An overview of assessment and upgrade processes

Modern terrorism is a major threat to United States’ facilities, interests, and personnel across the country and throughout the world. Although past events—such as the bomb attack on the Alfred P. Murrah Federal Building in Oklahoma City in 1995—have generated a wide knowledge of the consequences of bombing attacks, many public, federal, and corporate buildings still have minimal or no blast-resistance capacities and may not be able to survive the destructive effects of such attacks. Many government and public facilities were constructed before concern for bombing attacks was widespread and consideration to blast resistance was not necessary. The recent increase in attacks necessitates a need for the evaluation of many existing concrete and masonry structures to determine their survivability under blast loads.

Comprehensive studies of blast effects and mitigation began during World War II when the effect of near-field explosions on structures was studied. In the United States, the Cold War led to extensive evaluation of methods for increasing the blast resistance of military facilities. With new threats to commercial, public, and governmental facilities occurring in the 1990s, some of these protective technologies were imparted to various civilian engineering and construction entities, leading to extensive research on increasing the blast resistance of existing commercial and civilian structures.

Risky business

Because of the random nature of terrorist attacks, protection of existing buildings and their inhabitants can consume vast amounts of resources, cause disruptions to operations, and may affect building aesthetics, yet never offer a complete safety guarantee. Therefore, a blast mitigation or upgrade project should always be preceded with a risk assessment to determine the level of upgrade needed to mitigate damage from a potential attack. Some level of risk can be tolerated for certain facilities, while others must be protected at all costs. While some loss of property may be acceptable, the loss of human life or essential records and equipment is catastrophic.
Risk assessment is divided into three main components: threat assessment, which evaluates the likelihood of a threat; vulnerability assessment, which examines the vulnerability of a structure and also quantifies the threat in terms of impact of loss; and a risk analysis, which combines these two components to determine the overall consequences of an attack.
Threat assessment—A threat assessment takes into account the attractiveness of a facility as a target and the potential for damage, as well as the nature and method of possible attacks. Because the likelihood of an attack cannot be accurately foreseen, it is important to assess different, yet credible, threat scenarios. Methods of attacks (including unauthorized entry, explosive threats, and ballistic threats) will vary for each structure depending on importance, location, and access.
Vulnerability assessment—Once the threats are identified, a vulnerability assessment is performed to examine the potential impact and the degree to which facility operations would be impaired by an attack. The impact can range from minor (there is no significant impact on operations or loss of major assets) to devastating (the structure is no longer functional). The overall vulnerability of a structure is based on a combination of its attractiveness as a target and the level of defense in place. A deficiency in either of these characteristics can render a structure vulnerable to attack.
Risk analysis—Based on the results of the risk analysis, a building is typically classified into one of the following three vulnerability categories:
• High: The building has a high vulnerability level, and the potential for damage is great. Upgrades should be implemented as soon as possible.
• Moderate: The building has a moderate to high vulnerability level, and the damage could be noticeable to severe. Upgrades should be implemented in the near future.
• Low: The building has a low to moderate vulnerability level, and damage is likely to be minor. Upgrades will enhance security, but are not urgent.

Once the risk analysis is complete, necessary upgrades can be identified. In addition to evaluating layout, security, and access control, structural upgrades to mitigate blast effects often constitute the majority of blast upgrade work. In general, blast upgrade design and detailing for a structure requires a complex sequence of trade-offs. It must be balanced with other design constraints, such as initial and lifecycle costs, accessibility, aesthetics, constructability, materials, and efficiency. Because the probability of attack is still relatively low, even in today’s climate, the upgrade system should not interfere with building operations. However, it does need to be able to minimize loss of lives and business interruption in such an event. Therefore, a qualified team is usually sought out to perform the challenging blast upgrade project. Close coordination, responsiveness, and quick turnaround capabilities are key qualities to ensure a successful upgrade project.

Blast effects

Of the various types of potential terrorist attacks, explosive threats can be the most catastrophic in nature if the structure has not been protected or upgraded. Blast pressures can create loads on buildings that are many times greater than their design loads. Structural slabs may experience significant upward pressure that is not accounted for in typical design. Roof slabs may experience a downward overpressure, and foundations can experience blast-induced vertical and overturning forces. Buildings with relatively weak partition walls can be gutted very early during a blast—even at low-blast pressures. Structures with load-bearing walls or other walls systems that do not blow out easily could be completely destroyed, causing collapse of the structure.

Therefore, damage from blast loads can be categorized in two ways: localized effects or progressive collapse. Whereas localized effects are limited to the failure of a structural component or a group of components, progressive collapse involves the spread of an initial local failure from one structural element to another, causing extensive or total damage to the structure. In beams, slabs, and other bending members, after a structural component has been deflected by blast loads, it attempts to rebound. This action can cause brittle, shear failure if sufficient rebound resistance is not provided.

For columns, the reflected pressure may cause spalling and failure at their hinges if not properly protected. Therefore, columns are one of the main structural elements that need to be analyzed and upgraded if found deficient in their ability to resist a blast threat. If columns are supporting adjacent walls, the columns must be strengthened to resist reactions from these walls during the blast. Confinement of columns is essential to improving ductility, rotation, and shear capacity, as well as maintaining structural integrity and preventing progressive collapse.

Exterior walls of a structure could be subject to direct, reflected pressures from blasts. As such, load-bearing walls must be strengthened to prevent progressive collapse from the blast load. Non-load-bearing walls, on the other hand, must be strengthened to ensure that the walls will not fail in a ductile mode. Walls may also require shielding or a catching system to prevent damage and losses caused from debris generated by the blast. Further, the walls must be strengthened to resist loads transmitted by newly installed, blast-resistant windows and doors.

