Design of Commercial Buildings to Mitigate Terrorist Attacks Quiz

Hint

The purpose of this primer is to introduce concepts that can help building designers, owners, and state and local governments mitigate the threat of hazards resulting from terrorist attacks on new buildings. This primer specifically addresses four high-population, private-sector building types: commercial office, retail, multi-family residential, and light industrial. However, many of the concepts presented here are applicable to other building types and/or existing buildings. The focus is on explosive attack, but the text also addresses design strategies to mitigate the effects of chemical, biological, and radiological attacks.

Hint

From the standpoint of structural design, the vehicle bomb is the most important consideration. Vehicle bombs are able to deliver a sufficiently large quantity of explosives to cause potentially devastating structural damage. Security design intended to limit or mitigate damage from a vehicle bomb assumes that the bomb is detonated at a so-called critical location(see Figure 2-1). The critical location is a function of the site, the building layout, and the security measures in place. For a vehicle bomb, the critical location is taken to be at the closest point that a vehicle can approach, assuming that all security measures are in place. This may be a parking area directly beneath the occupied building, the loading dock, the curb directly outside the facility, or at a vehicle-access control gate where inspection takes place, depending on the level of protection incorporated into the design.

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Despite these uncertainties, it is possible to calculate the expected extent of damage and injuries to be expected in an explosive event, based on the size of the explosion, distance from the event, and assumptions about the construction of the building. Additionally, there is strong evidence to support a relationship between injury patterns and structural damage patterns.

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The design vehicle weapon size will usually be much smaller than the largest credible threat. The design weapon size is typically measured in hundreds of pounds rather than thousands of pounds of TNT equivalent. The decision is usually based on a trade-off between the largest credible attack directed against the building and the design constraints of the project. Further, it is common for the design pressures and impulses to be less than the actual peak pressures and impulses acting on the building. This is the approach that the federal government has taken in their design criteria for federally owned domestic office buildings. There are several reasons for this choice.

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Floor failure is common in large-scale vehicle-delivered explosive attacks, because floor slabs typically have a large surface area for the pressure to act on and a comparably small thickness. Floor failure is particularly common for close-in and internal explosions. The loss of a floor system increases the unbraced height of the supporting columns, which may lead to structural instability.

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• Use positive pressurization of primary egress routes, safe havens, and/or other critical areas.

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Historically, the preferred material for explosion-mitigating construction is cast-in-place reinforced concrete. This is the material used for military bunkers, and the military has performed extensive research and testing of its performance. Reinforced concrete has a number of attributes that make it the construction material of choice. It has significant mass, which improves response to explosions, because the mass is often mobilized only after the pressure wave is significantly diminished, reducing deformations. Members can be readily proportioned and reinforced for ductile behavior. The construction is unparalleled in its ability to achieve continuity between the members. Finally, concrete columns are less susceptible to global buckling in the event of the loss of a floor system.

Hint

• Three types of building damage can lead to injuries and possible fatalities. The most severe building response is collapse.
• For buildings that remain standing, the next most severe type of injury producing damage is flying debris generated by exterior cladding.
• Even if the building remains standing and no structural damage occurs, extensive injuries can occur due to nonstructural damage (see Figure 4-5). Typically, for large-scale incidents, these types of injuries occur to persons who are in buildings that are within several blocks of the incident. Although these injuries are often not life-threatening, many people can be affected, which has an impact on the ability of local medical resources to adequately respond. 

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Direct air-blast effects are damage caused by the high-intensity pressures of the air blast close to the explosion. These may induce localized failure of exterior walls, windows, roof systems, floor systems, and columns.

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Hint

Damage due to the air-blast shock wave may be divided into direct airblast effects and progressive collapse.

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To save lives, the primary goals of the design professional are to reduce building damage and to prevent progressive collapse of the building, at least until it can be fully evacuated. A secondary goal is to maintain emergency functions until evacuation is complete.

