Protection Levels. The entire building structure or
For higher levels of protection from blast, cast-in-place
portions of the structure will be assigned a protection level
reinforced concrete is normally the construction type of
according to the facility-specific risk assessment.
choice. Other types of construction such as properly
Protection levels for ballistics and forced entry are
designed and detailed steel structures are also allowed.
described in New Construction in this section. The
Several material and construction types, while not
following are definitions of damage to the structure and
disallowed by these criteria, may be undesirable and
exterior wall systems from the bomb threat for each
uneconomical for protection from blast.
protection level:
To economically provide protection from blast, inelastic or
Low and Medium/Low Level Protection - Major damage.
post elastic design is standard. This allows the structure
The facility or protected space will sustain a high level of
to absorb the energy of the explosion through plastic
damage without progressive collapse. Casualties will
deformation while achieving the objective of saving lives.
occur and assets will be damaged. Building components,
To design and analyze structures for blast loads, which
including structural members, will require replacement, or
are highly nonlinear both spatially and temporally, it is
the building may be completely unrepairable, requiring
essential that proper dynamic analysis methods be used.
demolition and replacement.
Static analysis methods will generally result in
unachievable or uneconomical designs.
Medium Level Protection - Moderate damage, repairable.
The designer should recognize that components might
The facility or protected space will sustain a significant
act in directions for which they are not designed. This is
degree of damage, but the structure should be reusable.
due to the engulfment of structural members by blast,
Some casualties may occur and assets may be damaged.
the negative phase, the upward loading of elements, and
Building elements other than major structural members
dynamic rebound of members. Making steel reinforcement
may require replacement.
(positive and negative faces) symmetric in all floor slabs,
Higher Level Protection - Minor damage, repairable. The
roof slabs, walls, beams and girders will address this
facility or protected space may globally sustain minor
issue. Symmetric reinforcement also increases the
damage with some local significant damage possible.
ultimate load capacity of the members.
Occupants may incur some injury, and assets may receive
Lap splices should fully develop the capacity of the
minor damage.
reinforcement.
Good Engineering Practice Guidelines
Lap splices and other discontinuities should be staggered.
Ductile detailing should be used for connections,
The following are rules of thumb commonly used to
especially primary structural member connections.
mitigate the effects of blast on structures. Details and more
There should be control of deflections around certain
complete guidance are available in the Technical Manuals
members, such as windows, to prevent premature failure.
listed in the New Techniques, Methods and References
Additional reinforcement is generally required.
section, and in the references below. The following guide-
lines are not meant to be complete, but are provided to
Balanced design of all building structural components is
assist the designer in the initial evaluation and selection of
desired. For example, for window systems, the frame and
design approaches.
anchorage shall be designed to resist the full capacity of
the weakest element of the system.
252 F A C I L I T I E S S T A N D A R D S
FOR THE
PUBLIC
BUILDINGS
SERVICE
8.7 Structural Engineering
Revised March 2005 PBS-P100
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