Unit Objectives

Explainthe basic physics involved during an explosive 
blast event, whether by terrorism or technological 
accident.

Explainbuilding damage and personnel injury resulting 
from the blast effects upon a building.

Performan initial prediction of blast loading and effects 
based upon incident pressure.


Unit VII: Explosive Blast

Units I-VI covered the Risk Assessment Process

Units VII and VIII explain Explosive Blast, CBR Agents, 
and their effects

Units IX and X demonstrate techniques for site layout and 
building design to counter or mitigate manmade threats 
and similar technological hazards


Blast Loading 

Factors

Explosive properties

..Type
..Energy output
(TNT equivalency)
..Quantity


FEMA 427, Figure 2-1: Schematic of Vehicle Weapon Threat 
Parameters and Definitions, p. 2-2



Typical Incident Pressure Waveform

FEMA426, Figure 4-1: Typical Pressure-Time History, p. 4-2



Incident and Reflected Pressure

ReflectedPressureEquivalent pressure occurs at Scaled Distance =
Distance / (Net Explosive Weight, TNT equivalent)1/3Blast energy lost at rate of volume increase in X, Y, and Z 
directionsIncidentPressure

FEMA 426, Figure 4-2: Reflected Pressure Coefficient vs. 
Angle of Incidence, p. 4-3

Reflected Pressure/Angle of Incidence

Typical Blast Impulse Waveform

FEMA 426, Figure 4-3: Typical Impulse Waveform, p. 4-4



Blast Loading Factors

Location of explosive 

relative to structure

..Stand-off distance
..Reflections and 
reflection angle
..Ground
..Buildings
..Identify worst case




Blast Compared to Natural Hazards

Higher incident pressures and relatively low impulse

..High explosive (C-4)
..Medium explosive


(black powder)

..Low explosive (gasoline)
..Aircraft or vehicle crash 
combines kinetic energy 
(velocity, mass), explosive
loads, and fuel/fire
..200 mph hurricane generates only 0.8 psi, but with 
very large impulse




Blast Compared to Natural Hazards 

Direct airblastcauses more 

localized damage

..Component breakage
..Penetration and shear
..Building’s other side farther away
..Reflections can increase damage 
on any side


Greater mass historically used 

for blast protection

..Greater mass usually detrimental 
during earthquake due to resonance




Factors Contributing to Building 
Damage 

First approximations based upon:

..Quantity of explosive
..Stand-off distance between building and explosive
..Assumptions about building characteristics



Types of Building Damage

Direct Air Blast

..Component failure
..Additional damage after breaching


Collapse

..Localized
..Progressive



Blast Pressure Effects 

FEMA 426, Figure 4-4: Blast Pressure Effects on a Structure, p. 4-7



Blast Pressure Effects 

FEMA 426, Figure 4-4: Blast Pressure Effects on a Structure, p. 4-7



Blast Pressure Effects 

FEMA 426, Figure 4-4: Blast Pressure Effects on a Structure, p. 4-7



Causes of Blast Injuries

Overpressure

..Eardrum rupture
..Lung collapse/failure


Blast Wave

..Blunt trauma, lacerations, and impalement



Causes of Blast Injuries

Fragmentation

Bomb or vehicle

Street furniture or jersey barriers

Building component failure

..Glass –predominant 
..Walls
..Floors




Murrah Federal Building, Oklahoma City



From Journal of American Medical Association, August 7, 1996

Murrah Federal Building, Oklahoma City


Text Box: The majority of deaths were due to the collapsing structure
Murrah Federal Building, Oklahoma City

From FEMA Oklahoma City Bombing Report 9-0300 / FEMA 277, 
August 1996

1.8 square miles

FEMA 426, Adapted from Table 4-1: DoD Minimum Antiterrorism 
Standards for New Buildings, p. 4-9

Text Box: Potential Injury Text Box: CONVENTIONAL CONSTRUCTION
FEMA 426, Adapted from Table 4-1: DoD Minimum Antiterrorism 
Standards for New Buildings, p. 4-9

Text Box: Potential Injury Text Box: CONVENTIONAL CONSTRUCTION
Nominal Range-to-Effect Chart

FEMA 426, Figure 4-5: Explosive Environments –Blast 
Range to Effects, p. 4-11

25

YIELD (˜TNT Equiv.)4,000 lb. 

Reflected PRESSURE 9,600 psi.

Stand-off 15 feet

YIELD (˜TNT Equiv.)20,000 lb. 

Reflected PRESSURE 800 psi.

Stand-off 80 feet

166 killed

Comparison of Stand-off

Murrah Federal Building

Khobar Towers

19 killed



FEMA 426, Figure 4-7: Blast Analysis of Building for Typical Large Truck 
Bomb Detonated in Building’s Parking Log, p. 4-12

Vulnerability Radii

Iso-Damage Contours

Stand-off required to 
prevent lethal glass 
injuryStand-off required to 
prevent lethal wall 
damageStand-off required to 
prevent collapse

FEMA 426, Figure 4-8: Relationship of Cost to Stand-off Distance,

p. 4-13

Cost Versus Stand-off

Blast Load Predictions

Incident and reflected pressure and impulse

..Software
..Computational Fluid Dynamics
..ATBLAST (GSA)
..CONWEP (US Army)
..Tables and charts of predetermined values



Pressure 

versus 

Distance

FEMA 426, Figure 4-10: Incident Overpressure Measured in Pounds Per 
Sq. Inch, as a Function of Stand-Off Distance and Net Explosive 
Weight, p. 4-17

250

Blast Damage Estimates

Assumptions -pressure and material

..Software -SDOF
..AT Planner (U.S. Army)
..BEEM (TSWG)
..BlastFX (FAA)
..Software -FEM
..Tables and charts of predetermined values



FEMA 426, Table 4-3: Damage Approximations, 
p. 4-19

Text Box: 3.5Text Box: Probable total destruction of most buildings (1)
Manchester Bombing



Summary

Explosive blast physics

Blast damage to buildings

Injury to personnel

Prediction of loading, damage, and injury

..Range-to-effect chart
..Incident pressure chart



Unit VII Case Study Activity

Explosives Environment, Stand-off Distance, and the 
Effects of Blast

Background

Purpose of activity: check on learning about explosive blast

Requirements

Refer to Case Study and FEMA 426

Answer worksheet questions