Centuries ago castles and moats addressed the need to keep a facility safe from an attacker. From those massive stone and wood structures, to the hardened reinforced concrete and sophisticated intrusion detection systems of the present, the principles of hardened structures have fundamentally remained the same: Identify the baseline threat and keep it at a safe distance, or create a structure as impervious as possible to that threat. Bruce Walton provides a broad, overall perspective on the problem of designing a hardened structure, and describes some of the techniques, fundamentals, and resources available.
Introduction
Protective structures over the years have relied on distance and mass for protection. For thousands of years, people have used caves and massive stone or wood structures to protect assets. Exterior walls had few openings because doors and windows are difficult to harden and defend. Defenders have used guards, fences, walls, ditches, hills, moats and other barriers to keep potential threats at a safe distance. Like ancient protective structures, most hardened structures today use massive construction of wood, rock, soil, or reinforced concrete with few windows or doors. Contemporary threats are kept at a safe standoff by operational and physical means similar to those used over the millennia. This article provides a broad, overall perspective on the problem of designing a hardened structure. A hardened facility design example is presented to demonstrate the procedure.
Definitions
The terms “hardened structure” and “protective structure” mean different things in different contexts, and lately with the increase in the terrorist threat, the common definitions have changed again. Antiterrorism Protection, Physical Security, and Hardened Structures are terms being used by many. The following definitions will hold for the bounds of this article:
Physical Security
Physical Security consists of measures taken to address criminal and vandal threats. Physical Security uses defensive measures that provide layers of detection and delay around an asset. The defensive layer must provide enough delay time to allow a response force to halt the attack. For the DOD, Physical Security is addressed primarily by policy that defines operational procedures, electronic security systems, and structural security measures to provide the required delay time. The assumption is that some minimal level of protection is required and risk is evaluated on an organization-wide basis with the assumption that there is always a criminal threat.
Antiterrorism Protection
Antiterrorism Protection addresses the design of both the building and the site to minimize the blast loads and weapon effects from terrorist threats to assets - usually people. This may mean the building is destroyed, but damage to assets is minimized. The actual threat to a specific asset is seldom known and it is unlikely that a specific asset will ever have a terrorist attack. The price people are willing to pay for protection from an unlikely threat of unknown magnitude has historically been very little in this country, but it is changing. As part of Antiterrorism Protection, blast hardening is sometimes done, but does not commonly meet the level of protection in the following definition of a hardened structure.
Hardened Structure
A Hardened Structure is usually designed to perform its primary mission after a wartime attack making hardening one of its primary requirements and a significant part of its cost. The facility is protected against a wide range of threats including forced entry, Chemical/Biological/Radiological (CBR), airblast, ground shock, penetration, fragmentation, and damage to the structure and equipment due to explosive loading. Designs must consider how camouflage, concealment and deception, active defense, and manned response can reduce or limit the effectiveness of the threat. The design assumptions are that during a war, the facility will be attacked and that it must survive and function after the attack. Almost all hardened structures inherently satisfy the requirements for both Physical Security and Antiterrorism Protection.
Likelihood of Protection
The conceptual differences between the three types of protective measures defined above are the likelihood of the protection actually being needed, the consequences of it not working, and the willingness of the user to pay for the protection. The government is willing to pay a limited price for physical security for all facilities and a high price for hardened structures for specific assets. In the past we funded antiterrorism protection at a low level because the likelihood was low, but in light of recent events, our population is re-evaluating this stance.
Designing for Wartime Threats
Designing facilities hardened for wartime threats is sometimes politically easier than designing normal facilities for the terrorist threat; because the users of the wartime hardened facilities understand the importance of hardening and are willing to give up things like large doors and windows, fancy interior finishes, and easy access. Some of the key aspects in design include:
Conventional Weapons
A wartime conventional weapon threat can range from airblast only to direct hits from precision-guided bombs and penetrators. Fully hardened facilities are designed to withstand a direct hit and detonation of a penetrating weapon. Semi-hardened facilities are designed to withstand small area weapons and near miss detonations of larger bombs. Other protected facilities are only designed to withstand airblast and fragments from bombs detonating at a distance.
Balanced Survivability
Whatever the threat, the designer tries to incorporate balanced survivability into the building. Balanced survivability is a condition wherein no significant facility failure mode has been overlooked or its importance underestimated, thus the facility has no “Achilles Heel.” Balanced survivability exists for a facility when all critical subsystems and resources required for accomplishing the facility’s mission are equally survivable at a specified threat level.
