The world faces a growing threat of terror attacks using bombs set to destroy infrastructure and hurt the public. Despite the political and social issues surrounding domestic terrorism, mitigating casualties and terror threats through engineering is a possibility, but a difficult one at that. In terms of civil engineering, designing for wind, snow, seismic, dynamic, and static loads are run of the mill practices. However, looking into the face of your structure being attacked by a bomb or terrorist is a real threat that civil engineers are conscious of. There is a limit to the extent where civil engineering design can overcome explosives, and designing structures to resist explosions indefinitely is just not feasible. There are, nonetheless, specifications and criteria which can be adjusted to help a building withstand explosions better than others.
First, let's look into how an explosion loads a structure, which will give evidence to the difficulty placed in explosive design. Buildings typically see slow loading or fast loading of smaller forces. Explosives, on the other hand, present high forces and high internal stresses in a relatively short period of time. To add to this, explosions are generally localized, which present an increased amount of force in a small relative area. In other words, explosively loaded structures face high stresses in short bursts. Oftentimes, terrorist attacks present successive explosions, which can also present building structures with localized fatigue points, causing failure.
In terms of design against explosives, think of military bunkers. They are thick walled fortresses with no winds – high-density zones with small internal space relative to wall thickness. Unfortunately, due to the nature of explosives, this is the best and most effective way to protect structures from blasts. While explosive loading tends to be an unpredictable science, there are methods where certain design features can reduce casualties and ensure greater odds for the survival of a structure.
[Image Source: Wikimedia]
One of the biggest methods of designing for explosives isn't even an engineering design choice, rather it is a qualitative method of determining which buildings assume the highest risk for attack. Once this is determined, engineers can access strategic locations to reinforce the structure if the construction and engineering budgets allow. Making every building bomb proof is impossible and not to mention financially inviable.
After threats are assessed and key location points are determined, civil and structural engineers can begin modeling how a blast and shockwave will radiate through the affected zone to determine courses of action. When the bomb is close to the ground, shockwaves dissipate spherically from the origin, and pressures decrease in intensity the further from the source. The increases dynamic air pressures resulting from explosions must be vented somewhere or damage to soft tissue can occur in rapid pace.
[Image Source: Wikimedia]
Generally, casualties are seen in explosive attacks from shrapnel or the shockwave, only in certain circumstances does building collapse before evacuation result from attacks. This means that the key point civil engineers need to design around are mitigating the increased pressure, and providing diversion channels from shrapnel.
In the past, designing for explosive loading has been near impossible given the finite and nearsighted eyes of human engineers. The rise of supercomputers and quantum computing may present a new age of protection from blasts inside buildings. Solving problems like where peak stresses will be in varying explosions and mitigating risks involve far too many variables for modern computers to solve in a feasible amount of time. Quantum computing may, however, allow problems like these to be solved relatively instantaneously.
To understand how an explosion can impact a material, civil engineers use dynamic modeling to virtually understand the effects of an explosion. An example of such can be seen in the simulation video below.
As you may have noticed, there isn't one way or even a concrete method to protect a building from explosive loading. This shouldn't scare you, though, as the growing computing and simulation power available to engineers may be able to decrease casualties and help bring the effects of explosive terrorism under some control.
Technical sources for this article include the Whole Building Design Guide, Composite structural panels subjected to explosive loading, and ASCE.
Written by Trevor English