Structural Engineering Considerations in Design of Secure Compounds

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On the morning of August 7, 1998, an unmarked truck drove up to the main gate of the United States embassy in Nairobi, Kenya. After a brief altercation between the men in the truck and the guards, the truck exploded, unleashing 2,000 pounds of TNT. The embassy was heavily damaged, and a neighboring building collapsed. At the same time, 500 miles away a similar truck drove a similar bomb to the gate of the U.S. embassy in Dar es Salaam, Tanzania, and detonated it. The coordinated attacks killed 224 people and wounded thousands more. Combined with the 1995 bombing of the Oklahoma City Federal Building, the East Africa bombings kickstarted a serious look into how to protect structures against deliberate threats.
This article will provide an overview of the structural engineering principles behind designing a secure compound, including why it’s necessary and how structural designers account for physical security measures in their designs of these types of structures. A secure compound is a campus that is designed to protect the inhabitants from deliberate threats and attacks. These sites are typically large, spread out, and built away from dense city centers. Some common examples of secure compounds are embassies, certain military facilities, federal agency campuses, some aviation facilities, and courthouses.

While it might seem uneconomical to design structures for seemingly rare events, attacks on government buildings and important structures are more common than one might think. The U.S. embassy in Beirut was bombed by Hezbollah in 1983, killing 63 people, including 17 Americans. The embassy was subsequently relocated to a new property in a suburban location. The following year, the new location was bombed as well, killing 23 more people. If we can design buildings to ensure the safety of occupants in extreme events like hurricanes and earthquakes, can we not design them to ensure safety in a terrorist attack as well?

Physical Security

Physical security design can be broken down into four primary components: 1) secure perimeters, 2) blast resistant design, 3) progressive collapse mitigation, and 4) forced entry and ballistic resistance.

The secure perimeter of a site typically consists of a solid concrete wall or a concrete knee-wall with a steel fence atop. Two main aspects go into this design: anti-ram and anti-climb. The goal of anti-ram design is to prevent a vehicle from penetrating the secure perimeter. Numerous threat levels exist for vehicular attacks, corresponding to various sizes and speeds of vehicle impact. The gold standard is the “K-12” barrier, which is tested and designed to stop a 15,000-pound vehicle traveling 50 mph. Examples of this would be a Ford F-350 towing its max capacity or a fully loaded box truck (think a large U-Haul).

Anti-ram barriers can be concrete walls, bollards, or wedge barriers. Anti-ram concrete walls are usually extended high enough above the ground to withstand vehicle impact and have a cantilevered retaining wall type footing designed to absorb the impact of the vehicle. Anti-ram bollards closely resemble standard traffic bollards, but the steel pipe is filled with concrete and features an extensive foundation. Bollards need to be closely spaced so that a vehicle cannot navigate through them, approximately 5-feet on-center.

Anti-ram wedge barriers are prefabricated products that allow for vehicular access control. A typical wedge barrier can be seen in Figure 3. The barrier is kept raised and only lowered when a vehicle is granted access to the site. Wedge barriers have an extensive concrete footing and are often combined with sliding gates or bi-fold gates, which serve as anti-climb protection.
Anti-climb means that the site perimeter must be designed so that someone cannot climb over the wall and gain access to the compound. Walls around secure compounds are typically both anti-ram and anti-climb. Entrance pavilions, where pedestrians and vehicles enter the site, are also designed to be anti-climb.

The goal of blast-resistant design is to prevent failure of structural and architectural components and to minimize injury to the building’s occupants should an explosive device be detonated nearby. This is accomplished through the hardening of structural elements, facade features, and architectural appendages that may become a hazard during a blast event (Figure 4). Blast design considerations include maintaining global stability of the structure, limiting damage, and mitigating flying debris.