Another consideration for concrete walls is the connection to the structure. Where typical cast-in-place systems are monolithic and have good connection capacity, connections of precast concrete wall panels typically have inadequate rebound resistance (pull out) and can cause brittle failure.

Un-reinforced masonry walls almost have no blast resistance. They are weaker and may be more brittle than cast-in-place or precast concrete, resulting in a high likelihood of producing flying debris when exposed to blast loading. In contrast, reinforced masonry walls have more ductile behavior and typically produce less debris. Blast strengthening of masonry walls should include adequate detailing to provide reinforcement anchorage to the building frame. Strengthening also should prevent the movement of the stiffened wall as one unit under a blast pressure by providing shear connections to the existing structural frame.

Conventional structural upgrades

The main objective of a blast-strengthening project is to minimize the loss of life by minimizing local structural failures, controlling flying debris, and preventing progressive collapse until the building can be evacuated. Several strengthening systems may be used to enhance the blast resistance of various structural elements.

Conventional strengthening systems typically rely on steel or concrete to shield elements and either replace weak components or increase their blast strength via a variety of methods, including reducing spans, increasing mass and/or strength with section enlargement, boundary condition modifications, or connection upgrades.

Of these methods, section enlargement is one of the oldest methods used, particularly on slabs, beams, and columns. For slab strengthening, bonded overlays (on top of the slab) or underlays (added below the slab) formed from reinforced concrete may be added to the existing slab to improve its response to explosions by increasing its mass. The addition of reinforcement can improve the strength and ductility of the slab and, therefore, its ability to absorb and dissipate the energy of the explosion. Reinforced concrete column caps and drop panels can be added to increase resistance to both high-punching shear forces caused by the dynamic effects of a blast and excessive bending moments generated at the column-slab interface. Column jacketing with reinforced concrete, along with steel jacketing, is often used to confine the columns, increasing their mass and upgrading out-of-plane flexural and shear resistance. Jacketing also can be used to resolve column-buckling issues that may arise due to failure of a column lateral support, such as a floor system.

Structural reconfiguration is another conventional method widely used to upgrade slabs and beams. Beam and wall spans are shortened by the installation of additional support members, constructed with steel or concrete. This plan increases the stiffness and resistance of beams and walls against blast loads. Similar methods can be used to increase the capacity of slabs by introducing new supporting members. Walls also can be strengthened with steel posts or straps, which reduce wall spans, thus increasing their flexural and shear resistances. Connection detailing is crucial for achieving adequate performance of the new system as well as force transfer from the upgraded member or the new system to the existing structural frame.

Shielding is another conventional method used to upgrade the blast resistance of the structure, in which the exterior walls are protected by cast-in-place or pre-cast elements and supported by the frame of the building. Exterior roofs may be shielded by constructing a new roof system placed over the existing one. The new roof system is used to resist the blast load, shielding the existing roof while the existing roof is left in place to provide environmental protection and as structural diaphragm. Using this technique, slabs also may be shielded by constructing a new slab above or below an existing one, tied to the existing frame. The latter may require enlarging the existing columns and even the foundations to allow for adequate connection detailing and load-transfer capacities.

However, such conventional techniques sometimes result in limitations that may present additional problems. Aside from its impact on space and aesthetics, the addition of mass contributes to the weight of the structure, which could place great demand on the existing frames and foundations. Adding stiffness to the structure could present challenges, as this may require significant labor during installation and the closure of part of, or even the full facility during blast upgrading.

Innovative solutions

It may seem that the conventional, heavier, stiffer systems are preferable to thinner, more flexible systems when it comes to mitigating blast loads, but the opposite is actually true—if these lighter systems are designed to provide the desired improvement in strength and ductility to prevent progressive collapse. Although the added mass of heavier systems is advantageous in mitigating the effects of explosions, they can produce brittle failure if not properly designed. Further, the additional mass can pass on significantly larger reactions to the vertical and lateral supporting systems of the structure.

Accordingly, while heavier systems are still preferred in high-risk buildings, flexible systems tend to work better in the majority of civilian buildings being designed to resist moderate blast effects. By permitting some permanent damage to the structural elements without significantly increasing the hazards to the occupants, these lighter, more cost-effective systems can increase strength while absorbing energy through improved deformation. Therefore, the lighter systems transmit lower forces into the connections and supporting lateral systems. This ultimately reduces the potential for more serious structural failures.

Two such light-weight, high-strength systems are steel reinforced polymer (SRP) and fiber reinforced polymer (FRP) composites. These thin laminates are externally bonded to structural elements with an epoxy adhesive and they significantly increases the structure’s blast capacity. This is because the high flexibility of such systems increases the deformation and energy dissipation characteristics of structural elements, such as slabs, beams, and exterior concrete and masonry walls.

Of course, strengthening a structure and adding flexibility are only part of the upgrade process—controlling fly-in debris from non-structural elements is just as crucial, because of the high loss of life it could cause. A protective system made with SRP reinforcement has proven to completely encapsulate debris from disintegrating walls subjected to high-explosive charges.

Conclusion

Structural strengthening is a "scientific art form" that involves the use of conventional, cement-based materials, as well as new techniques and materials. A blend of expertise, including technical (engineering), constructability (construction methods), aesthetics (architectural), and economics (return on investment) is needed to assess and effectively design a blast upgrade.

Tarek Alkhrdaji, Ph.D., is the engineering manager at Structural Group. Comprised of three dynamic and diversified companies—Structural Preservation Systems, VSL, and Pullman Power—Structural Group businesses perform a wide range of projects involving industrial facilities, commercial properties, public infrastructure, and municipal buildings. For more information, visit www.structural.net.