Blast Safety of the Building Envelope
Last updated: 10-19-2011
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This section addresses the mitigation of explosion effects on the exterior envelope of a new building designed to meet federal anti-terrorist design requirements. The recommendations given are primarily focused on meeting the ISC Security Design Criteria, but are also useful for understanding the anti-terrorist design requirements of other government agencies including the U.S. Department of Defense and U.S. Department of State. Although the concepts presented are for new buildings, many of the same concepts may be applied to retrofit of existing buildings. Both vehicle and hand delivered weapons targeting the exterior envelope are considered.
Existing criteria documents vary in the level of detail that they provide and all have room for interpretation. A 'blast consultant' with expertise in structural dynamics and experience with the governing criteria documents can be a valuable resource for the team throughout design and construction. Often blast consultants are required for projects which meet anti-terrorist design criteria if explicit computation of structural response to explosive loads is required. Design criteria will give the requirements that this specialist needs to meet such as the number of years of experience and formal technical training in structural dynamics.
Designing security into a building requires a complex series of trade-offs. Security concerns need to be balanced with many other design constraints such as accessibility, initial and life-cycle costs, natural hazard mitigation, fire protection, energy efficiency and aesthetics. Because the probability of attack is very small, there is a desire for security not to interfere with daily operations of the building. On the other hand, because the effects of attack can be catastrophic, there is a desire to incorporate measures that will save lives and minimize business interruption in the unlikely event of an attack. The countermeasures should be as unobtrusive as possible to provide an inviting, efficient environment, and not attract undue attention of potential attackers. Security design needs to be part of an overall multi-hazard approach to the design, to ensure that the solution for explosion effects does not worsen the behavior of the building for other hazards. Conversely, multi-use solutions which improve the building performance for blast and other considerations such as sustainability are to be encouraged (See WBDG Resource Page Balancing Security/Safety and Sustainability Objectives).
The primary design objective is to save the lives of those who visit or work in these government buildings in the unlikely event that an explosive terrorist attack occurs. In terms of building design, the first goal is to prevent progressive collapse which historically has caused the most fatalities in terrorist incident targeting buildings. Beyond this, the goal is to provide design solutions which will limit injuries to those inside the building due to impact of flying debris and air-blast during an incident, and to limit harm to innocent civilians near the building perimeter. Finally, we seek to facilitate the rescue/recovery efforts by limiting the debris blocking access to the building and potential falling debris hazards which could harm rescue workers. In some cases, secondary objectives may need to be considered such as maintaining critical functions and minimizing business interruption.
The recommendations given are solution-focused. They are intended for designers who are tasked with implementing federally mandated anti-terrorist design criteria into projects, recognizing that these requirements need to be balanced and integrated with many other design constraints such as sustainability, construction and life-cycle costs, constructability, architectural expression and natural hazards protection. To maximize the benefit provided by the recommendations, anti-terrorist considerations should be implemented at the earliest planning and design stages possible. This will ensure that the resulting design maximizes protection while integrating with other design considerations.
In this sub-section the threat, loads and damages resulting from explosions are explained.
The primary threat is a stationary vehicle weapon located along a secured perimeter line surrounding the building (see Figure 1). Depending on the accessibility of the site to vehicles there may be more than one line of defense to consider. The outermost perimeter line is often a public street secured against vehicular intrusion using barriers and with limited secured access points. The size of the vehicle weapon considered outside the perimeter line may vary from hundreds to thousands of pounds of TNT equivalent depending on the criteria used. Weapon sizes vary depending on the specific criteria used and may be obtained from the federal agency client on a need to know basis.
Figure 1. Vehicle Weapon Threat
This threat is to be considered on all sides of the building with a public street or adjacent property lines along the secured perimeter line. Because air-blast loads decay rapidly with distance, the highest loads are at the base of the building and decay with height. Benefit of these reduced loads is usually not realized in terms of reduced design requirements except for high rise structures.
The required building setback may vary from tens to hundreds of feet depending on the criteria governing design. This is a fundamental requirement of design and should not be taken lightly. All the other requirements given in the criteria document are based on this setback being met. If this requirement can not be met, the contracting agency needs to be contacted at the earliest time possible to discuss the implications. In many cases not meeting this setback will translate into more severe design requirements for the exterior envelope. In other cases a waiver needs to be obtained to proceed with the design.
Sometimes the criteria document will consider the stationary vehicle threat of a weapon that manages to pass through security screening along the perimeter. For instance, this threat may be located in an employee parking stall near the building, or at the loading dock. This weapon is typically smaller than the weapon considered along the outermost perimeter line because the vehicle entering the screened area is assumed to have been inspected. The size of the weapon is based on the maximum amount that could be carried in the vehicle without attracting attention. A minimum separation distance between secured surface parking areas and the building are specified in the criteria document.
To protect against moving vehicular attacks, it is recommended that the barriers along the secured perimeter have anti-ram capability consistent with the size of the weapon and the maximum achievable velocity up to 50 miles per hour. Typically, for portions of the building that are parallel to adjacent streets, a maximum velocity of 30 miles per hour is considered. For street corners, or "T" intersections, a velocity of up to 50 miles per hour is considered. The weight of the vehicle may vary from 4000 pound car to 15000 pound truck depending on the criteria used. Two typical anti-ram barriers types are shown in Figures 2 and 3.
Left: Figure 2. Anti-ram bollards; Right: Figure 3. Anti-ram knee wall
Landscaping features may also be used effectively to thwart a vehicle from ramming into the building by creating an obstacle course. Monumental stairs, permanent planters against the building, statues, concrete seating, water features and other features can be effectively used to resist vehicular intrusion. This often is the approach which has been used for some Design Excellence projects where architectural integrity is paramount. This method is most effective when there is sufficient setback to have several layers of devices between the street and the building. CPTED (Crime Prevention Through Environmental Design) concepts may also be effective.
A secondary threat that is sometimes considered is a hand carried weapon placed directly against the exterior envelope. This weapon may be carried in a briefcase, backpack or hand truck depending on the level of security screening provided outside the building.
Note that some design criteria documents assume that no weapon is able to pass through the outmost security point based on the operational measures implemented. This will need to be verified prior to proceeding with design. In this case, only the vehicle weapon is considered outside the secured perimeter line.
Figure 4. Air-blast pressures as a function of weapon size and distance
Explosive pressures used for design are typically much greater than the other loads considered. Fortunately they decay extremely rapidly with time and space. As a rule of thumb, the pressures generated increase linearly with the size of the weapon, measured in equivalent pounds of TNT, and decrease exponentially with the distance from the explosion (see Figure 4). The duration of the explosion is extremely short, measured in thousandths of a second, or milliseconds (see Figure 5). Effects of shock wave expansion and engulfment of the building are shown in Figure 6.
