Many government facilities were constructed before the concern about bombing attacks was widespread.

 

Although recent concerns about terrorism have generated a wider knowledge of the consequences of bombing, the majority of public, federal, and corporate buildings still has minimal 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 for blast resistance was unnecessary.

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

Surviving the blast

Of the various types of potential terrorist attacks, explosive threats can be the most catastrophic if the structure has not been protected or upgraded. Blast pressures can create loads on buildings many times greater than their design capacity. As a result, structural slabs may experience significant, unaccounted for, upward pressure, roof slabs endure downward overpressure, and foundations face blast-induced vertical and overturning forces. Buildings with relatively weak partition walls can be gutted very early during a blast (even at low pressures), while structures with load-bearing walls or other wall systems that do not blow out easily can be completely destroyed, causing collapse. Therefore, damage from blast loads can be categorized as either 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 damage. In beams, slabs, and other bending members, structural components attempt to rebound after being deflected by blast loads. This action can cause brittle shear failure when there is insufficient rebound resistance.

For unprotected columns, the reflected pressure can lead to spalling and failure at their connections. 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. When 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 are also 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 the walls 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, wall 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 wall panels typically have inadequate rebound resistance (pull out) that can cause brittle failure.

Non-reinforced masonry walls have almost 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 should also prevent the movement of the stiffened wall as one unit under a blast pressure by providing shear connections to the existing structural frame.

Risky business

Due to the random nature of terrorist attacks, the protection of existing buildings and their inhabitants never offers a complete safety guarantee, despite consuming vast amounts of resources, causing disruptions to operations, and possibly affecting building aesthetics. Therefore, a blast mitigation upgrade project should always be preceded with a risk assessment to determine the level of improvement needed to mitigate damage from a potential attack. Owners typically acquire the services of engineering and other specialty consultants to perform this type of risk assessment studies and provide recommendations for blast mitigation.  In addition, a number of How-To Guides are now available through the Federal Emergency Management Administration (FEMA) that provide means to assess risks and to make decisions about how to mitigate them.  The guides outline methods for identifying the critical assets and functions within buildings, determining the threats to those assets, and assessing the vulnerabilities associated with those threats.

Some 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 essential records, equipment, or human life 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 loss; and
• risk analysis, which combines these two components to determine the overall consequences.

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. Since the likelihood of an attack cannot be accurately foreseen, it is important to assess different, yet credible, scenarios. Methods of attacks (including unauthorized entry, as well as explosive and ballistic threats) vary for each structure depending on importance, location, and access.

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 (i.e. there is no significant impact on operations or loss of major assets) to devastating (i.e. the structure is no longer functional). The overall vulnerability is based on a combination of the structure’s attractiveness as a target and the level of defense in place.

Based on the results of the risk assessment, a building is typically classified into one of three 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; and
• 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. Blast upgrade design and detailing for a structure generally requires a complex sequence of tradeoffs. It must be balanced with other design constraints such as initial and lifecycle costs, accessibility, aesthetics, constructability, materials, and efficiency.

As the probability of attack is still relatively low, the upgrade system should not interfere with building operations. However, it must still minimize loss of lives and business interruption in the event of a blast. Therefore, a team including specialty consulting and contracting services is usually sought to perform the challenging upgrade project. Close coordination, responsiveness, and quick turnaround capabilities are key qualities to ensure a successful project.

Conventional upgrades

The main objective of a blast strengthening project is to minimize the loss of life by reducing local structural failures, controlling flying debris, and preventing progressive collapse until the building can be evacuated. Several strengthening systems can 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 various methods, including:

• reducing spans;
• increasing mass/strength with section enlargement;
• boundary condition modifications; or
• connection upgrades.

Of these methods, section enlargement is one of the oldest strategies applied, particularly on slabs, beams, and columns. For slab strengthening, bonded overlays (atop the slab) or underlays (below the slab) formed from reinforced concrete can be added, increasing the slab’s mass, strength and ductility. This, in turn, improves its ability to absorb and dissipate the energy of an 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 potentially arising due to failure of a column lateral support such as a floor system. Although effective, column jacketing with concrete may not be desired due to space limitation or aesthetic impacts.  Steel jacketing may require less space than concrete but may not be able to increase the column mass and may be harder to construct.

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 slab capacity 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 to transfer force from the upgraded member/the new system to the existing structural frame.

