By Jon A. Schmidt, PE, SECB, BSCP
On June 25, 1996, the detonation of a large truck bomb at Khobar Towers, an Air Force housing facility in Saudi Arabia, killed 19 U.S. military personnel and one Saudi, and injured 372 others from multiple nations. In the wake of this attack, the U.S. Department of Defense (DOD) initiated a process for developing new design and construction criteria for its buildings worldwide to improve their ability to withstand such incidents. The first outcome of this effort was a set of interim standards DOD issued in 1999 and designated as for official use only (FOUO), thus authorizing its distribution only to federal government agencies and their contractors.
The events of Sept. 11, 2001, accelerated DOD's efforts, resulting in the initial publication of Unified Facilities Criteria (UFC) 4-010-01, DOD Minimum Antiterrorism Standards for Buildings, on July 31, 2002. Since this document was approved for public distribution, it represented the first widely available guidance for mitigating the threat of terrorism against buildings. Revisions released on Oct. 3, 2003, and Jan. 22, 2007, further refined the various requirements and incorporated lessons learned from the widespread implementation of earlier versions. On Feb. 9, 2012, DOD promulgated the latest edition, which once again includes a number of significant clarifications and changes.
One thing remaining the same is that UFC 4-010-01 is mandatory for four categories of DOD buildings:
- Inhabited buildings, which are routinely occupied by 11 or more personnel at a population density of at least one person per 430 square feet (40 square meters) of gross area.
- Primary gathering buildings, which are inhabited buildings routinely occupied by 50 or more personnel. All areas of such a facility that meet the population density requirement for an inhabited building must be treated as primary gathering.
- Billeting, in which 11 or more unaccompanied personnel are routinely housed.
- High-occupancy family housing, which has 13 or more units per building.
All other buildings are characterized as low-occupancy and exempt from the standards. Certain building types are also specifically exempt from some or all of the standards even when they would otherwise qualify as inhabited, primary gathering or billeting. It is important to note that "routinely occupied" (or housed) is defined as any predictable pattern of activity within a building that terrorists could recognize and exploit. This means that National Guard and Reserve facilities must be classified in accordance with how they are used during unit training assemblies (UTA) and other scheduled activities, even if considerably fewer personnel are inside them on a daily basis.
Although there seems to have been a misconception to the contrary, there are no waivers from the requirements of UFC 4-010-01. As the new version states, "Commanders cannot deviate from these standards or accept risk based on their own authorities." Instead, any anticipated deviations must be evaluated and approved in accordance with service- or agency-specific guidance and accompanied by "plans, procedures, and compensatory measures to mitigate the risks associated with those situations."
UFC 4-010-01 applies to all new construction, but existing buildings need to satisfy the standards only under certain circumstances:
- Major investments — renovations, repairs, revitalizations and restorations — when costs exceed 50% of the plant replacement value as determined in accordance with UFC 3-701-01, DOD Facilities Pricing Guide for FY2011, based on a building of the same size.
- Change of occupancy level, when a building or portion thereof is converted from a lower occupancy to a higher occupancy.
- Window replacement, in which case new glazing must meet the relevant standards, possibly requiring frame modification or replacement and/or reinforcement of supporting structural elements.
- Building additions that are 50% or more of the gross area of the existing building, in which case the existing building must also meet all of the standards.
Projects programmed or designed in accordance with previous editions of UFC 4-010-01 need not be reprogrammed or redesigned in accordance with the revised version if they have already proceeded beyond the 35% completion stage, or if a design-build request for proposals has already been issued. However, even in these situations, adherence to the revised version is encouraged wherever possible.
The simplest and most cost-effective strategy for protecting a building from a vehicle bomb attack, which is by far the most common tactic of terrorists, is to maximize the standoff distance between the building itself and the postulated location of the explosion. UFC 4-010-01 has always specified two types of conventional construction standoff distances (CCSD), each of which corresponds to a different explosive weight that serves as the design basis threat:
- CCSDI is measured to the controlled perimeter, or to parking and roadways where there is no controlled perimeter. This is associated with Explosive Weight I (larger) and was previously 82 feet (25 meters) for inhabited buildings and 148 feet (45 meters) for primary gathering buildings and billeting.
- CCSDII is measured to parking and roadways within a controlled perimeter, and to trash containers. This is associated with Explosive Weight II (smaller) and was previously 33 feet (10 meters) for inhabited buildings and 82 feet (25 meters) for primary gathering buildings and billeting.
A controlled perimeter is "a physical boundary ... with sufficient means to channel vehicles to the access control points" where there is "a demonstrated capability to search for and detect explosives." This is usually the installation fence, but in some cases rugged or densely wooded terrain may qualify. The actual magnitudes of Explosive Weights I and II are specified in UFC 4-010-02, DOD Minimum Antiterrorism Standoff Distances for Buildings (FOUO).
