EXPLOSIVE BLAST     


This chapter discusses blast effects, potential school damage, injuries, levels of 
protection, stand-off distance, and predicting blast effects. Specific blast design 
concerns and mitigation measures are discussed in Chapters 2 and 3. Explosive events 
have historically been a favorite tactic of terrorists for a variety of reasons and this is 
likely to continue into the future. The DoD, GSA, and DOS have considerable 
experience with blast effects and blast mitigation. However, many architects and 
building designers do not have such experience. For additional information on ex-
plosive blast, see FEMA 426, Reference Manual to Mitigate Potential Terrorist Attacks Against 
Buildings, and FEMA 427, Primer for Design of Commercial Buildings to Mitigate Terrorist 
Attacks. See sidebar for important reference material on explosive blast.
4.1 BLAST EFFECTS
An explosion is an extremely rapid release of energy in the form of light, heat, sound, 
and a shock wave. A shock wave consists of highly compressed air traveling radially 
outward from the source at supersonic velocities. As the shock wave expands, pressures 
decrease rapidly (with the cube of the distance) and, when it meets a surface that is in 
line-of-sight of the explosion, it is reflected and amplified by a factor of up to thirteen. 
Pressures also decay rapidly over time (i.e., exponentially) and have a very brief span 
of existence, measured typically in thousandths of a second, or milliseconds. 
Diffraction effects, caused by corners of a building, may act to confine the air-blast, 
prolonging its duration. Late in the explosive event, the shock wave becomes negative, 
creating suction. Behind the shock wave, where a vacuum has been created, air rushes 
in, creating a powerful wind or drag pressure on all surfaces of the building. This wind 
picks up and carries flying debris in the vicinity of the detonation. In an external 
explosion, a portion of the energy is also imparted to the ground, creating a crater and 
generating a ground shock wave analogous to a high-intensity, short-duration 
earthquake.
In the context of other hazards (e.g., earthquakes, winds, or floods), an explosive 
attack has the following distinguishing features:
_ The intensity of the pressures acting on a targeted building can be several orders of 
magnitude greater than these other hazards. It is not uncommon for the peak 
pressure to be in excess of 100 pounds per square inch (psi) on a building in an 
urban setting for a vehicle weapon parked along the curb. At these pressure levels, 
major damages and failure are expected.
_ Explosive pressures decay extremely rapidly with distance from the source. 
Therefore, the damages on the side of the building facing the explosion may be 
significantly more severe than on the opposite side. As a consequence, direct air-
blast damages tend to cause more localized damage.
_ The duration of the event is very short, measured in thousandths of a second, or 
milliseconds. This differs from earthquakes and wind gusts, which are measured in 
seconds, or sustained wind or flood situations, which may be measured in hours. 
Because of this, the mass of the structure has a strong mitigating effect on the 
response because it takes time to mobilize the mass of the structure. By the time 
the mass is mobilized, the loading is gone, thus mitigating the response. This is the 
opposite of earthquakes, whose imparted forces are roughly in the same timeframe 
as the response of the building mass, causing a resonance effect that can worsen 
the damage.
4.1.1 Building Damage
The extent and severity of damage and injuries in an explosive event cannot be 
predicted with perfect certainty. Past events show that the unique specifics of the 
failure sequence for a building significantly affect the level of damage. Despite these 
uncertainties, it is possible to give some general indications of the overall level of 
damage and injuries to be expected in an explosive event, based on the size of the 
explosion, distance from the event, and assumptions about the construction of the 
building. 
Damage due to the air-blast shock wave may be divided into direct air-blast effects and 
progressive collapse. Direct air-blast effects are damage caused by the high-intensity 
pressures of the air-blast close in to the explosion and may induce the localized failure 
of exterior walls, windows, floor systems, columns, and girders. Progressive collapse is 
discussed in Section 3.2.
The air-blast shock wave is the primary damage mechanism in an explosion. The 
pressures it exerts on building surfaces may be several orders of magnitude greater 
than the loads for which the building is designed. The shock wave also acts in 
directions that the building may not have been designed for, such as upward on the 
floor system. In terms of sequence of response, the air-blast first impinges on the 
weakest point in the vicinity of the device closest to the explosion, typically the 
exterior envelope of the building. The explosion pushes on the exterior walls at the 
lower stories and may cause wall failure and window breakage. As the shock wave 
continues to expand, it enters the structure, pushing both upward and downward on 
the floors (see Figure 4-1).
Floor failure is common in large-scale vehicle-delivered explosive attacks, because 
floor slabs typically have a large surface area for the pressure to act on and a 
comparably small thickness. In terms of the timing of events, the building is engulfed 
by the shockwave and direct air-blast damage occurs within tens to hundreds of mil-
liseconds from the time of detonation. If progressive collapse is initiated, it typically 
occurs within seconds.
Glass is often the weakest part of a building, breaking at low pressures compared to 
other components such as the floors, walls, or columns. Past incidents have shown that 
glass breakage may extend for miles in large external explosions. High-velocity glass 
fragments have been shown to be a major contributor to injuries in such incidents. For 
incidents within downtown city areas, falling glass poses a major hazard to passersby on 
the sidewalks below and prolongs post-incident rescue and cleanup efforts by leaving 
tons of glass debris on the street. Specific glazing design considerations are discussed 
in Chapter 3.
4.1.2 Casualties and Injuries
Blast can cause significant casualties. During the bombing of the Murrah Federal 
Building, 168 people were killed. Severity and type of injury patterns incurred in 
explosive events may be related to the level of structural damage. The high pressure of 
the air-blast that enters through broken windows can cause eardrum damage and lung 
collapse. As the air-blast damages the building components in its path, missiles are 
generated that cause impact injuries. Airborne glass fragments typically cause 
penetration or laceration-type injuries. Larger fragments may cause non-penetrating, 
or blunt trauma, injuries. Finally, the air-blast pressures can cause occupants to be 
bodily thrown against objects or to fall. Lacerations due to high-velocity flying glass 
fragments have been responsible for a significant portion of the injuries received in 
explosion incidents. In the bombing of the Murrah Federal Building in Oklahoma 
City, for instance, 40 percent of the survivors in the building cited glass as contributing 
to their injuries. Within nearby buildings, laceration estimates ranged from 25 percent 
to 30 percent.
4.1.3 Levels of Protection
The amount of explosive and the resulting blast dictate the level of protection 
required to prevent a building from collapsing or minimize injuries and deaths.  Table 
4-1 shows how the DoD correlates levels of protection with potential damage and 
expected injuries. The GSA and the Interagency Security Committee (ISC) also use 
the level of protection concept. However, wherein DoD has five levels, they have 
established four levels of protection. The GSA and ISC levels of protection can be 
found in GSA PBS-P100, Facilities Standards for the Public Buildings Service, November 
2000, Section 8.6.
The levels of protection above can roughly be correlated for conventional 
construction without any blast hardening to the following incident pressures as shown 
in Table 4-2.
Table 4-2: Correlation of DoD Level of Protection to Incident Pressure

