Six Steps to Specifying Blast Products

The world of blast protection has been rapidly expanding and there are now numerous products and approaches to provide protection from explosive attack. Where there used to be two solutions, there may now be ten. It is critical that security and design professionals understand the strengths and limitations of the various mitigation measures, and how to select products appropriate for their project.

For exterior explosions, the most fragile and vulnerable elements make up the façade of the building: walls, windows, doors, louvers, etc.  These elements are most often not critical for the structural integrity of the building (unless the façade consists of load-bearing walls), but they can represent significant hazards to occupants in the event of an explosion.  For very large or close-in exterior or interior explosions, the structure of the building can be damaged as well.

The process of specifying blast resistant products can be simplified into 6 basic steps.

The following narrative provides a very brief description of each step.

Step 1:  Is Blast Design Required?
While security consultants and blast engineers can make recommendations on this subject, it is the owner who must make the final determination.  Project types that may require blast protection include:

• Government buildings.
• Data centers.
• Iconic buildings in urban environments.
• Multi-national corporate headquarters.
• Critical infrastructure.
• Crowded spaces (shopping malls, movie theatres, sporting arenas, etc.).
• Buildings located near any of the above.
• NGO compounds in high threat environments.

The types of threats to be considered in the design should also be identified at this juncture.  This includes the delivery methods (vehicle, backpack, package, etc.).   These delivery methods will lead to the establishment of the locations of the threats.

Step 2:  Identify Vulnerable Elements
Based on the types and locations of the blast threats, the vulnerable building elements can be identified.  These elements may include:

• Façade.
• Perimeter structure.
• Interior beams/columns/walls/slab systems (for internal or external threats).
• Roof systems.
• Emergency evacuation and rescue equipment.

Step 3:  Develop Blast Design Criteria
For each type of building element, blast design criteria must be developed.  The primary components of blast design criteria are:

• Blast load to use for design/analysis.
• Required response of the elements to the blast loads.
• Confirmation of compliance methods and requirements.
• Applicable existing blast design criteria documents (if any) to be used in the design.

Step 4:  Identify Other Design Criteria
Other design requirements may be incompatible with certain approaches to blast resistance, and in other cases they may reduce the number of available products appropriate for a specific project.  Other design criteria to consider may include:

• LEED.
• ADA accessibility.
• Fire resistance/fire egress requirements.
• Operability (e.g., with respect to windows).
• Historic preservation.
• Aesthetics.

Step 5: Supporting Structure Check
One of the critical things to remember when designing structures for blast resistance is that the individual building elements (e.g., door, window, wall, roof system) do not act alone.  All building elements are supported in some manner, and those supportive systems must be able to withstand the blast loads transferred from the original building elements.

Step 6: Develop Specifications
Whether part of an overall construction document package or as a standalone document for one-off procurements, a well-written and complete specification is key to getting the proper blast resistant products.  Specifications should include:

• Blast design criteria for the element being specified.
• Engineer/manufacturer/installer/testing facility qualification requirements.
• Other design criteria (or reference to other specification sections).
• Blast design guidelines/standards/testing standards.
• Calculation/test report submittals.

An Updated ASTM F2656 Standard Test Method for Crash Testing of Vehicle Security Barriers has been Released!

There are a number of changes between the previous (2007) version the current version (June 2015). These changes include:

• Consolidation of the Imperial and Metric versions into a single document with the limitation that units for a given rating system must be consistent (i.e., for a single test it is not permissible to use some stated metric criteria and some criteria that is converted from imperial to metric).
• Addition of new vehicle size rating classes (Full Size Sedan and Class 7 Cabover).
• Addition of 50 km/h [30 mph] rating for the Small Passenger Car and Pickup Truck.
• Removal of the P4 penetration classification from the rating system.

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 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).