The operative principle in all editions of UFC 4-010-01 is that when the site design provides all of the applicable CCSDs, the building's exterior components can be designed for the usual environmental loads without any special analysis and hardening for blast effects. In the revised version, various CCSDs are specified depending on the exterior wall construction. These distances, which are based on common ranges of perameters for each construction type, are provided in Tables 1 and 2. Wall and roof systems that do not fall within the prescribed ranges must be analyzed for blast even when the CCSDs are provided. The absolute minimum standoff distance is now 18 feet (5.5 meters) for Explosive Weight I and 12 feet (3.6 meters) for Explosive Weight II.
At first glance, it appears that greatly reduced standoff distances are now permissible for certain materials, such as reinforced concrete and reinforced masonry. While this is true with respect to the wall and roof systems, it is important to recognize that glazing systems and their supporting structural elements must still be designed for blast effects from the appropriate explosive weight at the actual standoff distance. Design teams must carefully evaluate the trade-off between the benefits of shorter standoff distances, especially for buildings with constrained sites, and the additional cost of hardening exterior windows and glazed doors.
One simple solution is to recognize that the industry's "standard" DOD glazing systems for primary gathering buildings have been designed for Explosive Weight I at a standoff distance of 148 feet (45 meters), which controls over Explosive Weight II at a standoff distance of 82 feet (25 meters). The revised version of UFC 4-010-01 states that Explosive Weight I does not need to be considered when the standoff distance to the controlled perimeter exceeds 200 feet (60 meters). It turns out that Explosive Weight II at a standoff distance of approximately 63 feet (19 meters) produces blast effects comparable to those of Explosive Weight I at a standoff distance of 148 feet (45 meters). Therefore, glazing systems and their supporting structural elements in primary gathering buildings located at least 200 feet (60 meters) within a controlled perimeter and at least 63 feet (19 meters) away from parking and roadways will not require hardening beyond what has been typical for such buildings in recent years.
Other Noteworthy Changes
Among other significant revisions to UFC 4-010-01:
- Design submittals are required to include a narrative explaining how each individual standard is met; calculations or test results for all blast-resistant glazing systems and any structural elements where CCSDs are not provided; and progressive collapse analysis where applicable.
- It is now explicit that "vehicle barriers provided to meet these standards are not required to stop moving vehicles," and that "Parking areas and roadways do not require physical barriers."
- Parking areas located closer than the CCSD to existing buildings must be controlled for pedestrian and vehicle access.
- Instead of being set at 33 feet (10 meters) across the board, the unobstructed space now extends out from the building to the applicable value of CCSDII. In this zone, landscaping, equipment and other features must not allow for concealment of explosive devices 6 inches (152 millimeters) or greater in height or width. A smaller unobstructed space may be validated through blast analysis.
- Occupied overhangs and breezeways are still discouraged, but now permitted without hardening as long as the unobstructed space requirements are applied to the areas underneath them.
- Exterior stairwells, covered or enclosed walkways, vestibules, and foyers are "not considered to be routinely occupied." As a result, their outer construction (including glazing) does not need to be analyzed or hardened for blast, but the inner building components behind them must be capable of mitigating any hazards resulting from their failure in an explosion.
- Exterior doors, even without glazing, must now be tested in accordance with ASTM F2247 using an equivalent static pressure based on the characteristics of the specified blast loading (explosive weight and standoff distance) and the structural properties of the door itself. Alternatively, untested doors may be positioned such that they will not be propelled into inhabited areas or in front of a wall with sufficient strength to withstand the door's impact; this solution is mandatory for overhead doors since it is "impractical" to harden them.
- Mail rooms are no longer required to be sealed.
- Where equipment rooms have air intakes for cooling that could admit blast pressures, interior walls must be designed for the same blast loads as exterior walls.
- Emergency air distribution shutoff switches must be located on each floor with a maximum travel distance of 200 feet (60 meters). Exhaust systems may continue to operate where shutting them down would violate building or fire codes, such as in kitchens.
- Requirements for utility distribution and installation have been deleted.
UFC 4-010-01 is probably the most widely used document in the industry for designing and constructing buildings to mitigate potential terrorist attacks. The revised version offers greater flexibility when it comes to establishing standoff distances that do not require blast analysis and hardening of exterior wall and roof components. However, exterior glazing systems and their supporting structural elements must still be evaluated and may become quite expensive at reduced standoff distances.
About the Author
Jon A. Schmidt, PE, SECB, BSCP, is director of antiterrorism services and an associate structural engineer in the Aviation & Facilities Group at Burns & McDonnell. He earned his bachelor's degree in civil engineering and master's degree in structural engineering from The George Washington University.