Level of 
Protection
Incident Pressure 
(psi)
High
1.1
Medium
1.8
Low
2.3

Figure 4-2 shows an example of a range-to-effect chart that indicates the distance or 
stand-off to which a given size bomb will produce a given effect (see Section 4.2). This 
type of chart can be used to display the blast response of a building component or 
window at different levels of protection. It can also be used to consolidate all building 
response information to assess needed actions if the threat weapon-yield changes. For 
example, an amount of explosives are stolen and indications are that they may be used 
against a specific building. A building-specific range-to-effect chart will allow quick 
determination of the needed stand-off for the amount of explosives in question, once 
the explosive weight is converted to trinitrotoluene (TNT) equivalence. Given an ex-
plosive weight and a stand-off distance, Figure 4-2 can be used to predict damage for 
nominal building construction.
For design purposes, large scale truck bombs typically contain 10,000 pounds or more 
of TNT equivalent, depending on the size and capacity of the vehicle used to deliver 
the weapon. Vehicle bombs that utilize vans down to small sedans typically contain 
4,000 to 500 pounds of TNT equivalent, respectively. A briefcase bomb is 
approximately 50 pounds, and a pipe bomb is generally in the range of 5 pounds of 
TNT equivalent. Research performed as part of the threat assessment process should 
identify bomb sizes used in the locality or region. Security consultants have valuable 
information that may be used to evaluate the range of likely charge weights.
Figures 4-3 and 4-4 show blast effects predictions for a high school based on a typical 
car bomb, and a typical large truck bomb detonated in the schoolÕs parking lot, 
respectively. A computer-based GIS was used to analyze the schoolÕs vehicular access 
and circulation pattern to determine a reasonable detonation point for a vehicle 
bomb. Structural blast analysis was then performed using nominal explosive weights 
and a nominal school structure. The results are shown in Figures 4-3 and 4-4. The red 
ring indicates the area in which structural damage is predicted. The orange and 
yellow rings indicate predictions for lethal injuries and severe injuries from glass, 
respectively. Please note that nominal inputs were used in this analysis and they are 
not a predictive examination.
In the case of a stationary vehicle bomb, knowing the size of the bomb (TNT 
equivalent in weight), its distance from the structure, how the structure is put together, 
and the materials used for walls, framing, and glazing allows the designer to 
determine the level of damage that will occur and the level of protection achieved. 
Whether an existing building or a new construction, the designer can then select 
mitigation measures as presented in this chapter and Chapters 2 and 3 to achieve the 
level of protection desired.
4.2 STAND-OFF DISTANCE AND THE EFFECTS OF BLAST  
Energy from a blast decreases rapidly over distance. In general, the cost to provide 
asset protection will decrease as the distance between an asset and a threat increases, 
as shown in Figure 4-5. However, increasing stand-off also requires more land and 
more perimeter to secure with barriers, resulting in an increased cost not reflected in 
Figure 4-5. As stand-off increases, blast loads generated by an explosion decrease and 
the amount of hardening necessary to provide the required level of protection 
decreases. 
The critical location of the weapon is a function of the site, the building layout, and 
the security measures in place. For vehicle bombs, the critical locations are considered 
to be at the closest point that a vehicle can approach on each side, assuming that all 
security measures are in place. Typically this is a vehicle parked along the curb directly 
outside the building, or at the entry control point where inspection takes place. For 
internal weapons, location is dictated by the areas of the building that are publicly 
accessible (e.g., lobbies, corridors, auditoriums, cafeterias, or gymnasiums). Range or 
stand-off is measured from the center of gravity of the charge located in the vehicle or 
other container to the building component under consideration. 
Defining appropriate stand-off distance for a given building component to resist 
