EXPLOSIVE BLAST     


This chapter discusses blast effects, building 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. Ingredients for homemade bombs are easily 
obtained on the open market as are the techniques for making bombs. Also, 
explosive events are easy and quick to execute. Vehicle bombs have the added 
advantage of being able to bring a large quantity of explosives to the doorstep of the 
target undetected. Finally, terrorists often attempt to use the dramatic component of 
explosions, in terms of the sheer destruction they cause, to generate media coverage 
in hopes of transmitting their political message to the public. 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 explosive blast, see FEMA 427, Primer for Design of Commercial Buildings to 
Mitigate Terrorist Attacks.
4.1 BLAST EFFECTS
When a high order explosion is initiated, a very rapid exothermic chemical reaction 
occurs. As the reaction progresses, the solid or liquid explosive material is converted 
to very hot, dense, high-pressure gas. The explosion products initially expand at very 
high velocities in an attempt to reach equilibrium with the surrounding air, causing 
a shock wave. A shock wave consists of highly compressed air, traveling radially 
outward from the source at supersonic velocities. Only one-third of the chemical 
energy available in most high explosives is released in the detonation process. The 
remaining two-thirds is released more slowly as the detonation products mix with air 
and burn. This afterburning process has little effect on the initial blast wave because 
it occurs much slower than the original detonation. However, later stages of the blast 
wave can be affected by the afterburning, particularly for explosions in confined 
spaces. As the shock wave expands, pressures decrease rapidly (with the cube of the 
distance) because of geometric divergence and the dissipation of energy in heating 
the air. 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 . An 
explosion can be visualized as a "bubble" of highly compressed air that expands until 
reaching equilibrium with the surrounding air.
Explosive detonations create an incident blast wave, characterized by an almost 
instantaneous rise from atmospheric pressure to a peak overpressure. As the shock 
front expands pressure decays back to ambient pressure, a negative pressure phase 
occurs that is usually longer in duration than the positive phase as shown in Figure 4-
1. The negative phase is usually less important in a design than the positive phase.
When the incident pressure wave impinges on a structure that is not parallel to the 
direction of the waves travel, it is reflected and reinforced, producing what is known 
as reflected pressure. The reflected pressure is always greater than the incident 
pressure at the same distance from the explosion. The reflected pressure varies with 
the angle of incidence of the shock wave. When the shock wave impinges on a 
surface that is perpendicular to the direction it is traveling, the point of impact will 
experience the maximum reflected pressure. When the reflecting surface is parallel to 
the blast wave, the minimum reflected pressure or incident pressure will be 
experienced. In addition to the angle of incidence, the magnitude of the peak 
reflected pressure is dependent on the peak incident pressure, which is a function of 
the net explosive weight and distance from the detonation.
Figure 4-2 shows typical reflected pressure coefficients versus the angle of incidence 
for four different peak incident pressures. The reflected pressure coefficient equals 
the ratio of the peak reflected pressure to the peak incident pressure (Cr = Pr / Pi). 
This figure shows that reflected pressures for explosive detonations can be almost 13 
times greater than peak incident pressures and, for all explosions, the reflected 
pressure coefficients are significantly greater closer to the explosion.
Impulse is a measure of the energy from an explosion imparted to a building. Both 
the negative and positive phases of the pressure-time waveform contribute to impulse. 
Figure 4-3 shows how impulse and pressure vary over time from a typical explosive 
detonation. The magnitude and distribution of blast loads on a structure vary greatly 
with several factors:
_		Explosive properties (type of material, energy output, and quantity of 
explosive)
_		Location of the detonation relative to the structure
_		Reinforcement of the pressure pulse through its interaction with the ground 
or structure (reflections)
The reflected pressure and the reflected impulse are the forces to which the building 
ultimately responds. These forces vary in time and space over the exposed surface of 
the building, depending on the location of the detonation in relation to the 
building. Therefore, when analyzing a structure for a specific blast event, care should 
be taken to identify the worst case explosive detonation location.
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 incident pressure to be in excess of 100 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. In an urban setting, however, 
reflections off surrounding buildings can increase damages to the opposite side.
_		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. A discussion of 
progressive collapse can be found in Chapter 3.
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 4-4).
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 shock wave and direct air-blast damage occurs within tens to 
hundreds of milliseconds 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 Injuries
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 Murrah Federal 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 minimizing 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 the 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 incident pressures 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 4-5 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, 
after the explosive weight is converted to TNT equivalence. 
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. Given an explosive 
weight and a stand-off distance, Figure 4-5 can be used to predict damage for nominal 
building construction.
Figures 4-6 and 4-7 show blast effects predictions for a building based on a typical car 
bomb and a typical large truck bomb detonated in the buildings parking lot, 
respectively. A computer-based Geographic Information System (GIS) was used to 
analyze the building's 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 building structure. The 
results are shown in Figures 4-6 and 4-7. The red ring indicates the area in which 
structural collapse 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 in 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 4-8. However, increasing stand-off also requires more land and 
more perimeter to secure with barriers, resulting in an increased cost not reflected in 
Figure 4-8. 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. Figure 4-9 shows how the impact of a blast will decrease as the stand-off 
distance increases, as indicated in the blast analysis of the Khobar Towers incident. 
Increasing the stand-off distance from 80 to 400 feet would have significantly limited 
the damage to the building and hazard to occupants, the magnitude of which is 
shown as the yellow and red areas in Figure 4-9. Additional concepts of stand-off 
distance are discussed in Section 2.3. 
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 Unified Facilities Criteria  DoD Minimum Antiterrorism 
Standards for Buildings, UFC 4-010-01, 31 July 2002).
The GSA and ISC Security Criteria do not require or mandate specific stand-off 
distances. Rather, they provide protection performance criteria. In order to 
economically meet these performance standards, they present recommended stand-
off distances for vehicles that are parked on adjacent properties and for vehicles that 
are parked on the building site (see GSA Security Criteria, Draft Revision, October 8, 1997, 
and ISC Security Design Criteria for New Federal Office Buildings and Major Modernization 
Projects, May 28, 2001). 
Site and layout design guidance as well as specific mitigation measures to enhance 
stand-off and enhance protection from explosive blast are discussed in Chapter 2. 
4.3 PREDICTING BLAST EFFECTS
4.3.1 Blast Load Predictions
The first step in predicting blast effects on a building is to predict blast loads on the 
structure. For a detonation that is exterior to a building, it is the blast pressure pulse 
that causes damage to the building. Because the pressure pulse varies based on stand-
off distance, angle of incidence, and reflected pressure over the exterior of the 
building, the blast load prediction should be performed at multiple threat locations; 
however, worse case conditions are normally used for decision-making.
For complex structures requiring refined estimates of blast load, blast 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. The overpressure is assumed to 
instantaneously rise to its peak value and decay linearly to zero in a time known as the 
duration time. In order to obtain the blast load, a number of different tools can be 
used. Tables of pre-determined values may be used (see GSA Security Reference Manual: 
Part 3  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 4-10 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 Figures 4-5 and 4-10 indicate the relative size of the vehicles that 
might be used to transport various quantities of explosives.
4.3.2 Blast Effects Predictions
After the blast load has been predicted, damage levels may be evaluated by explosive 
testing, engineering analysis, or both. Explosive testing is actively conducted by 
Federal Government agencies such as the Defense Threat Reduction Agency, DOS, 
and GSA. Manufacturers of innovative products also conduct explosive test programs 
to verify the effectiveness of their products. 
Often, testing is too expensive an option for the design community and an 
engineering analysis is performed instead. To accurately represent the response of an 
explosive event, the analysis needs to be time dependent and account for non-linear 
behavior.
Non-linear dynamic analysis techniques are similar to those currently used in 
advanced seismic analysis. Analytical models range from equivalent single-degree-of-
freedom (SDOF) models to finite element (FEM) representation. In either case, 
numerical computation requires adequate resolution in space and time to account 
for the high-intensity, short-duration loading and non-linear response. The main 
problems are the selection of the model, the appropriate failure modes, and, finally, 
the interpretation of the results for structural design details. Whenever possible, 
results are checked against data from tests and experiments on similar structures and 
loadings. Available computer programs include:
_ AT Planner (U.S. Army Engineer Research and Development Center)
_ BEEM (Technical Support Working Group)
_ BLASTFX (Federal Aviation Administration)
Components such as beams, slabs, or walls can often be modeled by a SDOF system. 
The response can be found by using the charts developed by Biggs and military 
handbooks. For more complex elements, the engineer must resort to numerical time 
integration techniques. The time and cost of the analysis cannot be ignored in 
choosing analytical procedures. SDOF models are suitable for numerical analysis on 
PCs and micro-computers, but the most sophisticated FEM systems (with non-linear 
material models and options for explicit modeling of reinforcing bars) may have to 
be carried out on mainframes. Because the design analysis process is a sequence of 
iteration, the cost of analysis must be justified in terms of benefits to the project and 
increased confidence in the reliability of the results. In some cases, an SDOF 
approach will be used for the preliminary design and a more sophisticated approach, 
using finite elements, will be used for the final design.
Table 4-3 provides estimates of incident pressures at which damage may occur.
Table 4-3: Damage Approximations