A balanced survivability assessment (BSA) determines the capability of a facility to survive against a specified threat spectrum and still perform its mission. The BSA is a systems approach to survivability, yielding recommendations that facility designers can use to make prudent investment decisions in light of what they consider to be the most critical systems and most worrisome threats. A BSA can be performed on a facility design or an operational facility, and it is ideal if a team trained in BSA techniques examines design drawings early to identify potential survivability flaws. Balanced survivability ensures that no threat is neglected, and that all threats are addressed consistently. Additional design considerations are re liability, maintainability and logistics.
Incorporating post-attack expedient measures for a facility's systems that could help it recover quickly after an attack (or pre vent further damage) should be considered. Such measures may include incorporating utility cutoffs, additional fire protection, adequate utility backup connections, and structural repair kits.
Site Planning
Key elements in planning the site include:
Dispersion Placing resources in irregular patterns, and using physical separation, orientation, staggering, and system component distribution will increase survivability. Dispersion greatly increases an attacker’s targeting difficulties, and reduces the chance of simultaneous or collateral damage from any single strike.
Orientation Hardened facilities should be oriented so their most vulnerable sides face away from nearby critical structures. Aircraft shelter entrances should not face each other or nearby critical facilities. This decreases the potential for damage to vulnerable sides of the structure if a nearby structure is hit. A critical review of the site, its surroundings, and the building’s orientation and location on the site should be performed. If this siting analysis shows an explosive threat is more probable from one direction, the facility should be oriented and/or the entrances located to minimize blast and fragment loads on the blast door.
Separation From a survivability standpoint, there is an optimum distance between hardened facilities, such that no two facilities can be attacked by a single weapon or be acquired by an airborne target acquisition system on a single pass. Siting facilities too far apart however, may degrade their operational performance.
Building Layout
Redundancy The survivability and overall operability of the protected system can be improved by incorporating redundant facilities, components, paths, and circuits into the system. In this manner, damage to one part of the system will not necessarily shut down the entire system, but instead shift the operation to a redundant part.
Footprint and Floor Plan The footprint of a hardened structure should be a rectangle, square, or other regular geometric shape that attenuates the effect of an explosive blast. Designers should avoid re-entrant corners that tend to amplify blast pressure and enhance a structure’s radar image. (Areas such as recessed entryways contain re-entrant corners.) Activities of a less critical nature should be located on the exterior of the building. Hallways should be located along the exterior wall. Compartmentalized functional areas (isolation zones) should be considered to prevent fire or internal bomb blasts from propagating from one area or zone to another. Compartmentalization can be accomplished both by careful functional zoning and by proper design of walls, internal blast doors, and other separations.
Exterior Openings Exterior openings include personnel and equipment access, fresh air ventilation, cooling, and combustion equipment intake and exhaust portals. Designers should anticipate the possibility of blast pressure, heat, dust, fragments, and toxic gases entering the facility through exterior openings, and take appropriate preventive measures. Entrance openings should be kept as small and few in number as possible to minimize shielding problems, but still satisfy operational and emergency ingress and egress requirements.
Proportioning components The structural design process has two major, interdependent phases: (1) selecting a trial structural configuration (arrangement, shape, and material), and (2) proportioning components to prevent failure under prescribed influences. The proportioning phase is calculational in nature, and therefore requires a numerical response threshold (performance criterion) for each failure mode (failure modes are established during design). Typical failure modes are those associated with airblast, fragmentation, spall, weapon penetration or perforation, shock motion, cratering, fire, suffocation, and CBR agents. For the various failure modes, the performance criteria quantify the survivability requirements of the protected system elements and functional spaces in terms of personnel tolerances, equipment tolerances, endurance periods, and post failure capabilities.
Terrorist Threats
Once a defined threat is specified, standard design procedures for hardened structures are applied. Even if no threat is defined, the DOD has determined that a minimum level of protection is warranted for all inhabited buildings, and Unified Facilities Criteria (UFC) 4-010-01 “DOD Minimum Antiterrorism Standards for Buildings” is applied. This standard establishes criteria for DOD-inhabited buildings to minimize the potential for mass casualties and progressive collapse from a terrorist attack. The overarching antiterrorism philosophy is that an appropriate level of protection can be provided for all DOD personnel at a reasonable cost, and reduces the risk of mass casualties. Full implementation of the standards provides a level of protection against all threats and significantly reduces injuries and fatalities for the threats upon which these standards are based. The costs for these protective measures are not significant for most projects. The primary methods used to achieve this outcome are to maximize the standoff distance, to construct superstructures resistant to progressive collapse, and to reduce flying debris hazards from glazing.
