The most effective way to protect a structure from blast loading is to make sure an explosion doesn't occur close to a structure. It's a philosophy known as "enforced stand-off" and explains why you sometimes find vehicle barriers or large bollards around major buildings.
But what if you cannot enforce stand-off for some reason? We need to understand the loading generated by an explosion very close to a structure, which is very demanding for experimentalists.
Understanding the threats
Near-field blast loads can generate pressures with magnitudes tens of thousands of times higher than the most severe hurricanes, changing on a microsecond timescale.
We're currently developing experimental approaches to measure these loads and give design engineers a better understanding of the threats and how to defeat them.
In principle, any building can be designed to resist, or at least, mitigate the effects of even the largest explosions.
Measures which can help range from laying out the building to minimise loading, detailing of connections to enhance resilience and careful choice of facade systems like blast resistant glazing.
The issue, as in all structural engineering, is balancing the cost of these measures with the anticipated level of risk.
Two or three decades ago, civilian engineers rarely considered these issues in their designs. But the combination of an increased perceived threat, and the development of more effective materials and structural systems means that consideration of blast resilience is now becoming commonplace, at least for buildings in major urban areas.
The Alfred P Murrah building
The classic example of how things have changed is the Alfred P Murrah building in Oklahoma City, USA, which was severely damaged by a large vehicle bomb in 1995, resulting in 168 deaths.
The building designers in the 1970s weren't to know this, but there were aspects of the design which made it critically vulnerable to this attack.
Specifically, engineers in those days weren't thinking of enforced stand-off, so the attacker was able to park a large vehicle bomb very close to the structure and the resulting blast loading on the nearest structural elements was huge.
And then there was no consideration of how a building would respond if one or more columns were destroyed, especially where those columns supported large transfer beams. In the Oklahoma City event, the loss of two or three ground floor columns led to the collapse of the whole front face of the building and catastrophic loss of life.
On the other hand, the initial survival of the World Trade Center towers in the 9/11 attacks, despite the loss of many exterior columns due to the aircraft impacts (before fire weakened the damaged structures) shows that buildings can resist even very severe impact (or blast) damage.
These towers weren't explicitly designed for that scenario but it was a matter of good fortune that the resilience of the structures enabled many thousands of occupants to get to safety before the eventual collapses.
The WTC towers' exteriors were exceptional in having such closely spaced columns and spandrel beams, resulting in a great capacity for load redistribution. That specific structural system is not suitable for most buildings, but this case reinforced a principle that civilian protection engineers have taken on board.
We've learned important lessons from these, and other unfortunate events, which mean that we're better able to develop more resilient structures today.
But the threats, the possible design solutions and the economics of risk and protection are constantly changing, and our role as researchers is to provide the background understanding to make protection still more efficient and effective in the future.