ISC Security Design Criteria for New Federal Office Buildings and Major Modernization Projects A Review and Commentary (2003) / Chapter Skim
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3. Blast Effects on Buildings and People: A Primer for Users of the ISC Security Design Criteria
3. Blast Effects on Buildings and People: A Primer for Users of the ISC Security Design Criteria
Pages 25-40
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From page 25... ...
The challenge for the building project team is to design and construct federal facilities that provide protection from terrorist explosive threats while at the same time offering desirable workspaces in attractive buildings that are well integrated with the surrounding neighborhood. When these structures are situated on urban sites, it is often difficult to restrict access to preserve effective standoffs distances.
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From page 26... ...
This explanation may help dispel some misconceptions about protective design and elicit more cooperative participation by all stakeholders in implementing effective design strategies from the outset. Stakeholders can readily see the benefits of such basic ways to protect buildings from bomb damage as establishing a secure perimeter, preventing progressive collapse, isolating internal threats from occupied spaces, and mitigating the glass and debris hazard.
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From page 27... ...
These waves reflect off the ground, adjacent structures, and other surfaces, reinforcing the intensity of the blast's effects. These reflections are most pronounced in dense urban environments where neighboring structures can create a canyon effect or where pockets of blast energy can become trapped in re-entrant (concave)
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From page 28... ...
When directed upward, this pressure may be extremely damaging to slabs and columns because it acts counter to the design used to resist gravity loads. Air-blast pressures within a building can actually increase as the pressure waves reflect from surfaces and can cause injuries to the occupants directly by means of physical translation, ear, lung, and other organ damage, or debris from building elements and contents.
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From page 29... ...
Large explosive devices detonated at relatively great standoff distances will produce a large but uniform pressure over the surface of the building; at lesser standoff distances, even a small explosive device can produce locally intense effects, such as shattering load-bearing columns. While the former scenario is likely to govern design of the facade to limit the formation of hazardous debris, the assumption of a smaller, close-in device is likely to control design of the first-floor load-bearing elements to prevent localized failure leading to progressive structural collapse.2 If a large explosive device is detonated close to the structure, global damage 2Progressive collapse occurs when a localized failure overloads adjoining members, causing them to fail, which in turn causes damage disproportionate to the originating localized failure.
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From page 30... ...
Although these smaller bombs lack the power to cause catastrophic structural damage, they can injure and kill people and cause localized damage, particularly to windows and nonstructural elements. To provide a proper basis for risk assessment and cost-benefit evaluation, the information used to define blast loads or expected charge weights must be as unambiguous as possible.
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From page 31... ...
Although there was little structural damage to the five-story reinforced concrete building, the explosion reduced much of the interior to rubble destroying windows, window frames, internal office partitions, and other fixtures on the rear side of the building. The secondary fragmentation from flying glass, internal concrete block walls, furniture, and fixtures caused most of the embassy casualties.
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From page 32... ...
PROTECTIVE DESIGN STRATEGIES Overview Physical protection for buildings involves four basic actions: Establish a secure perimeter. Prevent progressive collapse.
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From page 33... ...
Although it is theoretically possible to predict the effects of a certain charge weight of a known explosive at a specified standoff distance, the actual charge weight of explosive used by a terrorist, the efficiency of the chemical reaction, and the location cannot be reliably predicted. Thus the approach embodied in the ISC Security Design Criteria is to use predefined levels of protection based on an estimation of risk to the facility, taking into account its symbolic importance, mission criticality, and the consequences of loss.
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From page 34... ...
If the charge weight is small, even small increases in standoff distance will significantly reduce blast forces, but if the threat is large, the blast forces may overwhelm the structure despite the addition of 9 or 10 feet of standoff distance, and this measure may not significantly improve the survivability of the occupants or the structure. Glazing Systems 1 1 · 1 11 1 The building exterior is the first real defense against the effects of a bomb; how the facade responds will significantly affect the behavior of the structure and the safety of its occupants.
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From page 35... ...
In addition to hardening3 the members that compose a curtain wall system, attachments to the floor slabs, or spandrel beams, require special attention. These connections must be adjustable to compensate for fabrication tolerances, accommodate differential interstory drifts and thermal deformations, and yet be capable of transferring gravity, wind, and blast loads.
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From page 36... ...
Upgrading existing structures to prevent localized damage from causing a progressive collapse may not be easy using the alternate path method because loss of support at a column line would increase the spans of all beams directly above the zone of damage and require different patterns of reinforcement and different types of connection cletails than those typically used in conventional structural design. Alternatively, columns may be sized, reinforced, or protected to prevent critical damage from a nearby bomb.
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From page 37... ...
Other Building Spaces and Systems The walls surrounding loading docks, mailrooms, and lobbies into which explosives may be introduced before inspection must be hardened to confine an explosive shock wave and must permit the resulting gas pressures to vent into the atmosphere. The isolation of occupied spaces from these vulnerable locations requires adequate reinforcement as well as connections that can resist the collected blast pressures.
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From page 38... ...
The localized loads will deform exterior bays of the structure much more than interior bays; as the blast loading progresses, the shear forces in each story of the building may not necessarily be distributed through the diaphragms in proportion to the framing stiffHess. Furthermore, the characteristic patterns of loading and deformation in a blast event depend to a great extent on the standoff distances, which may be significantly different from those resulting from a seismic excitation
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From page 39... ...
It is generally understood that increasing the ductile behavior of details in response to strong ground motions will increase the ductile behavior in response to blast loading. Yet though seismic design codes contain very useful detailing information that may be directly applied in blast resistance design, it is generally acknowledged that the zones of plastic hinge formation and the extent of ductility demands for seismic response are not necessarily useful.
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From page 40... ...
1996. Physical injuries and fatalities resulting from the Oklahoma City bombing.
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