Figure 5. Air-blast as a function of time
Figure 6. Sequence of air-blast effects
Some other air-blast effects to be aware of include:
- Pressure acting on the side of the building facing the explosion is amplified by factors that can be ten times the incident pressure. This pressure is referred to as the reflected pressure. Since it is not known which sides of the building the explosion will act on typically, all sides need to be designed for the worst case.
- Air-blast pressures have a negative or suction phase following the direct or positive pressure phase. The negative phase pressures can govern response in low pressure regions causing windows to fail outward or sloped roof systems to fall off the building.
- Slender members such as exposed columns which have less surface area for the air-blast to act on tend to be more sensitive to drag effects rather than direct pressure loading because the air-blast tends to "wrap-around" these members lessening the time that the reflected air-blast is acting.
- Rebound of the exterior envelope components following the explosion can pull the façade members off the building exterior. Note that this effect is different from the negative pressure effects discussed above. Rebound refers to the reversal of structural motion due to vibration rather than the reversal of loading direction. Since the design objective is to protect occupants, failure of the exterior envelope in the outward direction may be acceptable provided that the hazards of falling debris post-event and blocked egress points are avoided.
- Immediately below the weapon, a crater will be formed which may cause damage to underground portions of the building which cause damage to the foundation and the sub-surface roof and foundation walls which extend beyond the line of the superstructure.
- In addition to the propagation of a pressure wave through the air, a proportion of the energy of the weapon is transmitted through the soil. This effect is analogous to a high intensity, short duration earthquake which can disturb the functionality of computers and mechanical/electrical equipment. For above ground explosions, this effect is negligible and is generally neglected in design.
Air-blast parameters for a defined weapon size and distance from the exterior envelope may be determined by using charts found in military handbooks or by using government sponsored software products such as CONWEP or ATBlast.
Figure 7. Khobar Towers Bombing, 1996
Damage due to explosions may be divided into direct air-blast effects and progressive collapse. Applicable criteria documents provide guidance regarding how to handle each of these design conditions.
Direct air-blast effects refer to damage caused by the high-intensity pressures of the air-blast close in to the explosive source. These may induce the localized failure of exterior envelope components. The severity of response is a function of the size of the weapon, its proximity to the exterior envelope and the construction materials used.
An example of an exterior envelope failure due to direct air-blast effects is the bombing of the Khobar Towers military housing complex in Daharain, Saudi Arabia in 1996 (see Figure 7).
Progressive collapse refers to the spread of an initial local failure from element to element, eventually resulting in a disproportionate extent of collapse relative to the zone of initial damage. Localized damage due to direct air-blast effects may or may not progress, depending on the design and construction of the building. 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 8).
Figure 8. Alfred P. Murrah Federal Building (Oklahoma City Bombing, 1995)
For a stationary vehicle weapon located outside the secured perimeter line, the building facade facing the explosion will be most affected, with the worst damage directly across from the weapon. For the design threat, there may or may not be some glass breakage allowed depending on the criteria used and the level of protection assigned. There may be some localized wall failure and/or wall damage as well. However, the frame will remain intact and the hazard presented by the damaged exterior envelope will be reduced. It is unlikely that this threat will initiate progressive collapse unless the threat is very large and/or very close to the building.
For hand-carried weapons placed next to the exterior envelope, the direct air-blast response will be more localized than for curbside vehicle weapons but more severe, with damages and injuries extending a bay/floor or two. More extensive damage, possibly leading to progressive collapse may occur if the hand carried weapon is strategically placed directly against a primary load bearing element such as a column. Critical members will need to be designed to resist progressive collapse by either considering the loss of the member on the structural response, or by designing the member explicitly for the defined explosive loading. The governing criteria will clarify design procedures.
Flying debris generated by non-structural portions of the exterior envelope also has the capability of causing damage to the building envelope. Though this loading is only referred to in passing in criteria documents for federal buildings, the governing agency may request that this be investigated in some circumstances. An example of where this issue would be raised is when sunshades or other lightweight materials are attached to the building exterior.
Progressive collapse can propagate vertically upward or downward from the source of the explosion, and it can propagate laterally from bay to bay as well. The design criteria address the issue of progressive collapse by using a variety of approaches including:
- indirect design by providing prescriptive design measures such as requiring special seismic detailing at column-spandrel connections, design of the roof for a prescribed static loading, or using two-way systems to improve redundancy.
- 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.
In some cases a separate criteria document is provided for progressive collapse prevention which outlines in detail what the designer needs to do to satisfy the requirements. Some solution concepts for meeting these requirements include:
- the use of 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
- design each floor level to carry its own weight
To save lives, the primary goals of the design professional are to reduce building damages and to prevent progressive collapse of the building, at least until it can be fully evacuated. A secondary goal is to maintain emergency functions until evacuation is complete. For mission critical facilities, where the facility must be functional rapidly after an incident, a higher level of protection is required. Finally, good anti-terrorist design is a multidisciplinary effort requiring the concerted efforts of the architect, structural engineer, security professional and the other design team members. It is also critical for security design to be incorporated as early as possible in the design process to ensure a cost effective, attractive solution.
Preventing the building from collapsing is the most important objective. Historically, the majority of fatalities that occur in terrorist attacks directed against buildings are due to building collapse. Collapse prevention begins with awareness by architects and engineers that structural integrity against collapse is important enough to be routinely considered in design. Features to improve general structural integrity against collapse can be incorporated into common buildings at affordable cost. At a higher level, design for progressive collapse can be accomplished by the alternate path method (i.e. design for removal of specific elements) or by direct design of components for air-blast loading or by the indirect method of prescribing design features which promote redundancy and ductility.
Explicit progressive collapse methodologies have been developed by the GSA and the Department of Defense for their facilities. The Department of State approach for progressive collapse has been largely prescriptive and is included with their criteria for blast resistant design.
Furthermore, building design may be optimized by facilitating evacuation, rescue and recovery efforts through effective placement, structural design, and redundancy of emergency exits and critical mechanical/electrical systems. Through effective structural design the overall damage levels may be reduced to make it easier for people to get out 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.
Beyond the issues of collapse, and evacuation/rescue our objective is to reduce flying debris generated by failed exterior walls, windows and other components to reduce the severity of injuries and the risk of fatalities. This may be accomplished through selection of materials and use of capacity design methods to proportion elements and connections. A well designed system will provide predictable damage modes, selected to minimize injuries.
