Blasts / Explosions

Explosions occur when an explosive material, either in a solid, liquid or gaseous state, is detonated.  Detonation refers to chemical reaction that rapidly progresses, at supersonic speeds, through the explosive material.  The material is converted to a high temperature and high pressure gas that quickly expands to form a high intensity blast wave.  Structures in the path of the blast wave are entirely engulfed by the shock pressures.  At any location away from the blast, the pressure disturbance has the following shape:

Almost instantaneously following the blast, the pressure within the blast radius rises to a peak pressure, Pso, (side on pressure or incident pressure).  The  side on pressure decays to ambient after the positive phase duration after which a negative phase duration occurs where pressure falls below ambient to a minimum value, -Pso.  The negative pressure phase of a blast wave is usually significantly smaller and longer in duration than the positive phase and is consequently generally ignored in blast resistant design.  Correspondingly, a typical design blast load is represented by a triangular loading with side on pressure, Pso, and a duration, to, characterized by the following graph:

The area under the pressure-time curve is the impulse of the blast wave.  For a triangular blast loading, the impulse is calculated as 0.5 Pso to.  To provide a comparison, a load for a vapor cloud explosion could be 14.5 psi at a 200 ms duration while a 1000 lb TNT explosion would result in a side on pressure of 14.5 psi at a 23 ms duration.  Knowing both pressure and impulse (duration) are critical because both cause damage.

The side of a structure facing a blast is subject to an increase in pressure over the side on pressure due to a reflection of the blast wave.  For side on pressures of 20 psi or less and with an angle of incidence of normal to the structure face, the reflected pressure can be estimated as (2 + 0.05Pso)Pso.

Structure Response

The response of a structure from the blast load is evaluated by the amount of deformation the structure’s components undergo. The models currently used for calculation include Single Degree of Freedom (SDOF), Multiple Degree of Freedom (MDOF) and Finite Element Analysis (FEA). Each structural component is evaluated against an acceptable response criteria. For buildings located in petrochemical refineries, ASCE provides the following response criteria:

Response Criteria	Description
Low Damage	Localized building/component damage.  Building can be used, however repairs are required to restore integrity of structural envelope.
Medium Damage	Widespread building/component damage.  Building cannot be used until repaired.  Total cost of repairs is significant.
High Damage	Building/component has lost structural integrity and may collapse due to environmental conditions (i.e. wind, snow, rain).  Total cost of repairs approach replacement cost of building.

A Pressure-Impulse (P-I) curve may be generated to show response over a wide range of blast loading. The P-I curve generally takes on the following form:

Facility Siting Analysis

OSHA PSM regulations (Standard 29 CFR 1910.119) require that a facility involved in the processing or storage of toxic or reactive highly hazardous chemicals which present a potential for a catastrophic event have a risk analysis performed on the facility.  The risk to building occupants from an external explosion, fire, and toxic release is evaluated.

OSHA is particularly concerned that facility siting address the spatial relationship between hazards and the locations of people, especially in occupied buildings.  Hazard zones inside a facility are identified and are rated on their level of risk.  Often the analysis yields that occupied buildings fall in these zones.  Facilities must make the assessment to retrofit existing structures, often cost prohibitive, or to purchase new buildings designed to address these threats.

Installation Designs – Anchored or Sliding

Many blast-resistant modular buildings are designed to be permanent installations with their anchorage and foundations designed to resist the total anticipated blast loads. This design approach can result in quite large foundations. Refer to Section ASCE [2008] for a further discussion on foundation design strategy for blast-resistant buildings.

However, in the case of modular blast-resistant steel buildings, some owners have taken the approach that the foundations and anchorages need only be designed for normal design loads (loads other than blast). In this case, the building’s anchorages are permitted to ‘break’ during a blast event (they act as anchorage ‘fuses’), but are designed to remain intact under other design loads. Alternatively, the building can be designed to be completely unanchored (free to slide) for both blast and other loading effects, subject to the local building official’s anchorage requirements for gravity, wind and earthquake loading. Whether a building should be anchored for blast, anchored for other loads (but not for blast) or completely unanchored (and free to slide), depends on the anticipated use of the building, whether or not potential down time is acceptable following a blast event, the amount of flexibility in the utility connections (power, water, wastewater, gas) and, most importantly, the owner’s tolerance to risk. Owners should make this decision on many factors, including risk, safety, cost, magnitude and probability of blast.

If the building is unanchored (free to slide) for blast loading, or only anchored for conventional loads with a structural anchorage fuse, the maximum sliding displacement, velocity, and acceleration of the building can be estimated using impulse-momentum first principles, simplified numerical integration methods or finite element analysis. The contents and personnel within the building should be assessed for these actions. In this case, the structural movement may result in impact/damage to attached utilities and building contents, as well as the possibility of injury to personnel, due to interaction with the structure, fixed equipment and internal moving objects (typically unrestrained and falling objects). In such interactions, the critical components of motion can be local accelerations, velocities and displacements that govern local forces and energies of impact, including the propensity to topple over and fall. Permanent fixtures and equipment should be designed to withstand the calculated local building motions as a result of blast loads. Anchorage and restraint techniques for nonstructural items have long been used for earthquake design (FEMA 412[2002], FEMA 413[2004], FEMA 414[2004], SMACNA[1998]). Attached utilities should also be designed to accommodate expected movements or fail in a safe manner.

As stated above, the decision as to whether or not these displacements, velocities and accelerations are acceptable to an owner depends on the anticipated use of the building, whether or not potential down time is acceptable following an event, flexibility in the utility connections and, most importantly, the owner’s tolerance to risk. Since the building will act as an external pressure barrier, the design of internals need only consider the effects of movement. Definitive assessment criteria for interactions with personnel are not available, but criteria do exist (Baker[1983]). Additional criteria for projectile impact (such as falling objects) are also available (TNO[1992]). Note that architectural and nonstructural components may become debris hazards. TM 5-1300 provides some guidance on tolerance of mechanical and electrical equipment as well as personnel. For sensitive and critical equipment that must function during and after the event, verification by shock testing with the induced motions consistent with expected structural motions may be needed.

For modular buildings that are free to slide, the calculated permissible sliding displacement sometimes has been limited to 12 in. (300 mm), but as stated above, this is very much an owner decision and is specific to the building being designed. In all cases, buildings that are not anchored for blast must have a high margin against overturning and the propensity to uplift should be calculated. In the case of significant uplift, application of pressure to the underside of the building should be considered, as this further adds to the overturning moment and magnitude of uplift.