Physical Security: Designing Buildings to Resist Explosive Threats  

Introduction

This section addresses the design of the structure of a building to withstand blast loads. The four basic physical protection strategies for buildings to resist explosive threats are 1) Establishing a secure perimeter (see WBDG Site Security Design Process); 2) Mitigating debris hazards resulting from the damaged façade (see also WBDG Physical Security: Design of Building Envelope for Blast Loads); 3) Preventing progressive collapse; and 4) Isolating internal threats from occupied spaces. Other considerations, such as the tethering of non-structural components and the protection of emergency services, are also key design objectives that require special attention. Generally, the size of the explosive threat will determine the effectiveness of each of these protective strategies and the extent of resources needed to protect the occupants. Therefore, determining the appropriate design threat is fundamental to the design process and requires careful consideration.

Description

Threat Definition

Comprehensive threat and vulnerability assessments, and risk analysis can help the design team understand the potential threats, vulnerabilities, and risks associated with a building as well as determine the design threat for which a building should be designed to resist. Usually, the definition of the design threat is based on history and expectation. However, it is limited by the size of the means of delivery. For example, a hand-carried device, if efficiently packaged, could occupy as little as half a cubic foot of space and could be easily concealed in a large brief case or small luggage and introduced deep into the structure where it could do considerable damage. As a result, screening stations at the entrances, mailrooms, and loading docks provide the best means of preventing hand-carried satchel threats from entering the occupied spaces. On the other hand, vehicle threat, which can carry significantly larger explosive charge weights, requires secured perimeters and comprehensive screening procedures for underground parking structures or loading docks.

Screening procedures, however, have limitations and the potential for threats to bypass their scrutiny must be recognized in the physical protection scheme. Therefore, the selection of the design level explosive threat depends on the features of the building, the site conditions, and the level of risk the client is prepared to accept.

Evaluating Risk

Terrorist activities and malevolent manmade events do not lend themselves to annual frequencies or statistical probabilities of occurrence, and are difficult, if not impossible to predict based on a mathematical model. Moreover, the intensity of any malevolent event is highly variable, and this significantly affects the consequences, should an event occur. It is therefore impossible to determine absolute project security risks. However, even if we cannot state with mathematical certainty how frequently a specific threat may occur, we can much more reliably relate how likely a specific event will occur relative to other specified events.

Relative risk is a function of an asset's importance, relative likelihood of a threat's occurrence and the consequence should the event occur. Importance of an asset is determined by its criticality to the operation of the facility (its ability to function) and is independent of design parameters. The relative likelihood (occurrence) of a threat event is related to the ease with which it can be carried out (accessibility). The consequences are related to extent of damage in response to a specified hazardous event and may be expressed in terms of damage (cost of repair), hazard to occupants (injuries or fatalities), public perception, and downtime (lost productivity). The vulnerability of a system is related to its features which may increase the ability for the threat to occur.

The criticality of a facility may be reduced through distributed redundancy and a facility's risk is further reduced by making a hazardous event less likely to occur and the consequences less extreme. Accessibility is reduced by maintaining the standoff distances to the critical structural components and introducing layers of access control and screening, much like a series of sieves, which are each capable of trapping a given type of threat while permitting smaller threats to evade detection. The consequence can be reduced by enhancements in physical protection. Methodology for calculating risk and conducting a TVRA (Threat, Vulnerability, and Risk Assessment) is provided in various standards including the FEMA-426 / BIPS-06 prepared by the Department of Homeland Security and ASIS SRA-2024 "Security Risk Assessment prepared by ASIS International. Additional information may be found on the Threat/Vulnerability Assessments and Risk Analysis WBDG resource page.

Blast Loading

Since this Resource Page focuses on explosive threat, one must first understand how a blast affects its surrounding environment. When an explosive device is detonated at or near the ground surface, shock waves radiate hemispherically and the peak intensity blast pressure decays as a function of the distance from the source. The incident peak pressures are amplified by a reflection factor as the shock wave encounters an object or structure in its path. Reflection factors depend on the intensity of the shock wave and the angle of obliquity of the shock front. However, when the explosion is within an occupied space, the confinement of the explosive by-products produces a quasi-static gas pressure that needs to be vented into the atmosphere.