Maximize Standoff Distance
Maximizing the standoff distance keeps the threat as far away from critical buildings as possible. It is the easiest and least costly method for achieving the appropriate level of protection to a facility. When standoff distance is not available, the structure needs to be hardened to give the same level of protection that it would have with a greater standoff. While sufficient space around a structure is not always available to provide the minimum standoff distances required for conventional construction, maximizing the available standoff distance will always result in the most cost-effective structural solution. Maximizing standoff distance also ensures that there is opportunity in the future to upgrade buildings to meet increased threats or to accommodate higher levels of protection. If minimum standoff distances are achieved, conventional construction should minimize the risk of mass casualties from a terrorist attack, with only a marginal impact on the total project cost.
Progressive Collapse Avoidance
Progressive collapse is a chain reaction of failures following damage to a relatively small portion of a structure. The resulting damage from a progressive collapse failure is out of proportion to the damage of the initial failed area. Consequences of progressive collapse are unnecessary loss of life and the entrapment of survivors in the collapsed structure. The UFC has provisions that minimize the ability of the structure to go into a progressive collapse mode of failure. Designing those provisions into the buildings before construction begins, or during a major renovation project is the most cost effective solution. All inhabited structures of three stories or more, are to have a progressive analysis performed. This analysis assures that the structure will remain stable when key members are removed and is accomplished by providing structural continuity, redundancy, or energy dissipating capacity (ductility) in the remaining members of the structure. There are two approaches to perform a progressive collapse analysis - the direct and the indirect methods.
Direct Design Approach Direct design explicitly considers structural resistance through the alternate path method or through the specific local resistance method. When a local failure occurs, such as the removal of a structural member, the alternate path method seeks to find a load path that will absorb the loads created. The specific local resistance method applies loads to the structure that must be accounted for in the design.
Indirect Design Approach Indirect design implicitly considers a structure’s resistance to progressive collapse by defining a minimum level of strength, continuity, and ductility for structural members. Typical guidance recommends using highly redundant structural systems such as moment resisting frames, continuity across joints so the member can develop the full structural capacity of the connected members, and design members that accommodate large displacements without complete loss of strength. Other design details that minimize the possibility that collapse of one part of the building will affect the stability of the remainder of the building should be incorporated.
Examples include designing floor systems with top and bottom steel to accommodate load reversal, and designing building additions to be structurally independent from the protected portions of the existing building.
Minimize Hazardous Flying Debris
A high number of injuries result from flying glass fragments and debris from walls, ceilings, and fixtures (non-structural features). Flying debris is minimized through the proper design and selection of appropriate building materials. The glazing used in most windows will break at very low blast pressures, creating hazardous, dagger-like shards. The simplest protection from flying debris is to minimize the number and sizes of windows used in the building design. Additional protection can be garnered by using enhanced window units. Blast-resistant window and door units must be purchased as complete, tested assemblies that include the glazing unit, door or window frame, and frame connections to the structure. When installed, these elements become an integrated structural system. The UFC requires that all glazing units use a 1/4-inch laminated glass in all new construction and major renovations.
Observations of Conventional Structures
Review of typical structures often reveals that structural members have different capacities during the positive and negative phases of the blast load. Also, these members can have significant blast load capacity, but the connections may not. Special provisions of the concrete and steel design codes need to be followed to make a structure perform well, even when a reasonable amount of standoff is provided. Conventional design of buildings results in balanced design for normal loads and usually a very unbalanced survivability for blast loads. Most buildings are initially designed for easy access and natural lighting, which results in numerous lightweight doors, and larger windows. Hardening doors and windows for blast and fragment loadings is difficult and very expensive, typically 2 to 10 times that of normal construction. This results in a significant increase in building cost. Typical roof construction is kept lightweight especially in high seismic areas and the lack of mass in these elements makes it difficult to design them for blast loads.
Bruce Walton, PE
Protective Design Center
US Army Corps of Engineers
Omaha, NE
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