To reduce the hazards associated with fragments being propelled into the building interior the envelope system is designed by keeping in mind the concepts of balanced design, ductile response, and redundancy. The use of dynamic non-linear structural analysis methods is also beneficial when designing the exterior envelope of a building to resist air-blast effects.
Intuitively, it may seem that heavier stiffer systems are preferable to thinner more flexible systems. However, lighter systems are generally preferable for mitigating explosive effects, provided that they are designed to be ductile, redundant, balanced, and can resist the design load with the required response. Although heavier systems have added mass which is advantageous in mitigating the effects of explosions, they may be more prone to brittle failure if not properly designed and can impart significantly larger loads into the supporting structure behind the envelope. The larger loads may cause structural failures and perhaps initiate progressive collapse. These more robust solutions have their place in high risk buildings or in localized areas closest to the threat.
However, the lighter more flexible systems tend to be preferred solutions in the majority of civilian buildings being designed to resist air-blast effects today. By permitting some permanent damage to the exterior envelope, which does not significantly increase the hazard to the occupants, it is possible to design lighter, more cost effective, systems that absorb energy through deformation, and transmit lower forces into the connections and supporting structure, thus reducing the potential for more serious structural failures.
The design approach to be used for the structural protective measures is to first design the building for conventional loads, then evaluate the response to explosive loads and augment the design, if needed, making sure that all conventional load requirements are still met. This ensures that the design meets all the requirements for gravity and natural hazards in addition to air-blast effects.
Take note that explosive load effects mitigation may make the design more hazardous for other types of loads and therefore an iterative approach may be needed. As an example, for seismic loads, increased mass generally increases the design forces, whereas for explosion loads, mass generally improves response. Careful consideration between the blast consultant and the structural engineer is needed to provide an optimized response.
As an air-blast is a high load, short duration event, the most effective analytical technique is dynamic analysis, allowing the element to go beyond the elastic limit and into the plastic regime. Analytical models range from handbook methods to equivalent single-degree-of-freedom (SDOF) models to finite element (FE) representation. For SDOF and FE methods, numerical computation requires adequate resolution in space and time to account for the high-intensity, short-duration loading and non-linear response. Difficulties involve the selection of the model, the appropriate failure modes, and finally, the interpretation of the results for structural design details. Whenever possible, results are checked against data from tests and experiments for similar structures and loadings.
Exterior envelope components such as columns, spandrels and walls can often be modeled by a SDOF system and then solving the governing equation of motion by using numerical methods. Handbook methods may be used to evaluate the peak displacement response of structural components using graphs that require only that the designer define a few parameters including the ultimate resistance, fundamental period, and elastic limit deflection. Other charts are available which provide damage estimates for various types of construction based on peak pressure and peak impulse based on analysis or empirical data. Military design handbooks typically provide this type of design information. The design of the anchorage and supporting structural system may be evaluated by using the ultimate flexural capacity of the member.
For SDOF systems, material behavior may be modeled using idealized elastic, perfectly-plastic stress-deformation functions, based on actual structural support conditions and strain rate enhanced material properties. The model properties selected provide the same peak displacement and fundamental period as the actual structural system in flexure. Furthermore the mass and the resistance function are multiplied by mass and load factors, which estimate the actual portion of the mass or load participating in the deflection of the member along its span.
For more complex elements, the engineer must resort to finite element numerical time integration techniques and/or explosive testing. The time and cost of the analysis cannot be ignored in choosing design procedures. Because the design process is a sequence of iteration, the cost of analysis must be justified in terms of benefits to the project and increased confidence in the reliability of the results. In some cases, an SDOF approach will be used for the preliminary design and a more sophisticated approach, using finite elements, and/or supported by explosive testing may be used for the final verification of the design.
A dynamic non-linear approach is more likely to provide a section that meets the design constraints of the project compared with a static approach. Elastic static calculations are likely to give overly conservative design solutions if the peak pressure is considered without the effect of load duration. By using dynamic calculations instead of static, we are able to account for the very short duration of the loading. Because the pressure levels are so high, it is important to account for the short duration to mitigate response. In addition, the inertial effect included in dynamic computations greatly improves response. This is because by the time the mass is mobilized, the loading is greatly diminished, enhancing response. Furthermore, by accepting that damage occurs we are able to account for the energy absorption of ductile systems that occurs through plastic deformation. Finally, because the loading is so rapid, we are able to enhance the material strength to account for strain rate effects.
Response is evaluated by comparing the ductility (i.e., the peak displacement divided by the elastic limit displacement) and/or support rotation (the angle between the support and the point of peak deflection) to empirically established maximum values which have been established by the military through explosive testing. Note that these values are typically based on limited testing and are not well defined within the industry at this time. Maximum permissible values vary depending on the material and the acceptable damage level. Some criteria documents do provide the design values that need to be met. Other criteria are silent on this topic or make a general reference to a source document.
Acceptable Damage Levels
Levels of damage computed by means of analysis may be described by the terms: minor, moderate or major depending on the peak ductility, support rotation and collateral effects. A brief description of each damage level is given below.
Minor: Non-structural failure of building elements as windows, doors, and cladding. Injuries may be expected, and fatalities are possible but unlikely.
Moderate: Structural damage is confined to a localized area and is usually repairable. Structural failure is limited to secondary structural members, such as beams, slabs and non-load bearing walls. However, if the building has been designed for loss of primary members, localized loss of columns may be accommodated without initiating progressive collapse. Injuries and possible fatalities are expected.
Major: Loss of primary structural components such as columns or transfer girders precipitates loss of additional adjacent members that are adjacent or above the lost member. In this case, extensive fatalities are expected. Building is usually not repairable.
Generally, moderate damage at the "Design Threat" level is a reasonable design goal for new construction. For buildings that need to remain operational post-event or are designated as high risk, minor damage may be the more appropriate damage level.
The building envelope is the most vulnerable to an exterior explosive threat because: it the part of the building that is closest to the weapon and it is the critical line of defense for protecting the occupants of the building. Progressive collapse provisions are perhaps the single most effective measure that can be implemented to save lives and should be considered above any other upgrades. Laminated glass is perhaps the single most effective measure to reduce extensive injuries.
Early consideration of man-made hazards will significantly reduce the overall cost of protection and the inherent protection level provided to the building. If protection measures are considered as an afterthought not considered until the design is nearly complete, the cost is likely to be greater because more areas will need to be structurally hardened due to poor planning. An awareness of the threat of man-made hazards from the beginning of a project also helps the team to decide early what the priorities are for the facility. Including protective measures as part of the discussion regarding trade-offs early in the design process often helps to prioritize goals.