The intensity of the blast pressures is therefore a function of the charge weight and the standoff distance to the protected space. Charges situated extremely close to a target structure impose a high impulse, high intensity pressure load over a localized region of the structure. This high intensity loading tends to shatter or shear through the structural materials. At greater distances, the intensity of the peak pressure is significantly reduced; however, the surface area over which it acts is much greater. As a result, the hazard potential is increased over a larger portion of the structure.

The shape of the building can also have a contributing effect on the overall damage to the structure (see Figure 2). Reentrant corners and overhangs are likely to trap the shock wave, and multiple reflections may amplify the air-blast. Large or gradual reentrant corners have less effect than small or sharp reentrant corners and overhangs. In general, convex rather than concave shapes are preferred for the exterior of the building. (i.e., the reflected pressure on the surface of a circular building decays more rapidly than on a flat building or "U" shaped building). Terraces which are treated as roof systems subject to downward loads require careful framing and detailing to limit internal damage to supporting beams.

Dynamic Analysis of Building Systems

The performance of building systems in response to explosive loading is highly dynamic, highly inelastic, and highly interactive. By controlling the flexibility and resulting deformations, structural or façade components may be designed to dissipate considerable amounts of blast energy. Structural and façade elements that initially deform inward due to the directly applied blast loads will subsequently rebound outward. Depending on the extent of inelastic deformation, the resulting rebound reaction forces may be equal to or less than the peak magnitudes from the initial loading. The detailing of these elements and their connections must consider the load reversals due to rebound.

The phasing of the different responses and the energy that is dissipated through inelastic deformation must be carefully represented in order to accurately determine the behavior. The 'sequential single-degree-of freedom (SDOF) model' approach, commonly used to analyze individual components, is likely to produce overly conservative designs, while an accurate representation of the structural system truly requires a complex 'multi-degree-of-freedom (MDOF) model.' These MDOF models may be developed using appropriate inelastic Finite Element software for which an explicit formulation of the equations of motion may be solved. The details of the finite element models, including the interaction between the various components of structural and façade systems, will determine the accuracy of the analysis. Only this approach will provide the most authentic representation of the system's ability to resist the dynamic blast loading AND provide the most economical design.

The behavior of structural materials, such as steel and aluminum, in response to explosive loading was the subject of intense investigation by the governments of the United Kingdom, Israel, and the United States of America. Some of these materials behave very differently when subjected to high strain rate loading than they do under static conditions. Furthermore, the inelastic deformation of these members depends on their section properties, shape functions, and extent of deformation. For compound sections composed of different pieces and materials, transformed section properties may be used to characterize an equivalent material and a combined or composite section property may be used to represent its structural resistance. Care must be taken to calculate composite section properties when strain compatibility between components can be justified and combined section properties when deformation compatibility between components is enforced.

Performance Standards

Performance standards identify the acceptable levels of damage that may be permitted in response to specified explosive events. The American Society of Civil Engineers publication Blast Protection of Buildings (ASCE 59-22) contains detailed consensus standards that define industry accepted design methodologies and response limits for the protective design of structures and façade systems. These standards represent the best practices and standards of care based on research and testing programs conducted by U.S. Government agencies (e.g., Department of Defense, Department of State, General Services Administration, etc.), universities and private research organizations. While project specific criteria may be based on risk assessment or other prescriptive security requirements, these standards help unify the methods and metrics that determine the effectiveness of a protective design upgrade. Analytical tools that evaluate the likely performance of structural and façade elements in response to blast loads are used to demonstrate compliance with established blast criteria or performance specifications.

In addition to specific design metrics, many of these specifications contain the criterion that the building system must be a balanced design. The objective of this criterion is to realize the capacity of all the materials, maximize the potential energy dissipated due to deformation, and manage the failure mechanisms. This is accomplished by assuring a controlled sequence of failure. While a consistent capacity design is desirable, it could have significant impact on the sizing of the members and the design of the connections between the different components.

