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Designing Buildings to Resist Explosive Threats

by Robert Smilowitz
Weidlinger Associates, Inc.

Last updated: 10-19-2011

Introduction

The four basic physical protection strategies for buildings to resist explosive threats are 1) Establishing a secure perimeter; 2) Mitigating debris hazards resulting from the damaged façade (see also WBDG Glazing Hazard Mitigation; 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

A. Defining the Design Threat

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.

Blast Loading

Because 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 highly 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.

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. 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 glass and the support mullions, 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.

Performance Standards

Analytical tools that evaluate the likely performance of curtain-wall façades in response to blast loads are used to demonstrate compliance with established blast criteria or performance specifications. Many of these performance 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. Depending on the specified performance conditions, the application of this criterion could have significant impact on the sizing of the members and the design of the connections between the different components.

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.

B. 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 by maximizing the keep-out 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.

To guarantee the maximum keep-out distance between unscreened vehicles and the structure, anti-ram bollards or large planters must be placed at the curb around the perimeter of the building. The site conditions will determine the maximum speeds attainable, and thus the kinetic energy that must be resisted. Both the bollard and its foundation must be designed to resist the maximum load. Conversely, if design restrictions limit the capacity of the bollard or its foundation, then site restrictions will be required to limit the maximum speed attainable. Furthermore, public parking abutting the building must be secured or eliminated, and street parking should not be permitted 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 threat is a large charge weight, 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.

Façade Protection

The building's exterior is its first real defense against the effects of a bomb. How the façade responds to this loading will significantly affect the behavior of the structure. Hardening of the façade is typically the single most costly and controversial component of blast protection, and may produce a dramatic change to the exterior appearance of the structure such as smaller window sizes and more rugged attachments. Moreover, given the large surface areas of most buildings, modest levels of protection may not be cost-effective. Therefore, it may be best to concentrate on improving the post-damaged behavior of the façade.

Except for very thick lights, most glazing materials and components designed to respond to the blast loads will most likely be damaged by the blast overpressures. To improve the post-damage behavior of the glazing system, one could specify laminated glass for new construction or apply anti-shatter film to existing glazing. While these features do little to improve the strength of the glass, they attempt to hold the shards of glass together and better protect the occupants from hazardous debris (see also WBDG Glazing Hazard Mitigation. Laminated glass possesses the best post-damage behavior, may be used with a wide variety of glazing materials and thickness, and provides the highest degree of safety to occupants. The effectiveness of Mylar films, on the other hand, depends on the method of application and the thickness of the film. Common film systems range from a simple edge-to-edge (daylight) application, to a wet glazed adhesion, to a mechanical attachment to the existing window frame. The mechanical attachments are most effective when they are anchored to the underlying structure. Regardless of the method, there are architectural issues and life-cycle costs associated with the use of anti-shatter films.

Equally important to the design of the glass is the design of the window frames. For the window to properly fail, the glass must be held in place long enough to fail. Short of that, the glazing will dislodge from the housing intact and cause serious damage or injury. The capacity of the frame system to resist blast loading should therefore exceed the corresponding capacity of the glazing, often referred to as the "glass fail first criteria." Factors of two to three, over the nominal capacity of the glass to resist breakage, may be required to design the frames. The bite, including the possible use of structural silicone sealant, must be adequate to assure the failed glass is retained within the frame. Depending on the façade, the mullions may be designed to span from floor to floor or tie into wall panels and must be capable of withstanding the reactions of a window loaded to failure. Finally, the walls to which the windows are attached must be designed to accept the reaction forces as well.