Site and Architectural Issues
The placement of the building on the site can have a major impact on its vulnerability. Ideally, the building is placed as far from the property lines as possible. This applies not only to the sides that are adjacent to streets, but the sides that are adjacent to adjoining properties since we can not be certain about who will occupy the neighboring properties during the life of the building. A common practice example of this is to creating a large plaza area in front of the building, but leaving little setback on the sides and rear of the building. Note that this practice generally increases the vulnerability of the other three sides. Also, if this approach is used, the exterior envelope may extend beyond the superstructure below ground which will also need to be considered as part of the protective design effort.
Figure 9. Affect of building shape on air-blast loading
The shape of the building can have a contributing effect on the overall damage to the structure (see Figure 9). Reentrant corners and overhangs are likely to trap the shock wave, which may amplify the effect of the air-blast. Note that 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 decay 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 damages to supporting beams.
Figure 10. U.S. Courthouse, Seattle, Washington
(photo courtesy of Frank Ooms/NBBJ)
It is recommended that the lobby and loading dock areas which are vulnerable to attack are placed exterior to the main structure to limit the potential of progressive collapse. An example of a building where the lobby is separate from the main structure is the U.S. Courthouse in Seattle, Washington (see Figure 10).
Generally simple geometries, with minimal ornamentation (which may become flying debris during an explosion) are recommended unless advanced structural analysis techniques are used. If ornamentation is used, it is recommended that it consists of a light weight material such as timber or plastic which are less likely to become lethal projectiles in the event of an explosion than for instance brick, stone or metal.
Soil can be highly effective in reducing the impact of a major explosive attack. Bermed walls and buried roof tops have been found to be highly effective for military applications and can be effectively extended to conventional construction. This type of solution can also be effective in improving the energy efficiency of the building. Note that if this approach is taken, no parking is to be permitted over the building.
The exterior walls are subject to direct reflected pressures from an explosive threat located directly across from the secured perimeter line. The objective of design at a minimum is to ensure that these members fail in a ductile mode such as flexure rather than a brittle mode such as shear. The walls also need to be able to resist the loads transmitted by the windows and doors. It is not uncommon for instance for bullet resistant windows to have a higher ultimate capacity than the walls to which they are attached. Beyond ensuring a ductile failure mode, the exterior wall may be designed to resist the actual or reduced pressure levels of the defined threat. Note that special reinforcing and anchors are provided around blast resistant window and door frames.
It may be advantageous to consider the reduction in pressure with height due to the increase in distance and the angle of incidence at the upper levels of a high rise building. If pressure reductions are taken into account at the upper floors, minimum requirements such as balanced design, ductile response and redundancy are to be met to reduce the hazard to occupants in case the actual explosion is greater than the design threat.
Various types of wall construction are considered below.
Poured-in-place Reinforced Concrete
Figure 11. U.S. Embassy, Kampala, Uganda
Ductile poured-in-place reinforced concrete will provide the highest level of protection. For high risk facilities which are vulnerable to large explosive threats this is the material of choice. Virtually all new U.S. embassies are constructed using this material (see Figure 11).
Ductile reinforced concrete is also is recommended for portions of lower risk buildings that do not meet required minimum setbacks or which house critical functions such as primary egress paths or high occupancy areas.
Historically, the preferred material for explosion mitigating construction is cast-in-place ductile reinforced concrete. This is the material that is used for military bunkers, and the military has performed extensive research and testing its performance. Reinforced concrete has a number of attributes that make it the construction material of choice. It has significant mass, which improves response to explosions because the mass is often mobilized only after the pressure wave is significantly diminished, reducing deformations. Members can be readily proportioned and reinforced for ductile behavior. The construction is unparalleled in its ability to achieve continuity between the members.
Note that for reinforced concrete to respond favorably to explosion loads, it must be detailed in a ductile manner such as is done in seismic zones. Non-ductile concrete design such is used in non-seismic design can perform badly, as is witnessed by the collapse of the Alfred P. Murrah Building in Oklahoma City. Some attributes of ductile design for blast design are as follows:
- Use symmetric reinforcement on both faces.
- Span the wall floor to floor rather than from column to column.
- Stagger splices away from high stress areas.
- Space bars no more than one wall thickness apart, but no less than one half the wall thickness apart.
- Use ductile special seismic detailing at connections.
- Use development lengths to develop the ultimate flexural capacity of the section.
- For progressive collapse prevention, consider the loss of exterior wall that measures vertically one floor height and laterally one bay width.
- Use closed ties or spiral reinforcing along the entire length of beams and columns including connections with a minimum bend angle of 135 degrees and a spacing not exceeding d/2.
For pre-cast panels, consider a minimum thickness of five inches exclusive of reveals with two-way reinforcing bars to increase ductility and reduce the chance of flying concrete fragments. This is to reduce the loads transmitted into the connections which need to be designed to resist the ultimate flexural resistance of the panels. In high pressure regions, using ribbed panels may be an effective way to resist the loads. These panels need to bear against the floor diaphragms.
The following are recommendations and considerations in designing pre-cast elements for air-blast resistance.
Reinforcement: Two-way, symmetric reinforcement is recommended to accommodate large deformations and rebound loads. For thin panels where it is difficult to place two layers of reinforcement, the use of two layers of heavy wire mesh, one layer of two-way reinforcement along the centerline, or staggered bars on either face may be considered. If a single layer of reinforcement is used, it is critical to design the section so that the steel yields before the concrete fails in compression to obtain a ductile response. Enhanced protection may be provided by placing Fiber Reinforced Polymers, geotextile materials, Kevlar or similar materials on the inside face to provide confinement, fragment restraint and added tensile capacity. If reinforcing bars are used, a layer of wire mesh on the interior face may help to further restrain concrete fragments from entering the space. Closely spaced bars also help fragment restraint, but care must be taken not to increase the ultimate flexural capacity too much so as to keep the reaction loads to a reasonable level. Note that centerline reinforcement will not work in the rebound direction due to the failure of the tension concrete during the positive phase. If the primary objective is to protect occupants and not passersby, then this may be acceptable. Above major egress points however, where debris outside the building presents an obstacle to ingress and egress post event, added protection is desirable.
Pre-tensioned or post-tensioned construction provides little capacity for abnormal loading patterns and load reversals. If these systems are used, it is recommended that reinforcing bars are added to the design to provide blast mitigating properties.
Load bearing systems: For load bearing pre-cast systems, panels need to designed to span over failed areas by means of arching action, strengthened gravity connections, secondary support systems or other means of providing an alternate load path.