Physical Protection Strategies and Features

Perimeter Protection

While it may be possible to predict effects of a certain charge weight at a specified standoff distance, the actual charge weight of the explosive used by a terrorist, the efficiency of the chemical reaction, and the source location cannot be reliably predicted. Given the uncertainties, the most effective means of protecting a structure is to keep the explosive as far away as possible from the property lines by maximizing the setback or standoff distance. However, this approach is only necessary if an analysis identifies the building to be at risk of attack as opposed to suffering collateral damage due to an attack on a nearby target. The setback applies not only to the sides that are adjacent to streets, but also to adjoining properties. Many buildings are designed with a large plaza area in front of the building while leaving little setbacks on the sides and rear of the building. Although vulnerability may be reduced at the plaza facing facade, the neighboring properties may change during the life of the building and this vulnerability may be increased over time.

Where the property does not provide sufficient setbacks, modifications to the surrounding site may be considered. Public parking abutting the building must be secured or eliminated, and street parking may be restricted adjacent to the building. Removing one lane of traffic and turning it into an extended sidewalk or plaza can gain additional standoff distance. However, the practical benefit of increasing the standoff depends on the charge weight. If the charge weight is small, this measure will significantly reduce the forces to a more manageable level. If the weight of explosive threat is large, the blast forces may overwhelm the structure despite the addition of nine or ten feet to the standoff distance and the measure may not significantly improve survivability of the occupants or the structure.

To guarantee the maximum standoff distance between unscreened vehicles and the structure, anti-ram elements must be located at the curb around the perimeter of the building. These elements may consist of plinth walls, planters, or bollards and are designed to prevent a vehicle from penetrating the secure perimeter based on the size of the design basis threat vehicle and maximum achievable velocity. Both the element and its foundation must be designed to resist the ramming load. Vehicle impact speeds will range based on the surrounding roadways with corners of the site providing the greatest vulnerability based on a straight-line approach or limited curvature to slow down the vehicle. A vector analysis of the site conditions will determine the maximum speeds attainable, and thus the kinetic energy that must be resisted.

Two representative anti-ram barrier types are shown in Figures 3a and 3b.

Landscaping features that can be robustly reinforced such as monumental stairs, planters with below grade foundations, statues, concrete seating, and water features may also effectively prevent a vehicle from ramming into the building by creating an obstacle course. This approach is most effective when several layers of devices can be constructed between the street and the building.

Even if anti-ram barriers are used to keep a vehicle borne explosive at a guaranteed standoff distance, a hand-carried weapon can still be placed directly against the exterior envelope. This weapon may be carried in a briefcase, backpack, or hand truck. The effective management of this threat requires a combination of operational security and screening around the building perimeter.

Preventing Progressive Collapse

A primary goal of blast mitigation is to prevent progressive collapse which historically has caused the most fatalities in terrorist incidents targeting buildings. Progressive collapse refers to the spread of an initial local failure from element to element, as adjoining members are overloaded, eventually resulting in a disproportionate extent of collapse relative to the zone of initial damage. An example of progressive collapse due to an explosive event is illustrated by the bombing of the Alfred P. Murrah Federal Building in Oklahoma City (see Figure 4).

Progressive collapse can propagate vertically upward or downward from the source of the initiating damage, or it can propagate laterally from bay to bay. A protective design may avoid structural systems that either facilitate or are vulnerable to a progression of collapse resulting from the loss of a primary vertical load-bearing member. Critical members may be designed to resist disproportionate collapse by either considering the loss of the member on the structural response (threat independent), or by designing the member explicitly for the defined explosive loading (threat specific). Although the American Society of Civil Engineers Standard for Mitigation of Disproportionate Collapse Potential in Buildings and Other Structures (ASCE 76-23) only considers threat independent design methods, the General Services Administration (GSA) permits the use of threat specific design methods.

The design criteria address the prevention of disproportionate collapse using one of the following approaches:

• indirect design by providing prescriptive design measures such as
  • requiring special ductile detailing at column-spandrel connections,
  • using two-way systems to improve redundancy
  • using moment frames for steel buildings
  • doubling the columns along the perimeter
  • using cables embedded in concrete spandrel beams
  • designing load bearing walls as deep beams to span across the damage
  • designing each floor level to carry its own weight following the removal of a column
• direct load design of the member to resist the explosive loads generated by design threat. For instance, the design of a column to resist the effects of a hand carried weapon placed directly against it.
• alternate load path design by considering the effect of loss of each perimeter column or other critical load bearing member on the stability of the building. Often corner columns present the greatest challenge to meeting the requirements for progressive collapse prevention.

