All western democracies are now acutely aware of the apocalyptic
consequences of a well-orchestrated attack on high-profile government
facilities and other related targets. Many of these buildings are historical,
ornate, listed and constructed using traditional techniques. Many of the modern
retrofitted reinforcement techniques used to protect these structures against
terrorist attacks are unsightly, intrusive or inappropriate. However, security
specialists are well aware that while there might be little that can be done to
defend a building against an aircraft attack, much can be done to defeat the
more traditional car bomb and bullet. The methods available to the structural
engineer to strengthen existing structures and provide resistance to the
effects of a blast attacks are discussed in this paper.
The design of civilian or commercial buildings to withstand the
effects of a terrorist blast is unlike the design of military installations or
the design of embassy buildings. The objectives of the Structural Engineering
Guidelines for the Design of New Embassy Buildings are to prevent heavy damage
to components and structural collapse. Adherence to the provisions of the
guidelines will minimize injuries and loss of life and facilitate the evacuation
and rescue of survivors. The blast-protection objective of any commercial or
public building must be similar to those of embassy structures, that is, to
prevent structural collapse, to save lives, and to evacuate victims.
Architectural and structural features play a significant role in
determining how the building will respond to the blast loading. These features
can include adjacent or underground parking, atriums, transfer girders, slab
configurations, and structural-frame systems. The keep-out distance is vital in
the design of blast resistant structures since it is the key parameter that
determines the blast overpressures that load the building and its structural
elements. The degree of fenestration is another key parameter as it determines
the pressures that enter the structure. The smaller the door and window
openings the Embassies and military structures occupy secure sites with
substantial keep-out distances better protected the occupants are within the
structure. Following these key parameters,
The probability that any single building will sustain damage
from accidental or deliberate explosion is very low, but the cost for those who
are unprepared is very high.
2.1 EXPECTED TERRORIST BLASTS ON STRUCTURES.
External
car bomb
Internal
car bomb
Internal
package
Suicidal
car bombs
2.2 MAJOR CAUSES OF LIFE LOSS AFTER THE
BLAST.
Flying
debris
Broken
glass
Smoke
and fire
Blocked
glass
Power
loss
Communications
breakdown
Progressive
collapse of structure
The goals of blast-resistant design are to:
Reduce
the severity of injury
Facilitate
rescue
Expedite
repair
Accelerate
the speed of return to full operations.
To resist blast loads,
The first requirement is to determine the threat. The major
threat is caused by terrorist bombings. The threat for a conventional bomb is
defined by two equally important elements, the bomb size, or charge weight, and
the standoff distance the minimum guaranteed distance between the blast source
and the target
Another requirement is to keep the bomb as far away as
possible, by maximizing the keepout distance.
No matter what size the bomb, the damage will be less severe the further the
target is from the source.
Structural hardening should actually be the last resort in
protecting a structure; detection and prevention must remain the first line
of defense. As terrorist attacks range from the
small letter bomb to the gigantic truck bomb as experienced in Oklahoma City,
the mechanics of a conventional explosion and their effects on a target must be
addressed.
4.1. MECHANICS OF A CONVENTIONAL EXPLOSION
With the detonation of a mass of TNT at or near the ground
surface, the peak blast pressures resulting from this hemispherical explosion
decay as a function of the distance from the source as the ever-expanding shock
front dissipates with range. The incident peak pressures are amplified by a
reflection factor as the shock wave encounters an object or structure in its
path. Except for specific focusing of high intensity shock waves at near 45°
incidence, these reflection factors are typically greatest for normal incidence
(a surface adjacent and perpendicular to the source) and diminish with the
angle of obliquity or angular position relative to the source. Reflection
factors depend on the intensity of the shock wave, and for large explosives at
normal incidence these reflection factors may enhance the incident pressures by
as much as an order of magnitude Charges situated extremely close to a target
structure impose a highly impulsive, high intensity pressure load over a
localized region of the structure; charges situated further away produce a
lower-intensity, longer-duration uniform pressure distribution over the entire
structure. In short by purely geometrical relations, the larger the standoff,
the more uniform the pressure distribution over the target. Eventually, the
entire structure is engulfed in the shock wave, with reflection and diffraction
effects creating focusing and shadow zones in a complex pattern around the
structure. Following the initial blast wave, the structure is subjected to a
negative pressure, suction phase and eventually to the quasi-static blast wind.
During this phase, the weakened structure may be subjected to impact by debris
that may cause additional damage