Understanding the blast loading expected at each structure, namely blast overpressures (i.e., pressures exceeding atmospheric pressure) and their duration, is the foundational step. Loads are determined by considering a line of threats around the site perimeter and enveloping the governing overpressures utilizing methodologies founded on empirical data or Computational Fluid Dynamics (CFD) analysis. In addition to explosive charge size and standoff distance, the geometry of a structure influences blast effects. Those with complex geometries typically create “confining” effects that produce higher design loads. Ultimately, the enveloped loads are implemented in a time-history analysis to verify the global and local response of the building structure and individual elements, respectively.

From a global standpoint, a blast event will induce base and story shear forces like wind or seismic loading. Blast story shears are developed considering the impulsive loads impacting a building surface area and the structure type. Forces are applied to the floor diaphragms based on surface tributary heights for each story down to the foundation, where the load attenuates. Coordination is required between the blast engineer and structural engineer to determine the governing lateral load for the building.

Ductile detailing for both architectural and structural components is incorporated into the design to induce plasticity in ductile failure mechanisms. For various architectural features, ductile detailing requirements are implemented when their response is not required to meet specific limits under blast (i.e., canopies, sunshades, railings, architectural fins, etc.). The standard of practice provides connections and anchorage that are designed to resist the ultimate flexural capacity of the element so that it will remain attached to the supporting structure.

Progressive collapse refers to a structural failure mechanism where local damage to a load-bearing element triggers a chain reaction, leading to the disproportionate collapse of a structure. UFC 4-023-03, Design of Buildings to Resist Progressive Collapse, outlines strategies to enhance a building's robustness and limit such failures. The design philosophy focuses on redundancy, ductility, and continuity to ensure the structure can withstand localized damage.
Progressive collapse design is threat-independent, meaning that unlike blast design, the principles and methods are not tailored to specific threats like explosions or impacts. Instead, the design focuses on ensuring the structure can maintain its integrity regardless of the cause of localized damage. Per the UFC, only buildings three stories or more are required to be designed for progressive collapse.

The two design procedures for progressive collapse are the Tie Force Method and the Alternate Path Method. The Tie Force Method ensures that structural elements are interconnected to redistribute loads in the event of localized damage. This approach mandates robust connections in steel buildings and continuous reinforcement in concrete buildings within the structure. The Tie Force Method is typically applied to buildings with regular configurations, such as low-rise structures, where enhanced connectivity effectively mitigates localized damage.

The Alternate Path Method involves designing the structure to provide alternative load paths if a primary structural element fails. This method ensures stability by redirecting loads through other members, maintaining the overall integrity of the building. The Alternate Path Method is suitable for complex structures, mid to high-rise buildings, or facilities with higher risk profiles, where additional redundancy is necessary beyond the Tie Force Method.

The three common structural analysis procedures for designing and assessing buildings for progressive collapse are the Linear Static Procedure (LSP), the Nonlinear Static Procedure (NSP), and the Nonlinear Dynamic Procedure (NDP). The LSP applies static loads to a simplified linear model of the structure, assuming linear elastic material behavior. The LSP has lower computation demands but does not accurately capture nonlinear behaviors or dynamic effects, which are vital for accurate and not overly conservative progressive collapse calculations. The LSP is usually used in early design stages to gauge the building’s response to progressive collapse or for simple, redundant structures where detailed dynamic analysis is not critical.

The NSP, also known as pushover analysis, applies increasing static loads to a nonlinear model. This method accounts for material nonlinearity and inelastic behavior, providing a more realistic representation of the structure's response to extreme loading. It’s a more accurate representation of structural behavior than LSP, including post-yield response, but has increased computational demands.

The NDP involves time-history analysis using nonlinear models subjected to dynamic loads. This method captures both nonlinear material behavior and dynamic effects. Of the three procedures, NDP offers the most accurate and comprehensive assessment of a structure's response to progressive collapse but also requires the highest level of model detail and is more computationally demanding.

By implementing these methods and procedures, the UFC 4-023-03 ensures that buildings can withstand local failures without experiencing total collapse. In the case where a threat cannot be eliminated, deferred, or stopped from damaging the building, designing for progressive collapse mitigation offers another way to limit catastrophic structural damage and save lives.

