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 considered in blast resistant design.

The duration of the shock wave is very short, measured in milliseconds 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.  Shock waves impart a significant positive load on building structures followed by a much smaller negative load (or suction action).  For walls, this results in inward and outward loadings and for roofs, this results in upward and downward loadings.

A number of factors contribute to how a building will respond to an explosive event, with some of the most critical being:

• The size of the explosive device.
• The distance from the explosive device to the building.
• The orientation of the blast wave with respect to the building.
• The type and quality of building construction.

The size of the explosive device, the distance from the bomb to the building (i.e., standoff distance), and the orientation with respect to the building determine the magnitude of the pressure (i.e., force over area) and the duration that the pressure acts on the building element.

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 existing buildings were not built with explosive loading in mind.  Therefore, just because a building does not respond well to an explosion does not necessarily mean that the building was poorly designed or constructed if there is significant damage or collapse after an explosion.

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 with unreinforced masonry (e.g., brick or concrete block units) tend to respond the worst to explosions, while concrete and steel framed buildings tend to respond the best.

In framed buildings, the windows and infill walls (i.e., material that fills in the space between the columns and beams) are the least resistant to blast forces, and can create hazardous flying debris.  In situations where a building does not collapse from an explosion, the majority of the injuries come from flying debris.

There are many retrofit approaches out there to mitigate blast loads and their resulting hazards.  However, they are by no means one size fits all.  Great care must be used when designing blast effects mitigation upgrades, as the upgrades themselves could add new burdens to the structure and make the situation worse rather than better.

The following are some examples of things to be cautious of with respect to upgrades:

• Adding stronger elements (e.g., windows, doors, or posts) may pose problems in terms of load distribution through an existing building’s diaphragm system, which could then overload the strength of the base building.
• Certain vendor products may provide strong blast performance in ideal conditions, but degrade when subjected to harsher environments.
• Blast walls, which are are often seen as a ‘golden bullet’ of protective design, are effective in a very narrow band of situations. Their effectiveness is highly dependent on the relational geometry of the explosive device, the wall, and the structure being protected.  If the proper geometry is not in place, blast walls provide little, if any, benefit and could even create adverse load reverberations that would increase the loads on the target structures.   Additionally, for a blast wall to be effective, it must be designed to resist the blast forces imposed by the explosion, this can be a difficult and costly design requirement to implement.

Potential issues such as these should be fully understood before designing blast resistant retrofits.

Learn more about blast effects and how to design buildings against them in our face-to-face class, October 19 – 23, 2015 (click here for more information).