explosive blast effects is difficult. Often in urban settings, it is either not possible or 
practical to obtain appropriate stand-off distance. Adding to the difficulty is the fact 
that defining appropriate stand-off distance requires a prediction of the explosive 
weight of the weapon. In the case of terrorism, this is tenuous at best.
The DoD prescribes minimum stand-off distances based on the required level of 
protection. Where minimum stand-off distances are met, conventional construction 
techniques can be used with some modifications. In cases where the minimum stand-
off cannot be achieved, the building must be hardened to achieve the required level 
of protection (see the DoD UFC Ð DoD Minimum Antiterrorism Standards for Buildings, 
UFC 4-010-01, 31 July 2002).
The first step in predicting blast effects on a building is to predict blast loads on the 
structure. Because blast pressure pulse varies based on stand-off distance, angle of 
incidence, and reflected pressure over the exterior of the building, the blast load 
predictions can be very complex. Consultants may use sophisticated methods such as 
Computational Fluid Dynamics (CFD) computer programs to predict blast loads. 
These complex programs require special equipment and training to run.
In most cases, especially for design purposes, more simplified methods may be used by 
blast consultants to predict blast loads. Tables of pre-determined values (see GSA 
Security Reference Manual: Part 3 Ð Blast Design & Assessment Guidelines, July 31, 2001) or 
computer programs may be used such as: 1
_ ATBLAST (GSA)
_ CONWEP (U.S. Army Engineer Research and Development Center)
Figure 4-6 provides a quick method for predicting the expected overpressure 
(expressed in pounds per square inch or psi) on a building for a specific explosive 
weight and stand-off distance. Enter the x-axis with the estimated explosive weight a 
terrorist might use and the y-axis with a known stand-off distance from a building. By 
correlating the resultant effects of overpressure with other data, the degree of damage 
that the various components of a building might receive can be estimated. The vehicle 
icons in Figure 4-6 indicate the relative size of the vehicles that might be used to 
transport various quantities of explosives.
The analysis of structures subjected to the effects of an explosion is very complex and 
requires an understanding of structural engineering, dynamics, strengths of materials, 
and specialized training in explosive effects. Such analysis should be performed by 
engineers who can conduct a complex analysis that is both time-dependent and 
accounts for non-linear behavior. In the absence of such an analysis of a specific 
structure, it is possible to provide rough approximations of building damages to be 
expected in an explosive event. Table 4-3 provides basic estimates of incident 
pressures at which different types of damage generally occur to buildings based on the 
incident pressures determined in Figure 4-6.


The following additional references are recommended:
_ Air Force Engineering and Services Center. Protective Construction Design Manual, ESL-TR-87-
57. Prepared for Engineering and Services Laboratory, Tyndall Air Force Base, FL. (1989).
_ U.S. Department of the Army. Security Engineering, TM 5-853 and Air Force AFMAN 32-1071, 
Volumes 1, 2, 3, and 4. Washington, DC, Departments of the Army and Air Force. (1994).
_ U.S. Department of the Army. Structures to Resist the Effects of Accidental Explosions, Army TM 
5-1300, Navy NAVFAC P-397, AFR 88-2. Washington, DC, Departments of the Army, Navy, 
and Air Force. (1990).
_ U.S. Department of Energy. A Manual for the Prediction of Blast and Fragment Loading on 
Structures, DOE/TIC 11268. Washington, DC, Headquarters, U.S. Department of Energy. (1992).
_ U.S. General Services Administration. GSA Security Reference Manual: Part 3 Blast Design and 
Assessment Guidelines. (2001).
_ Biggs, John M. Introduction to Structural Dynamics. McGraw-Hill. (1964).
_ The Institute of Structural Engineers. The Structural EngineerÕs Response to Explosive Damage. 
SETO, Ltd., 11 Upper Belgrave Street, London SW1X8BH. (1995).
_ Mays, G.S. and Smith, P.D. Blast Effects on Buildings: Design of Buildings to Optimize 
Resistance to Blast Loading. Thomas Telford Publications, 1 Heron Quay, London E14 4JD. 
(1995).
_ National Research Council. Protecting Buildings from Bomb Damage. National Academy Press. 
(1995)