Incorporating Forced Entry and Ballistic Resistance (FEBR) provides increased security for building occupants under various levels of threats from individuals that may have already breached the secure perimeter or entered a building. FEBR requires the hardening of certain walls, doors, and windows throughout a compound.

The various levels of FEBR protection are defined by the site owner or governing body. Some common levels of FEBR protection are FE 5, FEBR 15, and FEBR 60 (FE 5 excludes ballistic resistance). The numbers define the amount of time in minutes that the barrier must theoretically withstand attack before being penetrated. FEBR wall, door, and window designs are based on individually tested assemblies. For a 5-minute FE assembly, the test involves two men swinging sledgehammers and other tools at the door for 5 minutes. FEBR 15 minute and FEBR 60 tests are similar but involve artillery fire and less readily available, heavy-duty tools like battering rams. An example of an FEBR test is shown in Figure 5.

FEBR detailing is prescriptive, based on the tested configurations and usually requires no additional calculations from the structural engineer if pre-approved configurations are implemented. FEBR walls are typically concrete walls, structural steel stud-walls, or glass curtain walls. Curtain walls are used where dictated by the architectural requirements of the building and usually consist of thick glass windows supported by HSS steel mullions.

As described earlier, designing a site for physical security introduces many structural design considerations not common in conventional building design. These protective designs have several major effects on the final building design.

Perhaps the most consequential effect on a building from protective design is the increased weight of structure that comes as a result of designing for blast, progressive collapse, anti-ram, and FEBR. Whereas conventional buildings rarely use solid concrete partition walls or exterior walls, a secure building will likely feature many. Despite the cascading effects of a heavier building, a concrete wall is often most economical when compared with steel or glass wall options.

Considering progressive collapse when designing a building often results in the need for concrete beams spanning between columns and walls. Flat slab systems are typically impractical given the quantity of reinforcing needed, and structural steel framing systems are often uneconomical due to the number of moment connections needed to create a frame of such redundancy. In turn, the additional dead weight leads to larger foundations and significant seismic base shears. On secure compounds, seismic loads often govern over wind loads even in regions of high wind and low seismicity. In regions of high seismicity, seismic forces can be significantly higher than a conventional building of the same size.

Exterior building elements like canopies or architectural features requiring structural backup framing are often complex and challenging when a building is designed for blast loading. These elements, such as canopies, are often constructed from structural steel, which requires ductile detailing and capacity design of connections. A canopy or other exterior feature on a building designed for blast loading can lead to significant added cost.

Similarly, when curtain walls, storefront, and doors/windows must meet blast loading requirements or FEBR requirements, the assemblies become complex and heavy. Curtain walls often feature thick glass with large structural steel mullions and significant embed plates cast into the base structure. Curtain wall design is usually delegated to a supplier and carried out at the beginning of the construction phase. Thus, the timing and sequencing of the design and installation of these embeds can pose a scheduling challenge to the contractor.

Lastly, as the building construction industry targets reducing embodied carbon in design and construction of the built environment, hardening and strengthening the structure for physical security can be counterproductive to this goal, mostly due to the significant increase in concrete needed. To offset this, governments and owners can look to operational carbon reductions for their buildings, such as PV arrays and efficient mechanical operating systems.

Designing a secure compound poses unique challenges for practicing structural engineers. From designing for vehicular impact to reducing risk of progressive collapse after the removal of a primary load bearing element, the structural engineer must take on a larger role and responsibility in the design of a secure compounds. With proper knowledge of the design methodologies discussed in this article, structural engineers can incorporate the requirements for a secure compound into their design without compromising architectural intent, function, or beauty of a site. ■

About the Authors

Chris Heckmann, PE, is a senior project manager with Ehlert Bryan in Washington, DC, specializing in overseas U.S. diplomatic facility design.

Achraf Ayad, PE, is an assistant project manager with Ehlert Bryan in Washington, DC, specializing in overseas U.S. diplomatic facility design.

Leslie Duffy, PE, is an associate with KPFF’s Protective Design team in San Francisco, CA, specializing in blast engineering.