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

SOURCE: 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

Additional sources of information: 
_ 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. Fundamentals of Protective Design for Conventional Weapons, 
TM 5-855-1. Washington, DC, Headquarters, U.S. Department of the Army. (1986).
_ 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 Engineers 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 Typical pressure-time histor


Figure 4-2 Reflected pressure coefficient vs. angle of incidenc


The integrated area under the pressure verse time function is known as the impulse: 
 I = P(t)dt
 I = impulse (psi-ms or Mpa-ms)
 P = Pressure (psi or MPa)
 T = time (ms


Figure 4-3 Typical impulse wavefor


Figure 4-4 Blast pressure effects on a structur


SOURCE: NAVAL FACILITIES ENGINEERING SERVICE CENTER, USERS GUIDE ON PROTECTION AGAINST TERRORIST 
VEHICLE BOMBS, MAY 199


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. 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 - 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  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  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. 


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


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


SOURCE: DEFENSE THREAT REDUCTION AGENC


Figure 4-5 Explosives environments - blast range to effect


Figure 4- 6 Blast analysis of a building for a typical car bomb detonated in the buildings parking lot


Figure 4-7 Blast analysis of a building for a typical large truck bomb detonated in the buildings 
parking lot


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


Figure 4-8 Relationship of cost to stand-off distanc


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


Figure 4-9 Stand-off distance and its relationship to blast impact as modeled on the Khobar Towers 
sit


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


Figure 4-10 Incident overpressure measured in pounds per square inch, as a function of stand-off 
distance and net explosive weight (pounds-TNT


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