Figure 4-1 Blast pressure effects on a structur


Table 4-1: DoD Minimum Antiterrorism (AT) Standards for New Buildings

Level of 
Protection
Potential Structural Damage
Potential Door and 
Glazing Hazards
Potential Injury
Below AT 
standards
Severely damaged Ð frame 
collapse/massive destruction. 
Little left standing. 
Doors and windows fail and 
result in lethal hazards 
Majority of 
personnel suffer 
fatalities. 
Very Low
Heavily damaged Ð onset of 
structural collapse. Major 
deformation of primary and 
secondary structural members, 
but progressive collapse is 
unlikely. Collapse of non-
structural elements. 
Glazing will break and is 
likely to be propelled into 
the building, resulting in 
serious glazing fragment 
injuries, but fragments will 
be reduced. Doors may be 
propelled into rooms, 
presenting serious hazards. 
Majority of 
personnel suffer 
serious injuries. 
There are likely to 
be a limited number 
(10 percent to 25 
percent) of fatalities. 
Low
Damaged Ð unrepairable. Major 
deformation of non-structural 
elements and secondary 
structural members and minor 
deformation of primary structural 
members, but progressive 
collapse is unlikely. 
Glazing will break, but fall 
within 1 meter of the wall 
or otherwise not present a 
significant fragment hazard. 
Doors may fail, but they 
will rebound out of their 
frames, presenting minimal 
hazards. 
Majority of 
personnel suffer 
significant injuries. 
There may be a few 
(<10 percent) 
fatalities. 
Medium
Damaged Ð repairable. Minor 
deformations of non-structural 
elements and secondary 
structural members and no 
permanent deformation in 
primary structural members. 
Glazing will break, but will 
remain in the window 
frame. Doors will stay in 
frames, but will not be 
reusable. 
Some minor injuries, 
but fatalities are 
unlikely. 
High
Superficially damaged. No 
permanent deformation of 
primary and secondary structural 
members or non-structural 
elements. 
Glazing will not break. 
Doors will be reusable. 
Only superficial 
injuries are likely. 

SOURCE: THE DoD UNIFIED FACILITIES CRITERIA (UFC), DoD MINIMUM ANTITERRORISM STANDARDS FOR BUILDINGS, 
UFC 4-010-01, 31 JULY 200


SOURCE: DEFENSE THREAT REDUCTION AGENC


Figure 4-2  Explosives environments - blast range to effect


Figure 4-3    Blast analysis of a high school for a typical car bomb detonated in the schoolÕs parking 
lo


Figure 4-4    Blast analysis of a high school for a typical large truck bomb detonated in the schoolÕs 
parking lo


Figure 4-5 Relationship of cost to stand-off distance
SOURCE: U.S. AIR FORCE, INSTALLATION FORCE PROTECTION GUID


 1For security reasons, the distribution of these computer programs is limited.


SOURCE: U.S. AIR FORCE, INSTALLATION FORCE PROTECTION GUID


Figure 4-6  Incident overpressure measured in pounds per square inch, as a function of stand-off 
distance and net explosive weight (pounds-TNT


Table 4-3: Damage Approximations

Damage
Incident Overpressure 
(psi)
Typical window glass breakage
0.15 Ð 0.22
Minor damage to some buildings
0.5 Ð 1.1
Panels of sheet metal buckled
1.1 Ð 1.8
Failure of concrete block walls
1.8 Ð 2.9
Collapse of wood framed buildings
Over 5.0
Serious damage to steel framed buildings
4 Ð 7
Severe damage to reinforced concrete 
structures
6 Ð 9
Probable total destruction of most buildings
10 Ð 12

SOURCES: EXPLOSIVE SHOCKS IN AIR, KINNEY & GRAHM, 1985; FACILITY DAMAGE AND PERSONNEL INJURY FROM 
EXPLOSIVE BLAST, MONTGOMERY & WARD, 1993; AND THE EFFECTS OF NUCLEAR WEAPONS, 3RD EDITION, GLASSTONE 
& DOLAN, 1977