Applied Knowledge

Blast Basics

An explosion is a rapid release of energy in the form of light, heat, sound, and a shock wave. The shock wave travels outward, in all directions, from the source of the explosion and is the primary source of building damage. The duration of the shockwave is fast, measured in milli-seconds rather than seconds (think of a blink of the eye) and the forces imposed on anything it its path (be it a building or a person) are enormous – many times greater than hurricanes.
Many factors contribute to how a building will respond to an explosive event, the most critical are.
• The size of the bomb
• The distance from the bomb to the building
• The type of building construction.
The size of the bomb (also termed Net Explosive Weight) and the distance from the bomb to the building (standoff distance) determine the magnitude of the pressure (force over area) and the duration that the pressure acts on the building element. Pressures decrease as the standoff distance increases and durations tend to increase with greater standoff. Shorter durations and smaller pressures cause less damage than higher pressures and longer durations.
The type of construction is also a significant factor in how much damage a building will experience from an explosion. It is important to remember that the vast majority of buildings were not built with explosive loading in mind. Therefore, just because a building does not respond well to a bomb does not mean that the building was poorly designed. Blast loading is an extremely abnormal event and stresses buildings in ways never anticipated during the original design and construction. Buildings are generally designed to hold up gravity (downward) loads and lateral wind loads. In earthquake regions, they are also designed to withstand forces created by ground movements. Standard buildings are not designed to withstand large, aboveground shock waves of the magnitudes associated with explosions. Very lightweight buildings and buildings built of unreinforced masonry (brick or concrete block units) tend to respond the worst to explosions, while concrete and steel framed buildings built in high seismic zones tend to respond the best.

Upcoming Article

One of our own (Arturo Montalva) has an article scheduled to be published in the October issue of STRUCTURE Magazine. Arturo (along with his co-authors Jeff Baylor and Klaus Wittig) is presenting What Can Open Source Structural Software Do For You? The article discusses the use of Open Source Software which is extensively used in the Computer Science community but rarely in Structural Engineering. In this particular article, the authors present an overview of Calculix (available online), an FEM code similar to Abaqus(R) and primarily developed by Guido Dhondt and Klaus Wittig.

Did You Know…..?

Did you know… even if your building meets the required standoff distance, hardening measures are still most likely required to provide protection to occupants.
Required (or recommended) standoff distances (the distance between your structure and your protected perimeter) are developed by governments and agencies based on their analysis of the most likely threats that might be used against their facilities, the level of risk that is acceptable to their organization, and their ability to cost effectively harden their structures to the blast loads from an explosion at the prescribed standoff distance.
If you are involved in security planning and implementation for your facility, I recommend that you gather as much information as possible about the background of the prescribed standoff distances. Some of the information may not be available due to security concerns, but even a little information can help you better protect the people under your care.

• What vehicle or package size is the standoff distance based on?
This information can help you make decisions regarding physical protection, screening and surveillance in and around your building.

• Has your facility been designed or retrofitted to mitigate the effects of an explosion at the required standoff distance?
Standoff distance alone is rarely sufficient to offer adequate protection. Regularly designed buildings, while adequate for their original design intent, are not capable of providing protection against explosive forces.

• What level of protection is your building expected to provide based on the assumed charge size at the required standoff distance?
Even when buildings are designed to mitigate blast effects, they are not designed as bunkers, rather they are designed to provide a specific level of protection to the occupants. It can be important to know what level of protection was included in the design and what that means for sheltering in place, evacuation and other operational considerations.

• What building hardening measures, if any, were assumed to be implemented when the required standoff distance was decided upon?
Standoff distances are not developed in a vacuum and they often assume that certain blast mitigating measures are implemented (such as the installation of blast resistant windows or anti-shatter film). In order to get the intended benefit of the required standoff distance, it will be important to make sure that all the assumed mitigation measures are in place.