New facilities may be designed to accept the loss of an exterior column for one or possibly two floors above grade without precipitating further collapse. In these cases, the design requirements are intended to be threat-independent to protect against an explosion of indeterminate size that might damage a single column, which results in adequate redundant load paths in the structure should damage occur due to an unspecified abnormal loading. Columns may also be sized, reinforced, or protected to prevent critical damage in response to a closely located, specified explosive threat. The vulnerable concrete columns may be jacketed with steel plate or wrapped with composite materials and the vulnerable steel columns may be encased in concrete to protect the cross sections and add mass.

However, the upgrade of existing structures to prevent localized damage from developing into a progressive collapse may not be easily accomplished through the alternate path method. This is because the loss of support at a column line would increase the spans of all beams directly above the zone of damage and require different patterns of reinforcement and different types of connection details than those typically detailed for conventional structural design. Hardening key elements of existing structures to resist design level explosive threats is better for preventing disproportionate collapse than supplementing the capacity of the connecting beams and girders or upgrading them using the alternate path method. However, the effectiveness of these approaches is predicated on the operational and technical security procedures that will limit the magnitude of the explosive threat. This includes the establishment of effective perimeter protection, adequate screening of vehicles entering an underground parking facility or loading dock, and inspection of parcels that may be hand carried into the building. For more information on retrofitting existing buildings, see WBDG Retrofitting Existing Buildings to Resist Explosive Threats.

Although the primary goal for most buildings is to reduce building damage and to prevent disproportionate collapse, a secondary goal is to maintain emergency functions until evacuation is complete. Evacuation, rescue and recovery efforts may be facilitated through effective placement, structural design, and redundancy of emergency exits and critical mechanical/electrical systems. Overall damage levels may be reduced to make it easier for people to egress and emergency responders to safely enter. Multiple, easily accessible, protected primary egress routes, free of debris caused by exterior envelope failure will be key to reaching these goals. Refer to WBDG Physical Security: Design of Building Envelopes for Blast Loads for additional information related to blast-hardened envelopes.

Floor Slab Reinforcements

A reinforced-concrete, flat-plate structural slab is an economical system that provides for maximum use of vertical space, particularly for buildings in areas with height restriction. However, when subjected to a blast load, punching shear, and softening of the moment-resisting capacity of the slabs will reduce the lateral-load-resisting capacity of the system. Once the moment-resisting capacity of the slabs at the columns is lost, the ability of the slab to transfer forces to the shear walls is diminished and the structure is severely weakened. In addition to the failure of the floor slab, the loss of contact between the slab and the columns may increase the unsupported column lengths, which may lead to the buckling of those columns. Furthermore, the lateral load resisting system—which consists of the shear walls, the columns, and the slab diaphragms that transfer the lateral loads—may be weakened to such an extent that the whole building may become laterally unstable.

Conventional flat-plate design may be upgraded by paying more attention to the design and detailing of exterior bays and lower floors, which are the most susceptible to an exterior vehicle explosive threat, and the design of the spandrel beams, which tie the structure together and enhance the response of the slab edge. Drop panels and column capitols may be used to shorten the effective slab length and improve the punching shear resistance. If vertical clearance is a problem, shear-heads embedded in the slab will improve the shear resistance and improve the ability of the slab to transfer moments to the columns. Furthermore, the blast pressures that enter the structure through shattered windows and failed curtain walls will load the underside and subsequently the top surfaces of the floor slabs along the height of the building. Both the delay in the sequence of loading and the difference in magnitude of loading will determine the net pressures acting on the slabs. Consequently, there will be a brief time in which each floor will receive a net upward loading. This upward load requires that the slab be reinforced to resist loads opposing the effects of gravity.