Designing glazing systems capable of resisting a specified overpressure requires a cascade of costly upgrades to the façade, including really thick laminated glass, and relatively heavy frames and mullions. There are also major construction challenges such as reinforcement and steel embedments that get in the way of new cast-in-place reinforced concrete wall construction, and the substantial anchorage required to accommodate the large reaction forces. Moreover, attaching these window systems to existing walls may even be a physical impossibility. Because the improved capacity is likely to fall far short of the pressures associated with a realistic terrorist threat, it is recommended that for new construction with low threat criteria and limited budgets for blast protection, the engineer select the weakest laminated glazing that satisfies wind and serviceability requirements. In this way, the improved post failure behavior provides the occupants a measure of protection at a reasonable cost.

Curtain Wall Protection

Photo of a sample blast curtainwall

Fig. 1. Sample blast curtain wall engineered to take advantage of a flexible system. Some protective features may include: insulated glazing unit with laminated inner light; glazing adhered to mullion with structural silicone sealant; and curtain-wall frame with steel backup encased in aluminum.

A curtain wall is a nonbearing exterior enclosure that is supported by a building's structural steel or concrete frame and holds either glass, metal, stone, or precast concrete panels. Lightweight and composed of relatively slender extruded aluminum members, curtain-wall façades are considerably more flexible than conventional, hardened punched window systems. In a blast environment, the mullion support would absorb a portion of the blast energy and improve the performance of the glazing, allowing the glazing to sustain greater blast environments (although the mullions themselves should be designed to resist the forces collected by the glass).

It is important to take into account the inherent flexibility of curtain-wall systems when sizing members for blast loads and evaluating the glazing for hazard. This enables the engineer to both ascertain the true blast worthiness of the curtain wall as well as to properly calculate the reduced load transfer into supporting structural elements.

The design of curtain-wall systems to withstand the effects of explosive loading depends on the performance of the various elements that comprise the system. Curtain-wall response software, based on more sophisticated finite element methods than simplified Single-Degree-of-Freedom glass fragment hazard analytical approaches, was developed for the Department of Defense, Technical Support Working Group (TSWG) to accurately represent the capacity of the glazing and the supporting frame members. While the glazing may be the most brittle component, the performance of the system, and the reduction of hazard to the occupants depend on the interaction between the capacities of the various elements. In addition to hardening the individual members that comprise the curtain-wall system, the attachments to the floor slabs or spandrel beams require special attention. These connections must be adjustable to compensate for the fabrication tolerances and accommodate the differential inter-story drifts and thermal deformations as well as be designed to transfer gravity loads, wind loads, and blast loads.

Energy-Absorbing Catch Systems

Photo of a sample catch system

Fig. 2. Sample catch system

An alternative approach to blast protection takes the concept of a flexible curtain-wall system one step further by making full use of the flexibility and capacity of all the window materials to absorb and dissipate large amounts of blast energy while preventing debris from entering the occupied space. Energy-absorbing catch systems (a.k.a. Cable Protected Window Systems (CPWS)) work in such a way that as the glass is damaged it bears against a cable catch system, which in turn deforms the window frames. Extensive explosive testing, as well as sophisticated computer simulations, has demonstrated the effectiveness of these systems.

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.

Drawing of floor slabs

Fig. 3. Floor slabs

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.

Diagram showing column heads and a drop panel.

Fig. 4. Column heads

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 ensure that the tensile forces can 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 Reinforcements

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.

Diagram showing direct pressure being applied to the lower level of a structure and the effects being extreme bending underdirect pressure.

Fig. 5. Direct pressure

In conditions that cause uplift—the net upward load on the slab—the column's tension 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. For more information on retrofitting existing buildings, see WBDG Retrofitting Existing Buildings to Resist Explosive Threats.

Diagram showing the bottom slab being pressurized first, the lower pressure moved to the top of the slab and the new upward pressure on the slab will result in an uplifting of columns.

Fig. 6. Uplift

Weakened connection diagram showing the slab columns connections weakened under the blast pressure therefore allowing the unsupported columns to lead to a loss of stability.

Fig. 7. Weakened connection

Preventing Progressive Collapse

In addition to façade debris hazards, building occupants may be vulnerable to heavier debris resulting from structural damage. Progressive collapse occurs when an initiating localized failure causes adjoining members to be overloaded and fail, resulting in an extent of damage that is disproportionate to the originating region of localized failure. 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.