Connections: Ductile connections should be used. In the event the actual air-blast loading is higher than the design load, the connections and supporting structure needs to be able to accept the loads transmitted by the panel loaded to its ultimate flexural capacity. Reaction loads from the windows at ultimate capacity are to be included in the calculation of connection design loads. Using this approach, every panel with a different configuration will have a different set of loads used for designing the connections. Note that small panels will have higher reaction loads than larger panels using this method due to their increased stiffness. Standardizing the panel sizes greatly simplifies the connection load determination.
Also, the connections need to provide sufficient lateral restraint for the panels to accept large deformations. Depending on the design details of the connection used, lateral restraint design will require consideration of in-plane shear, buckling, flexure and/ or tension loads in the design. Punching shear through the panel also needs to be checked for the ultimate flexural capacity of the panel. The connection should provide a direct load path from the panel into the supporting structure to minimize P-delta effects.
In seismic areas where connector rods are used to permit large in-plane motion it is recommended that buckling due to out-of-plane motion be checked.
Floor-to-Floor panels with continuous, bearing type connections directly into the floor diaphragms are preferred. Multi-story panels directly bearing on the floor diaphragms may also be considered. Connections into exterior columns or spandrel beams are discouraged to avoid the possibility of initiating structural collapse of the exterior bay. Redundant gravity connections are strongly recommended to prevent falling debris if a single connection fails.
Connections should be checked for rebound loads. It is conservative to use the same load in rebound as for the inward pressure. More accurate values may be obtained through dynamic analysis or charts provided in military handbooks.
Specifications: Specifications for pre-cast elements can be either in the form of a performance requirement, with the air-blast pressures and required performance provided, or as a prescriptive specification with equivalent static pressures provided. The equivalent static pressures are computed based on the peak dynamic response of the panel for the defined threat. The performance specifications give the pre-cast contractor more flexibility to provide the systems with which they are most familiar. However, it requires that the contractor either have in-house dynamic analysis capability or have a relationship with a blast engineer who can work with them to customize the most cost-effective system.
On the other hand, as static equivalent pressures are based on the specific panel's response to the air-blast load. Changing dimensions, reinforcement, or supported elements would require recalculation of the static equivalent load and are therefore not recommended. However, when using the static equivalent loads, the designer may proceed normally with the lateral design process, using a load factor of one.
Note that equivalent static values are different from quasi-static values which assume a displacement ductility less than one. The equivalent static values are based on computations that are non-linear, with ductilities in excess of one.
For structural pre-cast systems, the connections are the critical issue to be addressed. Connections need to have adequate development lengths and strength to permit the panel to reach its ultimate flexural capacity. Extensive use of transverse walls or an 'egg crate' type of design is an effective way to achieve the needed lateral support (see Figure 7). Special seismic detailing is recommended for structural pre-cast connections.
For CMU block walls, use 8 inch block walls, fully grouted with vertical centered reinforcing bars placed in each void and horizontal reinforcement at each layer. Connections into the structure are to resist the ultimate lateral capacity of the wall. For infill walls, avoid transferring loads into the columns if they are primary load carrying elements. The connection details may be very difficult to construct. It will be difficult to have all the blocks fit over the bars, near the top, and it will be difficult to provide the required lateral restraint at the top connection. A preferred system is to have a continuous exterior CMU wall that laterally bears against the floor system. For infill walls, use development lengths which develop the full capacity of the section. For increased protection, consider using 12 inch blocks with two layers of vertical reinforcement. Also, consider using CMU block units that encourage a homogenous response such as an 'I' shaped unit with inner and outer faces that are connected with a small strut.
Brick load bearing walls resist blast mostly through mass and so thicker solid walls on the order of 18 inches or so can perform well at pressure levels less than 10 psi or so. Brick walls with Dynamic structural response are computed by using a kinematic model with bearing providing resistance at the hinge points.
Masonry is considered a much more brittle material that may generate highly hazardous flying debris in the event of an explosion and is generally discouraged for new construction.
Metal Stud Systems
For metal stud systems, use metal studs back to back and mechanically attached, to minimize lateral torsional effects. To catch exterior cladding fragments, attach a wire mesh, steel sheet, or Fiber Reinforced Polymer to the exterior side of the metal stud system. The supports of the wall should be designed to resist the ultimate lateral out-of-plane bending capacity load of the system.
If a single stud system is used, 16 gauge systems with depth of 6 inches, and laterally braced are preferred.
Special care is required at the connections to insure that failure does not occur prior to the stud reaching its ultimate capacity. Deeper, thicker channels are preferred. Additional lateral support using angles or other method may be needed along the interior to prevent failure.
Enhanced protection may be provided by placing Fiber Reinforced Polymers, geotextile materials, Kevlar or similar materials on the inside face cladding to provide confinement, fragment restraint and added tensile capacity. These materials are to bear directly against the metal studs.
Timber should not be used for blast mitigating solutions. Timber is too light and fragile to resist the types of loads discussed here.
Brick veneers, bris soleil, sunshades and other non-structural elements attached to the building exterior are to be avoided to limit flying debris and improve emergency egress by ensuring that exits remain passable. If used they should be designed using lightweight materials with connections designed to resist the capacity of the element.
For the exterior frame, there are two primary considerations. The first is to design the exterior columns to resist the direct effects of the specified threats. The second is to ensure that the exterior frame has sufficient structural integrity to accept localized failure without initiating progressive collapse. To meet these goals column spacing should be limited to 30 feet and floor heights should be limited to not greater than 16 feet wherever possible.
Because columns do not have much surface area, air-blast loads on exposed stand alone columns that are not supporting adjacent wall systems tend to be mitigated by "clearing-time effects". This refers to the pressure wave washing around these slender tall members and consequently the entire duration of the pressure wave does not act upon them. On the other hand, the critical threat is directly across from them, so they are loaded with the peak reflected pressure which is typically several times larger than the incident or overpressure wave that is propagating through the air.
For columns subject to a vehicle weapon threat on an adjacent street, buckling and shear are the primary effects to be considered in analysis. For a very large weapon close to a column, shattering of the concrete due to multiple tensile reflections within the concrete section can destroy its integrity. Circular columns shed load more rapidly than rectangular columns and can be beneficial.
Buckling is a concern if lateral support is lost due to the failure of a supporting floor system. This is particularly important for buildings that are close to public streets. In this case, exterior columns should be capable of spanning two or more stories without buckling.
Attempting to circumvent progressive collapse prevention provisions given in criteria documents by using a glass façade with the first line of columns inset a few feet into the building interior is not acceptable.