The ductility demands and shear capacity required to resist multiple-load reversals often force the engineer to provide beams to span over critical sections of the slab. The inclusion of beams will greatly enhance the ability of the framing system to transfer lateral loads to the shear walls. The slab-column interface should contain closed-hoop stirrup reinforcement properly anchored around flexural bars within a prescribed distance from the column face. Bottom reinforcement must be provided continuous through the column. This reinforcement serves to prevent brittle failure at the connection and provides an alternate mechanism for developing shear transfer once the concrete has punched through. The development of membrane action in the slab, once the concrete has failed at the column interface, provides a safety net for the post-damaged structure. Continuously tied reinforcement, spanning both directions, must be detailed properly to enable the tensile forces to be developed at the lapped splices. Anchorage of the reinforcement at the edge of the slab or at a structural discontinuity is required to guarantee the development of the tensile forces.

In all, the slab should be designed to prevent a punching shear failure that may in turn develop into a progressive collapse. Although research has shown that punching shear failures at interior columns are more likely to result in a progressive collapse than a failure at an exterior column, the external bay around the perimeter of the structure must be hardened at all intersecting columns for the external car bomb threat.

Column Reinforcement

For blast consideration, the distance from the explosion determines, to a great extent, the characteristics of the loading on a structure. For example, buildings located at a substantial distance from a protected perimeter—approximately 100 ft. or more—will be exposed to relatively low pressures fairly uniformly distributed over the façade; buildings located at shorter distances from the curb—most typical in urban environments—will be exposed to more localized, higher intensity blast pressures. Due to direct blast pressures, the columns of a typical building, which are designed primarily to resist gravity loads with no special detailing for ductility demands, may experience severe bending deformations in addition to the axial loads that the columns support. To enhance protection, the columns must be designed to be sufficiently ductile to sustain the combined effects of axial load and lateral displacement.

In conditions that cause uplift—the net upward load on the slab—the column will experience a brief tensile force. Conventional reinforced concrete columns not designed to resist the combined effects of bending may be prone to damage under these conditions. The lower-floor columns must therefore be designed with adequate ductility and strength to resist the effects of direct lateral loading from the blast pressure and impact of explosive debris. Reinforced concrete columns may be designed to resist the effects of an explosion by providing adequate longitudinal reinforcement, staggering the bar splices, and providing closely spaced ties at plastic hinge locations. Steel columns may be sized to withstand the lateral loads and column splices may be detailed to develop the plastic moments of the section. Existing concrete columns may be encased in a steel jacket or wrapped with a composite fiber to confine the concrete core and increase the shear capacity. On the other hand, existing steel columns may be encased in concrete to add mass and prevent a premature buckling of the thin flanges.

Transfer Girder Requirements

Transfer girders and the columns supporting transfer girders are particularly vulnerable to blast loading. Transfer girders typically concentrate the load-bearing system into a fewer number of structural elements, which contradicts the concept of redundancy desired in a blast environment. Typically, the transfer girder spans a large opening, such as a loading dock, or provides the means to shift the location of column lines at a particular floor. Damage to the girder may leave several lines of columns, which terminate at the girder from above, completely unsupported. Similarly, the loss of a support column from below will create a much larger span that bears critical loads. Transfer girders, therefore, create critical sections the loss of which may result in a progressive collapse. If a transfer girder is required and vulnerable to an explosive loading, then the girder should be designed to be continuous over several supports. There should be substantial structure framing into the transfer girder to create a two-way redundancy, thereby an alternate load path in the event of a failure. The column connections, which support the transfer girders, should be designed to resist both the shear forces and the axial tension forces that may result from large inelastic deformations.

Internal Partition Reinforcements

The lobby, loading dock, and mailroom are vulnerable locations where an explosive device like a hand delivered package bomb may be introduced prior to inspection and screening. These spaces are best located exterior to the main structure to limit the impact to adjacent spaces and the potential of progressive collapse. The lobby of the U.S. Courthouse in Seattle, Washington is an example of a building whose lobby is separate from the main structure (see Figure 11).