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. 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. For more information on retrofitting existing buildings, see WBDG Retrofitting Existing Buildings to Resist Explosive Threats.

Alternatively, columns may be sized, reinforced, or protected to prevent critical damage as a result of the design threat charge weight that may be located in close proximity to them. 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. For the upgrade of existing structures, these approaches are better for preventing progressive 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.

Catenary diagram showing the effects of post catenary action.

Fig. 8. Catenary

Transfer Girder Reinforcements

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, totally 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. So 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 as Type 2 connections to provide sustained strength despite inelastic deformations.

Transfer girders diagram showing: 1-blast pressure on the transfer girder, 2-transfer girder collapses, 3-trigger collapse of other parts of the system.

Fig. 9. Transfer girders

Overall Lateral Resistance

The conventional lateral loads—wind and seismic zone 1 forces—to which most buildings are designed are minimal. 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.

Diaphragm action diagram. The first part shows a typical floor buckling in the middle during seismic activity. The second shows the effects of a blast on a non-uniform blast surface.

Fig. 10. Diaphragm action

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. Provisions have been established for the design of structures to resist seismic forces that ensure both the ductility of the members and the capacity of the connections to undergo large rotations without failing. For example, the provisions of Chapter 21 of the American Concrete Institute 318 were devised to improve the behavior of reinforced concrete structures subjected to large inelastic deformations.

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.

Internal Partition Reinforcements

The walls surrounding loading docks, mailrooms, and lobbies—where explosive threats, like a hand delivered package bomb, may be introduced prior to inspection and screening—must be hardened to confine the explosive shock wave and permit the resulting gas pressures to vent into the atmosphere. Specific modifications to the features of these unprotected spaces can prevent an internal explosion from causing extensive damage and injury inside the building. 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. The isolation of occupied spaces from these vulnerable locations and any other unsecured spaces, such as basements and underground parking garages, requires both adequate levels of reinforcement as well as connection details capable of resisting the collected blast pressures. 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.

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. 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.

Seismic Protection vs. Blast Protection

It is often stated that blast damage would be reduced if 'seismic-like' construction standards were adhered to. However, this should not be taken to say that a structure designed to resist the effects of strong ground motions would perform well in response to an explosive loading. It is true that seismic building design details enhance the ductility of structures and thereby increase their capacity to sustain plastic hinges and withstand large rotations. Furthermore, for reinforced concrete structures, closely spaced stirrups improve the confinement of the core and increase the shear capacity of the section. Yet, it is important to understand that the nature of the blast loading and the structure's response to it is very different from a seismic event.

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. Chapter 21 of the American Concrete Institute 318 was developed to improve the behavior of reinforced concrete structures subjected to large inelastic deformations. 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. Furthermore, 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. Therefore, it should not be assumed that a structure adhering to the governing building codes' recommended provisions for seismic design or designed to withstand a strong ground motion is sufficient to resist the prescribed blast loading or prevent subsequent progressive collapse. For more information on seismic design, see WBDG Seismic Design Principles.

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 Zone IV restraints.

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

Private Sector Guidelines

Public Testing Institutions

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

WBDG

Building Types

Federal Courthouse, Land Port of Entry, Office Buildings, Research Facilities

Design Objectives

Aesthetics, Cost-Effective, Functional / Operational, Productive, Secure / Safe, Sustainable

Products and Systems

Section 07 92 00: Joint Sealants, Building Envelope Design Guide: Cast-in-Place Concrete Wall Systems, Masonry Wall Systems, Precast Concrete Wall Systems, Thin Stone Wall Systems, Fenestration Systems, Glazing, Windows, Curtain Walls, Sloped Glazing, Exterior Doors, Atria Systems

Federal Security Criteria Centers

Organizations and Associations

Publications