Confinement of concrete, using columns with closely spaced closed ties or spiral reinforcing, will improve confinement, shear capacity. It also will improve the performance of lap splices in the event of loss of concrete cover, and greatly enhance column ductility. The potential benefit to cost for providing closely spaced closed ties in exterior concrete columns is amongst the highest. Closed ties are to be used in columns and spandrels for the entire span and across connections with a maximum spacing of d/2 and a minimum bend angle of 135 degrees. Some other recommendations for reinforced concrete are given in wall subsection.
For steel columns, splices should be placed as far above grade level as practical. It is recommended that splices at exterior columns, which are not specifically designed to resist air-blast loads, employ complete penetration welded flange. Welding details, materials, and procedures should ensure toughness. Moment frame design is also recommended. As a fail safe protection measure for reducing the chances of progressive collapse, place an angle seat an inch or so beneath the spandrel to stop it from falling. It this concept is used, it is to be in conjunction with whatever the criteria document requires in terms of analysis and design.
For a package weapon, column breach is a major consideration. To mitigate this threat, some suggestions include:
- Do not use exposed columns that are fully or partially accessible from the building exterior. Arcade columns should be avoided.
- Use an architectural covering that is at least six inches from the structural member. This will make it considerably more difficult to place a weapon directly against the structure. Because explosive pressures decay so rapidly, every inch of distance will help to protect the column.
- Use steel plates at the base of concrete columns where it is accessible. Plates need to extend several feet above the accessible location.
- Encase steel columns with concrete and add a plate around the perimeter near the base if necessary.
Load bearing ductile reinforced concrete wall construction without columns can provide a considerable level of protection if adequate reinforcement is provided to achieve ductile behavior. This may be an appropriate solution for the parts of the building that are closest to the secured perimeter line (less than twenty feet). Masonry is considered a much more brittle material that is capable of generating highly hazardous flying debris in the event of an explosion and is generally discouraged for new construction.
Spandrel beams of limited depth generally do well when subject to air-blast. In general, edge beams are very strongly encouraged at the perimeter of concrete slab construction to afford frame action for redistribution of vertical loads and to enhance the shear connection of floors to columns. Confinement of concrete spandrels using spirals or closely spaced closed ties such as is used for columns is recommended. Transfer conditions are to be avoided.
Windows, once the sole responsibility of the architect become a structural issue once explosive effects are taken into consideration. In designing windows to mitigate the effects of explosions they are first to be designed to resist conventional loads and then to be checked for explosive load effects and balanced design.
Balanced or capacity design philosophy means that the glass is designed to be no stronger than the weakest part of the overall window/wall system, failing at pressure levels that do not exceed that of the frame, anchorage and supporting wall system. If the glass is stronger than the supporting members, than the window is likely to fail with the whole panel entering into the building as a single unit, possibly with the frame, anchorage and the wall attached. This failure mode is considered more hazardous than if the glass fragments enter the building, provided that the fragments are designed to minimize injuries. By using a damage limiting approach, the damage sequence and extent of damage is controlled.
Windows are typically the most vulnerable portion of any building. Though it may be impractical to design all the windows to resist a large scale explosive attack, it is desirable to limit the amount of hazardous glass breakage to reduce the injuries. Typical annealed glass windows break at low pressure and impulse levels and the shards created by broken windows are responsible for many of the injuries incurred due to large scale explosive attack.
Designing windows to provide protection against the effects of explosions can be effective in reducing the glass laceration injuries. For a large-scale vehicle weapon, this pressure range is expected on the sides of surroundings buildings not facing the explosion, or for smaller explosions where pressures drop more rapidly with distance. Generally we do not know which side of the building the attack will occur on so all sides need to be protected. Window protection should be evaluated on a case by case basis by a qualified blast consultant to develop a solution that meets established objectives. A number of generic recommendations are given in Figure 12 for the design of the window systems to reduce injuries to building occupants.
Figure 12. Preferred failure modes for windows
To limit glass laceration injuries, there are several approaches that can be taken. One way is to reduce the number and size of windows. If blast resistant walls are used then fewer and/or smaller windows will cause less air-blast to enter the building thus reducing the interior damage and injuries. Specific examples of how to incorporate these ideas into the design of a new building include: limiting the number of windows on the lower floors where the pressures are higher due to an external explosive threat; using an internal atrium design with windows facing inward not outward; clerestory windows which are close to the ceiling, above the heads of the occupants; and angling the windows away from the curb to reduce the pressure levels.
It may be advantageous to consider the reduction in pressure with height due to the increase in distance and the angle of incidence at the upper levels of a high rise building. If pressure reductions are taken into account at the upper floors, minimum requirements such as balanced design, ductile response and redundancy are to be met to reduce the hazard to occupants in case the actual explosion is greater than the design threat.
Glass curtain wall systems have been determined in recent explosive tests to perform surprisingly well to low levels of explosive loads. These systems have been shown to accept large deformations without the glass breaking hazardously compared to rigidly supported punched window systems. Some design modifications may be required to the connections, details and member sizes to optimize the performance.
Glass walls or windows at emergency exits are to be avoided to facilitate egress. Wire glass is to be avoided because of the severity of the injuries it may cause if it becomes flying debris.
Government design criteria generally specify either the threat or the loading pressure and impulse that blast mitigating windows need to be designed for. Pressure levels given vary from about 4 psi up to about 40 psi depending on the criteria document.
Typically, projectile impact loads are not considered for air-blast like they are for wind loads. However, Dade County certified windows for hurricanes may have a higher level or inherent blast resistance compared with other conventional window types. Impact resistant systems need to be checked to ensure that they meet the air-blast design criteria.
Glass is often the weakest part of a building, breaking at low pressures compared to other components such as the floors, walls, or columns. Past incidents have shown that glass breakage and associated injuries may extend many thousands of feet in large external explosions. High-velocity glass fragments have been shown to be a major contributor to injuries in such incidents. For incidents within downtown city areas, falling glass poses a major hazard to passersby and prolongs post-incident rescue and clean-up efforts by leaving tons of glass debris on the street. At this time, exterior debris is largely ignored by existing criteria.