The walls of these spaces must be hardened to confine the explosive shock wave and permit the resulting gas pressures to vent into the atmosphere. This hardening can be achieved by designing the slabs and erecting cast-in-place reinforced-concrete walls, with the thickness and reinforcement determined relative to the appropriate threat. Connection details of the hardened walls will also need to be capable of resisting the blast pressures. Underground parking may also be susceptible to an explosive device that is concealed inside a vehicle, possibly unknown to the driver and should be isolated from occupied spaces. These structural designs must be integrated with the remainder of the structural frame to make sure they do not destabilize other portions of the gravity load-bearing system. In general, the hardening scheme should be coordinated with the security operations including screening policies.

Overall Lateral Resistance

The conventional lateral loads—wind and lower seismic design category forces—to which most buildings are designed are minimal in comparison to blast loads. These minimal lateral load requirements may be resisted by a combination of shear walls, braced frames, and moment-resisting frame action. At each floor level, the slab diaphragms transfer the lateral loads to the lateral-load resisting system. Each component of the lateral-load resisting system must be checked to determine its adequacy to resist blast loads. Depending on the results of a blast analysis, the individual elements of the lateral-load resisting system may require modification.

Buildings with an irregular floor plan will induce large torsional effects on the lateral-load resisting system. Typically, symmetrical buildings behave better when subjected to blast or seismic loading. If the shear core is centrally loaded a large demand is placed on the diaphragm action of the floor slab to transmit the lateral loads from the perimeter of the floor into the central shear walls. This effect can be more critical for blast load than for seismic load. Seismic base motions are typically applied over the entire foundation; blast loads resulting from a close-in explosion tend to impose higher intensity loads over a more concentrated region. Although the total base shears may be nominally the same, the lateral-resisting behavior is not. The usual rigid diaphragm action might not be suitable in such a localized blast situation and a full three-dimensional analysis of the building might be required.

The ability of structures to resist a highly impulsive blast loading depends in great measure on the structural detailing of the slabs, joists, and columns that provides for the ductility of the load-resisting system. The structure has to be able to deform inelastically under extreme overload (i.e., dissipate large amounts of energy) prior to failure. In addition to providing ductile behavior, there needs to be a well-distributed lateral-load resisting mechanism in the horizontal floor plan. The use of several shear walls distributed throughout the building will improve the overall seismic as well as the blast behavior of the building. If adding more shear walls is not architecturally feasible, a combined lateral-load resisting mechanism can also be used. A central shear wall and a perimeter moment-resisting frame will provide for a balanced solution. The perimeter moment-resisting frame will require strengthening the spandrel beams and the connections to the outside columns. This will also result in better protection of the outside columns. For more information on seismic design, see WBDG Seismic Design Principles.

Seismic Protection vs. Blast Protection

While seismic building design details enhance the ductility of structures increasing their capacity to sustain plastic hinges and withstand large rotations, it should not be taken to say that a structure designed to resist earthquakes would perform well in response to an explosive loading. It is important to understand that the nature of the blast loading and the resulting structural response is very different from a seismic event (see Figure 13).

Explosion loads act directly on the exterior envelope whereas earthquakes load buildings at the base of the building. Consequently, the focus is on out of plane response for explosions and in plane response for seismic loads (see Figure 14).

Explosion loads are characterized by a single high pressure impulsive pulse acting over milliseconds rather than the vibrational loading of earthquakes which is acting over seconds (see Figure 15).

Explosion loads generally cause localized damage whereas seismic loads cause global response (see Figure 16).

Lateral loads resulting from strong ground motions are proportional to the mass, which is distributed throughout the building. Conversely, blast design relies, to some extent, on the inertial resistance of massive structural elements. Finally, seismic resistance is distributed globally throughout the structure whereas blast hardening must provide protection against localized explosive loads.

