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By Nancy A. Renfroe, PSP and Joseph L. Smith, PSP
Today’s environment of
increased risks requires
protective design and risk
management for most facilities.
This article presents an
overview of facility risk
assessments and focuses
on the critical area of
window hazard mitigation.
Because glass often causes the majority
of injuries from a bomb blast, mitigation
can significantly reduce those risks
for employees or building visitors.
The Basics of Facility Risk Assessments
All facilities face some risks associated
with natural events, accidents, or
intentional (criminal or terrorist) acts to
cause harm. Regardless of the nature of
the threat, facility owners have a
responsibility to manage associated
risks. Introducing properly designed
hazard mitigation measures can both
protect occupants and significantly
reduce facility recovery by maintaining
the facility’s envelope during an event.
The best course of action is to start
with a multi-hazard risk assessment
performed by a qualified security professional.
The following are
descriptions of assessment types:
- Threat Assessment: A threat assessment
evaluates the potential aggressors and
the type of tactics they are most likely
to employ. Consider a complete
spectrum of threats including natural
(earthquakes, floods, etc.) and manmade
(accidents, terrorist acts, etc.).
For threats involving explosives and other weapons of mass destruction,
quantify the type and/or size of
device. The result of the threat assessment
is a list of credible threats
and/or attack scenarios.
- Physical Security Assessment: A physical
security assessment consists of an
evaluation of the existing countermeasures.
These include fixed
countermeasures (i.e., locks and barriers)
as well as operational and
procedural countermeasures. Include
suggestions for upgrades to existing
countermeasures to meet a protection
goal. Often, a physical security assessment
uses a set of minimum standards
to evaluate existing conditions and
make upgrade recommendations.
- Vulnerability Assessment: A vulnerability
assessment quantifies the potential
impact of specific threat scenarios
based on existing or planned conditions.
The vulnerability assessment
should evaluate potential damage to
assets and injury to people from each
attack scenario. This provides a baseline
for determining the potential
benefits from security and structural
upgrades. The vulnerability assessment
may include detailed analysis of
the loss from an explosive, chemical
or biological attack. You will need
professionals with specific training
and experience in these areas to perform
these detailed analyses. Figure 1
shows a sample of the type of output
you can generate from a detailed
explosive analysis. It shows the potential
damage to a facility from an
explosive attack before and after a
window mitigation project. You can
model similar assessments for other
risks and mitigation projects.
- Risk Assessment: A risk assessment
incorporates the threat assessment,
the physical security assessment and
the vulnerability assessment to evaluate
the potential risks associated
with each threat. The objective of the
risk assessment is to pair the impact
of loss with vulnerability to help the
owner evaluate the acceptability of
different levels of overall risk. This
allows someone to effectively manage
their risks. A risk assessment may or
may not include detailed vulnerability
assessments experts perform (a
blast assessment, chem/bio assessment,
etc.). Obtaining these detailed
assessments may be part of the recommendations
from a risk
assessment. For planned facilities,
you can use a risk assessment to help
develop design criteria.
You can apply threat/vulnerability
assessments and risk analysis to any
facility or organization. Most risk
assessments include the following steps:
- Identify the asset(s) to protect
- Perform a threat assessment to
determine potential threats
- Perform a vulnerability assessment
to evaluate the existing hazard
mitigation measures
- Quantify the level of risk associated
with each threat or asset
- Identify potential upgrades to
reduce risks
- Re-evaluate risk reduction based
on potential upgrades
You should repeat the risk assessment
process every three to five years, or
when significant changes occur. This
could include changes in facility occupancy
or mission, neighboring
facilities, crime rates in the area, etc.
While the best defense against death
and injury from hazardous events is prevention,
this is not always possible.
Therefore, we must learn how to mitigate
the hazards associated with specific
events.
Mitigating the Risks from Hazardous
Window Failure
Hazardous glass fragments are undeniably
a major source of injury and
death in many explosive attacks. A total
of 759 people were injured in the
bombing of the A.P Murrah Federal
Building in Oklahoma City. Of those,
319 were inside the building and 440
were outside or in neighboring buildings.
An Oklahoma State Department
of Health survey found among the 405
injured respondents, 66 percent attributed
their injuries to flying glass or
falling on broken glass.
The department also found slightly
more than five percent of the people
injured from hazardous glass and wall fragments died. Ten percent of those
exposed to high-hazard glass or wall
fragments suffered serious injury with
permanent disabilities and 85 percent
sustained serious but recoverable
injuries. Not only that, but glass-related
injuries aren’t limited to the targeted
facility. There was widespread glass
breakage for more than a mile around
the Murrah Building.
Mitigating the risk of flying glass can
significantly reduce injuries from a
blast. The first step is to ensure a balanced
window system design. For the
overall system to provide adequate protection,
the glazing, frames and
anchorage must all be able to survive
the blast loading. If any one part of the
system fails, the entire system fails.
Similarly, the supporting wall must be
able to handle the loads the window
system transfers to it. If the window
system has a higher capacity than the
supporting wall, when the wall fails it
may blow the entire window system
into the facility.
The United States General Services
Administration developed a method of
evaluating the protection various window
configurations offer. This National
Standard is similar to the rating
schemes the British use. The standard
uses five performance conditions to
indicate the location of fragments
and/or shards after failure. We have
defined those performance conditions in
Figure 2.
Most highly critical at risk facilities
such as a military headquarters or power
plant operations room would require a
performance condition of 1. Childcare
centers require a performance condition
of 2 or better. Major office buildings or
courthouses typically require a performance
condition of 3.
There are several options for increasing
the capacity and/or reducing the
hazards from the glazing portion of a
window system. These include: selecting
an appropriate type of glass,
applying security window film,
installing blast curtains/shields and
using laminated glass.
Types of glass include:
- Plain-float annealed glass is the most
common glass type commercial construction
uses. It has an ultimate
design stress of 4000 psi and is the
most hazardous.
- Heat-strengthened glass is partially
tempered, which increases the ultimate
design stress to 7600 psi. It
gives a higher strength and slightly
reduces the hazard.
- Thermally-tempered glass is fully tempered glass, which increases the
ultimate stress to 16000 psi. It fails in
small cube shaped fragments that are
the least hazardous.
Tests have shown applying security
window film on the interior glazing
surface can significantly reduce the hazard.
Security windows film is typically
4, 6, 7, or 11 mils (1/1000th of an inch)
thick. There are four basic methods to
apply this film, which we illustrate and
explain in Figure 3.
Daylight installation is the lowest
cost and mechanically attached is generally
the highest. The range is roughly
$6-8 per square foot for daylight installation
and $10-20 per square foot for
mechanically attached.
Tests also show that engineered laminated
glass can provide very high levels
of protection at blast pressure/impulse
levels far greater than blast curtains
and/or films. Manufacturers are also
developing new technologies to reduce
the hazards from an explosive event.
These innovative and cost-effective products
and procedures will significantly
enhance our ability to protect people.
Include a qualified blast and security
specialist in the earliest stages of project
design to help you obtain the desired
levels of protection and to minimize the
impact on architecture and cost.
For more information on protecting
facilities from blast, wind and other
hazards visit the following websites:
This article was published in the Disaster Resource GUIDE for Facilities (Fall 2006).
About the Authors
Nancy A. Renfroe, PSP, is the Director, Security
Risk Management for Applied Research Associates.
Joseph L. Smith, PSP, is the Sr. Vice
President and Director of Security Engineering
and Applied Sciences for Applied Research
Associates. The authors can be reached at:
Applied Research Associates, Inc.; (601)-638-
5401 or nrenfroe@ara.com and jsmith@ara.com
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