Shielding is another method often employed to enhance a structure’s blast resistance, in which the exterior walls are protected by cast-in-place or precast elements and supported by the building frame. Exterior roofs may be shielded by constructing a new roof system placed over the existing one—the new system resists the blast load, shielding the pre-existing roof assembly left in place to provide environmental protection and serve as a structural diaphragm. Using this technique, slabs can also 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, the aforementioned conventional techniques sometimes result in limitations that present additional problems. Aside from its impact on space and aesthetics, the addition of mass contributes to the structure’s weight, which could place great demand on the existing frames and foundations. Adding stiffness to the structure can also present challenges, as this occasionally requires significant labor during installation and the closure of part of the facility (or even its entirety) during blast upgrading.

Innovative solutions

It might seem as if the conventional, heavier, stiffer systems are preferable to thinner, more flexible systems when it comes to mitigating blast loads, but the opposite is also true—provided the lighter systems are designed to offer the desired improvement in strength and ductility. Although the added mass of heavier systems is advantageous in mitigating the effects of explosions, they can be more brittle in failure when improperly designed. Further, they can pass on significantly larger reactions to the structure’s vertical and lateral supporting systems.

Accordingly, while heavier systems are still preferred in high-risk buildings, flexible systems tend to work better in the majority of projects 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, thereby transmitting lower forces into the connections and supporting lateral systems. This ultimately reduces the potential for more serious structural failures. Two such lightweight, high-strength systems—steel-reinforced polymer (SRP) and fiber-reinforced polymer (FRP) composites—are thin laminates externally bonded to structural elements with an epoxy adhesive that significantly increases blast capacity.¹

SRP composites can incorporate a family of reinforcements made from ultra-high-strength twisted steel wires. This revolutionary reinforcing material provides the ability to put ultra-high-strength steel (approximately 3.1 million kPa [450 ksi]—approximately eight times stronger than steel reinforcing bars) inside or outside virtually any material. Similarly, FRP composite systems are composed of unidirectional high-strength continuous fibers coated in an epoxy matrix. Fiber/epoxy laminates, noted for their high strength-to-thickness ratio, establish structural integrity in a manner similar to steel plates, yet their tensile strength capacity is eight to 10 times traditional structural steel. Both composite systems are comparable in cost and are typically more expensive that conventional structural steel and steel reinforcement.  However, due to their high strength and lightweight, the higher cost of composite materials is offset by reduced material, labor, and heavy machinery requirements. It is more likely to find that a composite strengthening solution is cheaper and faster to install than conventional systems.  

Both composites have proven successful in a variety of high-risk situations. Fiber reinforcement has been used for 30 years in aerospace, military, and manufacturing applications where low weight, high-tensile strength, and non-corrosive structural properties are required. In U.S. Navy explosion tests, SRP composites have proven to be an efficient blast-resistant upgrade for structures. The high shear strength of the steel wires allows for anchorage of the system to provide continuity of reinforcement and a continuous path for flow of forces resulting from the blast. The high flexibility of such systems also helps increase 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, due to the high loss of life it could cause. A catching system made with steel wire reinforcement can completely encapsulate debris from disintegrating walls subjected to high explosive charges. [are there any test results or reports we can attribute?] [most of these tests were done and government agencies or at their request.  As such test reports are considered confidential but can be shared under certain conditions and requirements] Additionally, wood panels laminated with these reinforcements have been used to control debris and reduce the potential injury of personnel inside the building. In ballistic scenarios, specially designed SRP laminates can both break up the projectile and catch the resulting fragments. The ultra-high-strength wires contained in the material act to slice projectiles, spreading the energy from the threat and lowering the loads for the material designed to catch the fragments.

Conclusion

Blast assessment and strengthening design is infinitely more complex than new construction and should not be treated lightly. Structural strengthening is a ‘scientific art’ 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. This explains the trend of employing the design-build approach for these types of sensitive projects.

Challenges often arise due to the unknown structural state characteristics, such as load path, materials properties, and the size/locations of existing reinforcement or prestressing. The degree to which the upgrade system and the existing structural elements share the loads must be evaluated and properly addressed in the upgrade design, detailing, and implementation methods. Experience has shown engaging specialty engineering and contracting firms familiar with the critical aspects of a strengthening project can ensure the most cost-effective and long-lasting results.

Notes

¹ For more information on applications for these products, see “Strengthening and Repairing Concrete” by this author and Jay Thomas (The Construction Specifier, September 2004).

Author

Tarek Alkhrdaji, Ph.D., is an engineering manager with the specialty contracting firm, Structural Group. He has been involved in numerous projects involving structural repair and strengthening, blast mitigation, as well as in-situ load testing. Dr. Alkhrdaji is an active member of the International Concrete Repair Institute (ICRI), the American Society of Civil Engineers (ASCE), and the American Concrete Institute (ACI) committees on fiber-reinforced polymer (440) and strength evaluation (437). He can be contacted via e-mail at talkhrdaji@structural.net.