The desirable features of earthquake-resistant design—that is, the provision for ductility in member response and connection details, and redundancy in the ability to redistribute extreme loads to lesser-loaded elements—are equally desirable in blast design. In both cases, it is the obligation of the engineer to guarantee that the full capacity of the section be realized and that no premature failure, resulting from inadequate confinement of a reinforced concrete section or the local buckling of steel sections, prevents the structure from transferring the loads to the foundation. Provisions have been established for the design of structures to resist seismic forces that provide both the ductility of the members and the capacity of the connections to undergo large rotations without failing. Chapter 18 of the American Concrete Institute 318 was developed to improve the behavior of reinforced concrete structures subjected to large inelastic deformations. Similarly, ANSI/AISC 341-16 Seismic Provisions for structural Steel buildings is a consensus standard for the design and construction of structural steel and composite structural steel/reinforced concrete buildings detailed for seismic resistance. It is recommended that those provisions be adhered to in designing the blast load resisting structural components. However, the required extent of confinement and ductility, and the location of the stress concentrations which form as a result of blast loading will not be the same as for structures subjected to a seismic event.

Alternative Construction Materials to Resist Explosive Threats

A variety of materials, not traditionally used in building construction, may provide alternatives to conventional blast hardening solutions. Among these alternatives there are shock attenuating chemically bonded ceramics (SA/CBC), and composite systems comprised of carbon, aramid and polyethylene fibers and resin. These materials are well-developed systems currently in use for the prevention of sympathetic detonation of explosives in munitions storage depots (SA/CBC materials) as well as in the seismic retrofit of reinforced concrete columns in highway bridges in California (carbon fiber wrapping). In the latter application, carbon fiber wrappings were found to have advantages over conventional steel jacketing of columns due to problems with weld seams and corrosion. Carbon fiber materials are also used to reinforce the top surface of existing reinforced concrete slabs in response to blast uplift pressures and rebound response. Spray-on elasto-polymers have been demonstrated to protect unreinforced masonry walls by providing a ductile membrane that enables these brittle elements to sustain large deformations without fragmenting and throwing hazardous debris.

In retrofit scenarios where conventional structural treatments may be too heavy or too labor intensive, composite materials may be attractive alternatives because of their lightweight and high tensile strength. However, full scale and component testing are required to collect data on the performance of these materials in blast scenarios as well as in different structural configurations. Ultimately, a set of analysis procedures and structural engineering guidelines are needed in order for engineers to specify such materials in both the retrofit of structures and in new construction.

Nonstructural Components

Nonstructural building components, such as piping, ducts, lighting fixtures and conduits, must be sufficiently tied back to structural elements to prevent failure of the services and falling debris hazards. To mitigate the effects of in-structure shock, due primarily to the infilling of blast over-pressures through damaged windows, these nonstructural systems should be located below the raised floors or tied to the ceiling slabs with seismic restraints. The requirements for the attachment of non-structural components for the different Seismic Design Categories are detailed in Chapter 13 of ASCE 7. These requirements are based on importance factors, which may be similarly related to the design of non-structural systems in response to blast loads.

Emerging Issues

New research and information regarding innovative structural materials and systems to be considered for blast mitigating design of civilian buildings is rapidly evolving. The solutions presented herein are based on nominal modifications to conventional construction to enhance overall protection levels. Generally, this approach will provide a cost-effective solution. If innovative solutions are used, then the explosive test reports should be reviewed to verify the system performs as described. If the site-specific application is significantly different from the tested specimen, then a computational analysis or explosive test may be required to verify the results.

Department of Defense agencies have sponsored explosive tests on conventional building materials, as well as some innovative solutions and have made some of this information available to the public. The Defense Threat Reduction Agency and the Army Corps of Engineers have been active in this area. An internet search is recommended to locate the latest information that has been made available.

Multi-hazard approaches to blast mitigation technologies and compatibilities between blast resistance and sustainability suggest synergies may be beneficial to both. While timber stick-built construction is commonly used in conventional structures, its use in blast design is rare as timber does not have the material properties necessary to withstand blast loading. However, there has been recent blast testing and research conducted in Canada that suggests that there may be viability in utilizing cross-laminated timber (CLT) under very low blast loading. The US Army Corps of Engineers published technical report PDC-TR 18-02 "Analysis Guidance for Cross-Laminated Timber Construction Exposed to Airblast Loading" which provides guidance and performance criteria for CLT.

Relevant Codes and Standards

Federal standards and criteria are widely recognized as the primary source of guidelines for the design of buildings to resist explosive threats. Because of the uniqueness of each building's mission, functional requirements, and physical security design objectives, there are limited codes and standards that apply to blast mitigation design.