As part of the damage limiting approach, glass failure is not quantified in terms of whether breakage occurs or not, but rather by the hazard it causes to the occupants. Two failure modes that reduce the hazard posed by window glass are:
- glass that breaks but is retained by the frame
- glass fragments exit the frame and fall within 3 to 10 feet of the window
Figure 13. Illustration showing performance level conditions for windows
The glass performance condition is defined based on empirical data from explosive tests performed in a cubical space with a 10 foot dimension (see Figure 13). The performance condition ranges from 1 which corresponds to not breaking to 5 which corresponds to hazardous flying debris at a distance of 10 feet from the window. A description of each of these performance levels is given in Table 1. Generally a performance condition 3 or 4 is considered acceptable for buildings that are not at high risk of attack. At this level, the window breaks, fragments fly into the building but land harmlessly within 10 feet of the window or impact a witness panel 10 feet away no more than 2 feet above the floor level. The design goal is to achieve a performance level less than 4 for 90% of the windows.
Table 1. Performance Conditions for Windows
|Performance Condition||Protection Level||Hazard Level||Description of Window Glazing|
|1||Safe||None||Glazing does not break. No visible damage to glazing or frame.|
|2||Very High||None||Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable.|
|3a||High||Very Low||Glass cracks. Fragments enter space and land on floor no further than 1 meter (3.3 feet) from window.|
|3b||High||Low||Glazing cracks. Fragments enter space and land on floor no further than 3 meters (10 feet) from the window.|
|4||Medium||Medium||Glazing cracks. Fragments enter space and land on floor and impact a vertical witness panel at a distance of no more than 3 m (10 feet) from the window at a height no greater than 2 feet above the floor.|
|5||Low||High||Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 3 meters (10 feet) from the window at a height greater than 0.6 meters (2 feet) above the floor.|
The preferred solution for new construction is to use laminated annealed (i.e., float) glass with structural sealant around the inside perimeter. For insulated units, only the inner pane needs to be laminated. The lamination holds the shards of glass together in explosive events, reducing its potential to cause laceration injuries. The structural sealant helps to hold the pane in the frame for higher loads. Annealed glass is used because it has a breaking strength that is about one-half that of heat strengthened glass and about one-fourth as strong as tempered glass thus reducing the loads transmitted to the supporting frame and walls. Using annealed glass becomes particularly important for buildings with light weight exterior walls using for instance, metal studs, dry wall and brick façade. Use the thinnest overall glass thickness that is acceptable based on conventional load requirements. The preferred interlayer thickness is 60 mil unless otherwise specified by the criteria. This layup has been shown to perform well in low pressure regions (i.e., under about 5 psi). If a 60 mil polyvinyl butaryl (PVB) layer is used, the tension member forces into the framing members need to be considered in design.
To make sure that the components supporting the glass are stronger than the glass itself, we specify a window breakage strength that is high compared to what is used in conventional design. The breakage strength in window design may be specified as a function of the number of windows expected to break at that load. For instance, in conventional design, it is typical to use a breakage pressure corresponding to 8 breaks out of 1000. Where we are certain of a lot of glass breakage, a pressure corresponding to 750 breaks out of 1000 is used to have increased confidence that the frame does not fail too. Design criteria vary broadly on the specified number of breaks to use for design. Glass breakage strengths may be obtained from window manufacturers.
Smaller glass panes generally have higher capacities than larger panes which can significantly increase the loads transmitted to the frames. One way to reduce the loads transmitted to the frames is the use false or non-structural mullions.
There are several government sponsored software products available to evaluating the response of window glass, including HAZL, WinGard and WinLAC. These codes are made available to government contractors who have government projects requiring this type of analysis.
Glass block is generally not recommended because of the heavy projectiles these walls may create due to failure at the mortar lines. However, there are blast rated glass block products that are available in which each glass blocks are framed by a steel grate system.
The concepts presented here are for both vision and spandrel glass panels. If a shadow box is provided behind the spandrel panel, the connections need to be designed to resist the capacity of the box structure.
The frame members connecting adjoining windows are referred to as mullions. These members may be designed in two ways. Either a static approach may be used where the breaking strength of the window glass is applied to the mullion, or a dynamic load may be applied using the peak pressure and impulse values. A static approach may lead to a design that is not practical. Using this approach, the mullion can become very deep and heavy, driving up the weight and cost of the window system. It may also not be consistent with the overall architectural objectives for the project.
Sometimes cables or steel bars or tubes are placed behind the glass to prevent the glass from entering the interior. The State Department refers to this as the 'muntin' system when the glass bears against steel bars arranged in a cruciform shape. The Defense Department refers to a similar system with a single bar placed behind the glass as a 'catch-bar' system. For these types of systems the steel members are attached using full penetration welds and are able to experience large ductile deformations. Structural wood mullions have negligible resistance and should not be used for blast mitigating designs.
As with frames, it is good engineering practice to limit the number of interlocking parts used for the mullion.
Frame and Anchorage Design
The window frames need to retain the glass so that the entire pane does not become a single large unit of flying debris. It also needs to be designed to resist the breaking stress of the window glass.
To retain the glass in the frame, a minimum of a ¼ inch bead of structural sealant (e.g., silicone) is used around the inner perimeter of the window. The allowable tensile strength should be at least 20 psi. Also, the window bite (i.e., the depth of window captured by the frame) needs to be at least ½ inch. The structural sealant recommendations should be determined on a case-by-case basis. In some applications, the structural sealant may govern the overall design of the window systems.
Frame and anchorage design is performed by applying the breaking strength of the window to the frame and the fasteners. In most conventionally designed buildings, the frames will be aluminum. In some applications, where the windows are designed to resist high pressures, steel bar inserts, cable inserts or built-up steel frames may be used. Also, in lobby areas where large panes of glass are used, a larger bite with more structural sealant may be needed.
For reinforced concrete construction designed to resist high pressure loads, as is typical for embassy construction, anchorage of the steel window frames is provided by steel studs welded to a steel base plate. For this type of construction, the frame is typically constructed using a steel stop at the interior face and an angle with an exposed face at the exterior face. The frame is attached to the base plate using high strength fasteners. Coordination is required to ensure that the fastener locations for the steel frame, the steel studs and the rebar cage are properly arranged.
For masonry walls, metal straps are recommended for anchoring the window into the wall.
Inoperable window solutions are generally recommended for air-blast mitigating designs. However, there are operable window solutions that are conceptually viable. For instance designs where the window rotates about a horizontal hinge at the head or sill and opens in the outward direction. For this design, the window will slam shut in an explosion event. If this type of design is used, the governing design parameter may be the capacity of the hinges and/or hardware.
Supporting Wall Design
A similar approach may be used for checking the supporting wall response. It does not make sense and is potentially highly hazardous to have a wall system that is weaker than windows that it is supporting. Remember that the maximum strength of any wall system needs to be at least equal to the window strength. If the walls are unable to accept the loads transmitted by the mullions the mullions may need to be anchored into the structural slabs or spandrel beams. Anchoring into columns is generally discouraged because it increases the tributary area of lateral load that is transferred into these members and may cause instability.
