosive event into several steps:
- The explosive material (whether liquid or solid) “detonates”, which means it gets converted from its original state into a hot, high-pressure gas
- In order to reach equilibrium with the surrounding air, the product of the explosion expands rapidly, creating a shock wave made of highly compressed air
- This shock wave travels radially outward from its source (the explosive material) at supersonic speed. As it expands, pressure drops rapidly due to the energy dissipating and because of geometric divergence. This typically happens in milliseconds.
- The initial shock wave is only one third of the total chemical energy release. The remaining two thirds are released more slowly, as products of the detonation create an “afterburning” process, slower than the original blast and detonation.
- These secondary shock waves (or “stress waves”) also carry a huge amount of energy, and will pass through surfaces, organs and tissues, creating tremendous damage.
- At the site of the explosion, a vacuum (or blast wind) will also be created, due to the rapid expansion of the blast. If the blast can be seen as a “push” effect, then the vacuum is more of a “pull”, since it pulls in the surrounding atmosphere. This high-intensity vacuum can cause objects, glass, or debris to be draw back towards the explosion site.
Predicting damage: What are the factors that determine the effects of a blast?
The magnitude and distribution of a blast load, and their effect on the buildings and people we are trying to protect, will vary based on several factors.
- The first thing to take into account is the amount and type of explosives used: Higher quantities/yield of explosive materials tend to lead to a bigger, more destructive event.
- Second, the specific type and composition of the material (e.g., TNT, C4, Semtex, HMX) will also dictate the strength of the explosion.
- Third, the distance between the detonation and the structure (the “stand-off” distance) also plays a critical role.
- Fourth, the building’s architecture and construction materials are key to predicting how they will protect against a blast. Blast-resistant buildings will generally withstand explosions better than “regular” buildings. It is not a guarantee, of course.
But those criteria alone aren’t enough to predict the potential damage that will result from a blast. One must also take into account the pressure created after the initial explosion, and the physical interaction (including the angle of incidence) with the surrounding structures.
There are two types of pressures one has to worry about when considering explosive events:
- The peak incident pressure, which is determined by the explosives themselves and the distance to the area of impact. Put simply, this is the power of the original blast itself.
- The reflected This particular type of pressure varies based on the angle of surface impacted by the shock wave. For example, when the blast wave and surface are parallel, the reflected pressure is minimized. But when the blast wave hits a structure at a 90-degree angle, the reflected pressure is extremely strong. Reflected pressure is what one must really take into account when thinking of damage to buildings. Depending on the angle of incidence, reflected pressures for detonations can be almost 13 times greater than peak incident pressures.
When talking about pressure, one must also take into account the impulse. In physics, the impulse is the measurement of a given force over a certain time interval. Keep in mind that even though explosions take place over a very short amount of time, they’re not instantaneous. Pressure increases drastically, then drops and fluctuates—but does not disappear immediately.
The easiest way of understanding impulse and pressure would be to look at a graph such as this one:
In the graph above, we can see the peak pressure as the point on the waveform where pressure is at its highest (Pmax). The stress waves are the pressure over time. Everything below the curve would be the impulse itself. We can see it fluctuating over time, following the pattern of the pressure.
Here’s another graph, which focuses on stand-off distance and overpressure—the pressure caused by a shock wave above normal atmospheric pressure. As we can see, the effects of a blast can be felt at smaller or greater stand-off distances, depending on the quantity of explosive used. The maximum amount of overpressure represented here (2000 kilopascal) spreads much farther with bigger amounts of TNT.
This is why it’s crucial to take into account pressure, impulse, distance, and all the various energies released during an explosion into account and to evaluate them. We need to fully understand these explosive events—and the physical effects they can have—in order to best help protect structures and minimize the amount of energy that could be imparted on them.
Bringing explosive threats to buildings: Delivery methods
When considering threats to buildings—and the people inside or nearby—we must also be aware of the delivery methods of the explosive payload.
For explosive threats, there are two general delivery categories:
- Vehicle bombs, which can be referred to as Vehicle-Borne Improvised Explosive Devices (VBIED), Large Vehicle-Borne IEDs (LVBIED), and Very Large Vehicle-Borne IEDs (VLVBIED). This delivery method typically involves large quantities of explosives, detonated at a critical location. The size (regular, large, very large) refers to the size of the vehicle and the expected payload (e.g., a sedan carrying 200 kilos of explosives compared to a semi-trailer carrying several tons)
- Hand-delivered, also referred to as Personal-Borne IEDs (PBIED). While loads tend to be smaller than VBIED, the impact can be just as destructive—if not more. Bombs delivered by people can be detonated inside of buildings, can be easier to conceal, and may be detonated as less-secured locations.
Blasts damage to buildings and people
Structures don’t exist in a vacuum. They’re frequently populated, and many of them are not isolated—there could be other buildings around, and passersby in the area. Therefore, the threats from explosive events should be broken down into two major parts: damage mechanisms to buildings, and injury mechanisms to people.
For buildings themselves, there are three major damage mechanisms:
- The explosion itself, or direct air-blast effects. The damage caused by this first step will be determined by the terms we defined and the steps discussed in the first part of this article. The detonation, the shock waves, vacuum, the quantity and type of explosives, and the pressure (both incident and reflected) created by the explosive event. The direct air-blast may also result in flying debris damaging the structure even further.
- Collapse: Once a building has been hit (either directly or indirectly), parts of its structure are at risk of collapse. This is the most severe building response to an attack. This collapse may be immediate (e.g., a wall or beam being destroyed by the blast) but it may also be a progressive collapse. This happens when localized structural failure spreads to other elements of the buildings, leading to a chain-reaction that leads to more damage and/or collapse.
- Damage to nearby structures: Blasts targeted one building often have an impact on neighboring buildings. Either the explosion itself is powerful enough to reach nearby structures (e.g., a shock wave causing windows to shatter), or the collapse of the main target will damage surrounding buildings, or debris will be propelled with enough force to hit those buildings.
The injury mechanisms to people are, by severity:
- General collapse: According to FEMA, past incidents have shown that collapses have lead to the most extensive number of fatalities. Most fatalities are due to blunt impact and crushing. While survivors may survive the initial collapse, it’s quite frequent for victims to end up trapped under concrete slabs.
- Partial collapse: The failure of an exterior wall, floor collapse, and other parts of the building collapsing (while it remains standing) can also lead to deadly injuries, such as skull fractures and concussions.
- Flying debris: Glass windows, fixtures, and other building materials quickly become dangerous after a blast, leading to lacerations and abrasions and worse. While seen as less severe than other injury mechanisms, flying debris frequently lead to many wounded, since a shock wave can reach buildings hundreds of meters away and shatter their windows.
How to protect people and structures against this growing threat?
From Oklahoma to Nairobi, it’s all too clear that blast-based attacks are a significant threat to both buildings and people.
Their effectiveness, the damage they can inflict, the attention they draw, and the ease with which they can be executed have made car bombs and hand-delivered bombs a staple of terrorist attacks over the past few decades. Embassies, tourist locations, and government buildings continue to be hit multiple times a year, all over the globe.
We hope this article helped you better understand the nature of explosive events. In our next blog, we’ll take a closer look at how we can protect ourselves against this growing threat.
While we can’t always prevent terrorists from reaching their destination, we can prepare our buildings to better withstand explosions—for example, by increasing the distance between explosive device and structure to reduce the blast effect, or directly improving the structure itself by making it stronger, more massive and/or more flexible. Be sure to check in again to learn more about the importance of structural design, and how to ensure that a building’s perimeter, shell, and cell can help mitigate explosive attacks.