Federal Guidelines

• Department of Defense
  • Unified Facility Criteria (UFC)
    • UFC 1-200-01, DoD Building Code
    • UFC 4-010-01, DoD Minimum Anti-Terrorism Standards for Buildings—Establishes prescriptive procedures for Threat, Vulnerability and Risk assessments and security design criteria for DoD facilities.
    • UFC 4-020-01, DoD Security Engineering Facilities Planning Manual—Provides requirements for security and anti-terrorism.
    • UFC 4-023-03, Design of Buildings to Resist Progressive Collapse
• Department of State
  • Overseas Building Operations (OBO) Design Standards including Appendix P: Physical Security Standards Handbook
  • Overseas Building Operations—International Codes Supplements (OBO-ISC)
• General Services Administration (GSA)
  • PBS-P100 Facilities Standards for the Public Buildings Service
• Department of Homeland Security (DHS)
  • The Risk Management Process—An Interagency Security Committee Standard including Appendix A, Appendix B, and Appendix C— Defines criteria for determining facility security level, determination of the design basis threat, and provides physical security countermeasures for all nonmilitary federal facilities
• Department of Veterans Affairs (VA)
  • Physical Security and Resilience Design Manual (PSRDM)

Private Sector Guidelines

• ACI 318 Building Code Requirements for Structural Concrete and Commentary, Chapter 21, American Concrete Institute
• ASCE/SEI 59 Blast Protection of Buildings by the American Society of Civil Engineers.
• Blast Effects on Buildings, 2nd Edition by David Cormie, Geoff Mays and Peter Smith. London: Thomas Telford Publications, 2009.
• Design of Blast Resistant Buildings in Petrochemical Facilities. American Society of Civil Engineers, 2010.
• Handbook for Blast Resistant Design of Buildings by Donald O. Dusenberry, 2009.
• Structural Design for Physical Security by Task Committee on Structural Design for Physical Security of the Blast, Shock, and Impact; Committee of the Dynamics Effects Technical Administration; Committee of the Structural Engineering Institute of ASCE.

Public Testing Institutions

• ASTM International
• Underwriters Laboratories (UL)

Private Testing Laboratories

• Many private laboratories with expertise in protective glazing systems testing are also available. Contact the Protective Glazing Council for additional information and referral.

Additional Resources

Federal Security Criteria Centers

• Defense Threat Reduction Agency—Department of Defense (DOD) Anti-terrorism body—Pentagon's J34
• Department of Homeland Security—The Federal Protective Service (FPS)
• Cybersecurity and Infrastructure Security Agency (CISA)
• U.S. Army Corps of Engineers, Protective Design Center
• U.S. Department of Defense

Organizations and Associations

• American Concrete Institute (ACI)
• American Institute of Steel Construction (AISC)
• American Society of Civil Engineers, (ASCE)—Publishes papers and guidelines related to blast design in a structural engineering publications database.
• Federal Emergency Management Agency (FEMA)—Offers guidance on man-made disaster threat assessment and mitigation planning.
• Federal Facilities Council (FFC) Standing Committee on Physical Security & Hazard Mitigation
• Glass Association of North America (GANA)
• International Window Film Association (IWFA)
• National Concrete Masonry Association (NCMA)
• Protective Glazing Council
• Safety Glazing Certification Council (SGCC)

Publications

• Architectural Design for Security and Security and Technology Design by Donald M. Rochon. June 1998.
• BIPS 05 Preventing Structures from Collapsing by Department of Homeland Security (DHS). 2011.
• BIPS 06 / FEMA 426 Reference Manual to Mitigate Potential Terrorist Attacks Against Buildings by the Department of Homeland Security, October 2011.
• Designing for Crime and Terrorism, Security and Technology Design by Randall I. Atlas. June 1998.
• FEMA 427 Primer for Design of Buildings to Mitigate Terrorist Attacks by the Federal Emergency Management Agency.
• The Avoidance of Progressive Collapse, Regulatory Approaches to the Problem, PB-248 781 by the Department of Housing and Urban Development, Division of Energy, Building Technology and Standards, Office of Policy Development and Research, Washington D.C. 20410, October 1975.