Steel Solutions Center

12.2. Physical Security

12.2.1. What are the general objectives of defensive design involving a terrorist attack?

The main objective of defensive (protective) design of a civilian facility is to minimize casualties and damage. Life safety should be the primary design parameter. In certain situations it is also necessary to provide for the functional continuity of the facility. For example, a hospital must function after an attack in order to provide services for critical patients. Similar requirements apply to fire and police stations. While it is impossible to design all buildings against all threats, it is possible to design some buildings to be resistant to some threats. Defensive design often conflicts with aesthetics, accessibility, fire safety regulations and budgetary constraints.

12.2.2. What defensive strategies can be employed to reduce risks of terrorist attacks involving explosions?

The first step in the defensive design process is to establish the probable risk and the parameters of the threat to a facility. Risk of “collateral damage” to nearby buildings should also be considered. It is then possible to consider countermeasures (defensive strategies) to the threat. Common external blast threats are car, van or truck bombs. Internal blast scenarios involve a smaller explosive charge packed in a letter or a brief case, or a car bomb in a parking garage.

One way to protect a building from a possible attack is to make weapon delivery difficult. A set back distance and a secure fence around the building can serve this purpose. However, this approach often is not viable in a city where buildings adjoin other buildings along busy streets. In these cases, measures such as surveillance, limits on traffic movement and guards can enhance protection.

In the design of upgrades and retrofits of existing facilities, countermeasures that involve establishing a defensive perimeter (fences, bollards, etc.) and positioning the building at some distance from this secure perimeter often are not possible. Instead, threat countermeasures include the relocation of important functions to safer areas of the building. Other measure include hardening the mail area, moving people from external walls to inner offices, replacing or strengthening windows and window frames, hardened safety rooms, hardening portions of the building, or moving the entire operation to a more secure facility. In all circumstances, defensive strategies must incorporate some measures of facility-access control, contingency planning and emergency training for all occupants.

12.2.3. What is the difference between physical and operational security measures?

Physical security measures, also called passive security measures, include actions such as perimeter protection with walls, fences, bollards, planters and intrusion-detection alarms. It also includes actions like hardening the structure or portions thereof to mitigate blast effects if perimeter protection is not sufficient.

Operational security measures, also called active security measures, involve actions such as intelligence, surveillance and guards.

12.2.4. What specific blast effects are considered in defensive structural design?

As mentioned previously, in an explosion produced by a vehicle bomb, part of the energy is released in the form of thermal radiation, and part is coupled into the air as air blast and into the ground (soil) as ground shock.

For above-grade structures subject to surface attack and airbursts, air blast is the primary mechanism producing the potential for damage and casualties, and this is the loading that is used in design.

For buried or below-grade structures, depending on weapon yield, ground shock can be an additional design effect.

12.2.5. What is a “stand-off” distance? What is a “height of burst” (HOB)?

Stand-off distance refers to the direct, unobstructed distance between a weapon and its target. Height of burst refers to aerial attacks. It is the direct distance between the exploding weapon in the air and the target. For a bomb capable of being detonated above a target, an optimum height produces the maximum coverage by a given level of pressure, resulting in maximum damage. This is referred to as the optimum HOB.

12.2.6. How large are design blast loads? How are blast loads evaluated?

Selection of the blast charge size W is based on the perceived risk to the design building and any buildings nearby. Various factors play a role here, such as the social and economic significance of the building, security measures that deter terrorists, and data from previous attacks on similar facilities. The minimum standoff distance R is determined from the layout of a building’s surroundings and reflects the expectation of how close to the building the design charge could explode.

W and R are two necessary inputs for the scaled distance parameter Z = R/W0.33 that is used to determine “equivalent” design pressure impulses using published curves [see Ref.10]. For greater accuracy, computer programs such as AT Blast are available for free download at

Blast loads are applied to external building cladding if it is assumed to transfer the loads to structural elements. Where windows, doors and external walls are not expected to remain intact, blast loads also should be applied to internal structural elements. Floor slabs especially should be checked for uplift-pressure impulse. Blast loads usually are not factored and used in combination with unfactored gravity loads.

For more information see Reference 1

[1] “Structures to Resist the Effects of Accidental Explosions,” Dept. of the Army Tech. Manual, TM5-1300, Dept. of the Navy Pub. NAVFAC P-397, Dept. of the Air Force Manual, AFM 88-22, June 1969

12.2.7. What are the most popular and cost-effective methods for upgrading existing buildings for physical protection?

Some level of blast resistance is required for new Federal Buildings. Existing Federal Buildings undergoing expansion also must include blast resistance. In each case the General Services Administration (GSA) establishes design requirements. Specific actions can involve: protecting windows; installing a secure perimeter fence and/or hardening a portion of the building; and determining the likelihood of progressive collapse and designing against it. There is no comparable, universal guidance in the civilian sector. However, some of the guidance developed by the Federal Government is available to the general public.