The design professional is able to reduce building damage by incorporating access controls that allow building security to keep large threats away from the building and to limit charge weights that can be brought into the building.

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Preventing the building from collapsing is the most important objective.

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Because air-blast pressures decrease rapidly with distance, one of the most effective means of protecting assets is to increase the distance between a potential bomb and the assets to be protected. The best way to do this is to provide a continuous line of security along the perimeter of the facility to protect it from unscreened vehicles and to keep all vehicles as far away from critical assets as possible.

Hint

The vehicle weight used for the design of barriers typically ranges from 4000 lb for cars up to 15,000 lb for trucks. Impact velocities typically range from 30 mph for oblique impact areas (i.e., where the oncoming street is parallel to the curb) up to 50 mph where there is straight-on access (i.e., where the oncoming street is perpendicular to the curb).

Hint

1. Indirect Method: Consider incorporating general structural integrity measures throughout the process of structural system selection, layout of walls and columns, member proportioning, and detailing of connections to enhance overall structural robustness. In lieu of calculations demonstrating the effects of explosions on buildings, one may use an implicit design approach that incorporates measures to increase the overall robustness of the structure. 
2. Alternate-Load-Path Method: Localize response by designing the structure to carry loads by means of an alternate load path in the event of the loss of a primary load-bearing component. 
3. Specific Local-Resistance Method: Explicitly design critical vertical loadbearing building components to resist the design-level explosive forces. Explosive loads for a defined threat may be explicitly considered in design by using nonlinear dynamic analysis methods. 

Hint

The exterior walls provide the first line of defense against the intrusion of the air-blast pressure and hazardous debris into the building. They are subject to direct reflected pressures from an explosive threat located directly across from the wall along the secured perimeter line. If the building is more than four stories high, it may be advantageous to consider the reduction in pressure with height due to the increased distance and angle of incidence. The objective of design at a minimum is to ensure that these members fail in a ductile mode such as flexure rather than a brittle mode such as shear. The walls also need to be able to resist the loads transmitted by the windows and doors. It is not uncommon, for instance, for bullet-resistant windows to have a higher ultimate capacity than the walls to which they are attached. Beyond ensuring a ductile failure mode, the exterior wall may be designed to resist the actual or reduced pressure levels of the defined threat. Note that special reinforcing and anchors should be provided around blastresistant window and door frames.

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Multi-family residential buildings are unique because they tend to house more elderly, handicapped, and children than do office buildings, which tend to have more able-bodied occupants within working age (18-65). Office buildings of course can have a certain percentage of less-able-bodied populations, depending on the tenancy (e.g., medical offices, social services, or child care centers), and such populations need to be accounted for in the design of these buildings as well. In any case, the occupancy will have a major effect on the evacuation and rescue efforts.

Hint

The initial construction cost of protection has two components: fixed and variable. Fixed costs include such items as security hardware and space requirements. These costs do not depend on the level of an attack; that is, it costs the same to keep a truck away from a building whether the truck contains 500 or 5000 lbs. of TNT. Blast protection, on the other hand, is a variable cost. It depends on the threat level, which is a function of the explosive charge weight and the stand-off distance. Building designers have no control over the amount of explosives used, but are able to define a stand-off distance by providing a secured perimeter.

Hint

Early consideration of man-made hazards will significantly reduce the overall cost of protection and increase the inherent protection level provided to the building. If protection measures are considered as an afterthought or not considered until the design is nearly complete, the cost is likely to be greater, because more areas will need to be structurally hardened due to poor planning. An awareness of the threat of manmade hazards from the beginning of a project also helps the team to decide early what the priorities are for the facility. For instance, if extensive teak paneling of interior areas visible from the exterior is desired by the architect for the architectural expression of the building, but the cost exceeds that of protective measures, than a decision needs to be made regarding the priorities of the project. Including protective measures as part of the discussion regarding trade-offs early in the design process often helps to clarify such issues.