Some window/wall designs will require additional support around the windows. For clerestory windows, the supporting wall is acting largely as a cantilever and will need to be supported with vertical braces spanning floor to ceiling. For punched wall systems with narrow pilasters between them, vertical braces may also be needed. For lighter wall systems like metal stud systems, double studs framing the window are recommended.
The balanced design approach is particularly challenging in the design of ballistic resistant and forced entry resistant windows, which consist of one or more inches of glass and polycarbonate. These windows can easily become stronger than the supporting wall. In these cases, it windows may need to be designed for the design threat air-blast pressure levels and implicitly accept that for larger loads balanced design conditions will not be met.
Doors are handled differently in different criteria. Most criteria neglect the response of door systems. This may be for several reasons. Doors that are capable of resisting air-blast loads can be very expensive. Also, doors are typically in transitory areas where people do not stay for very long. Some concepts for increasing the inherent resistance of doors are as follows:
- Use double steel doors with internal cross braces
- Orient doors to open outward so that they bear against the jamb during the positive pressure loading phase
- Fill the jambs with concrete to increase their strength
- Increase number of fasteners used to connect door into wall system
- Do not station guards or other persons directly behind doors
- Position doors so that they will not be propelled into rooms if they fail
In general sliding glass doors should be avoided. The typical failure mode for these door systems is along the channel supporting the framed glass. If these systems are used, this channel needs to be able to restrain the glass and to have anchors that are designed to resist the strength of the glass.
For revolving doors, use laminated glass and provide an increased bite in the frame.
Louvers are another type of opening to consider. These members should be designed with connections that are able to resist the flexural capacity of the louver. A catch system behind the louver is another approach using a well anchored steel grate.
Air intakes that are at or close to the ground level should always have grates so that weapons can not be lobbed into them. Also consider using a sloped grating for horizontal air intakes so that a potential weapon can roll off prior to detonation. Air intakes at or near ground level locations where chemical, biological, or radiological (CBR) attack may occur are to be avoided. (See WBDG Design Guidance Page Security for Building Occupants and Assets)
The primary loading on the roof is the downward air-blast pressure. The exterior bay roof system on the side(s) facing an exterior threat is the most critical. Roof systems which are low and therefore closer to the explosion will be subject to higher pressures than high roof systems. The air-blast pressures on the interior bays are less intense and may require less hardening. Secondary loads include upward pressure due to the air-blast penetrating through openings and upward suction during the negative loading phase. The upward internal pressures may have an increased duration due to multiple reflections of the internal air-blast wave. It is conservative to consider the downward and upward loads separately.
To provide redundancy, roof bay dimensions less than or equal to 30 feet are preferred.
The preferred system is to use cast-in-place ductile reinforced concrete with beams in two directions. If this system is used, beams should have continuous, symmetrical top and bottom reinforcement with tension lap splices. Shear ties should develop the bending capacity of the beams and be closely placed along the entire span. All ties are to have a 135 degree bend minimum. Two way slabs are preferred.
Somewhat lower levels of protection are afforded by conventional steel beam construction with a steel deck and concrete fill slab. The performance of this system can be enhanced by use of normal weight concrete fill, increasing the gauge of welded wire fabric reinforcement, and making the connection between the slab and beams with shear connector studs. Tension membrane behavior along the edges should be considered in the design of the connections. Since it is anticipated that the slab capacity will exceed that of the supporting beams, beam end connections and supporting columns should be capable of developing the ultimate flexural capacity of the beams to avoid brittle failure. Beam to column connections should be capable of resisting upward as well as downward forces.
Pre-cast and pre/ post-tensioned systems including hollow plank are generally viewed as less desirable due to the lack of ductility. Pre and post-tensioned roof systems are discouraged due to the lack of ductility associated with these systems. If they are used, a system that has continuous bond with the concrete is preferred, with anchors which are designed to be protected from direct air-blast effects. Also, additional mild reinforcement top and bottom is recommended to ensure a ductile response. Connections need to be designed to resist both the direct and uplift forces.
Concrete flat slab/ plate systems are also less desirable because of the potential of shear failure at the columns. Where flat slab/ plate systems are employed, they should include features to enhance their punching shear resistance. Continuous bottom reinforcement should be provided through columns in two directions to retain the slab in the event that punching shear failure occurs. Edge beams should be provided at building exterior.
Lightweight systems, such as untopped steel deck or wood frame construction are considered to afford negligible resistance to air-blast. These systems are prone to failure due to their low capacity for downward and uplift pressure.
In general, the roof systems should be designed to resist the actual loads associated with the defined threat. Because roof systems are high up and not exposed to the reflected wave, they are subject to lower pressures than the walls. Sloped roofs may be subject to somewhat higher pressures and are usually of lighter construction than flat roof systems making them particularly vulnerable to air-blast effects. For this reason, sloped systems should be avoided.
Gravel on roof systems may become air-borne debris similar to what happens for wind loads. However, because of the severity of explosive loads and the short duration compared with wind, this is considered a secondary effect that is not explicitly addressed in the criteria documents.
Skylights in roof systems create a falling fragment hazard to occupants below. These should be designed with a catch system beneath or designed to remain in the frame for the design load. Ideally, skylights should be placed as far from the weapon as possible to keep the pressures low. Similar concepts apply for atria with multistory glass walls.
Parapets, roof mechanical room enclosures, and tile roof systems exterior to the building are generally not a primary concern since they are exterior to the building. Generally these members are designed to sustain heavy damage but not become flying debris. Although roofing aggregate may become a flying hazard, in the context of an explosion event this hazard is not significant enough to warrant much concern.
For buildings that are very close to the secured perimeter, there is the possibility of the foundations becoming undermined by the cratering effects. However, if this is an issue, then generally, this will be accompanied by heavy damages to the superstructure as well. If the crater reaches the building, then the most cost effective option may be to increase the building setback.
Ground shock effects are generally a secondary effect since most of the energy of a vehicle weapon is transmitted to the air rather than the soil. The weapon would have to be placed underground to have a significant effect on the structure. Currently underground weapons are not considered by the governing federal criteria for civilian buildings.
There are significant benefits to placing secured areas below grade in terms of mitigating explosion effects from an exterior weapon. The massiveness and softness of the soil provides a protective layer than significantly reduces the impact on the structural systems below grade. Berms can also be effectively employed for above ground portions of the building.
For buildings with below grade portions that are adjacent to the building creating a plaza level at ground level, keep in mind that the roof systems of these underground areas will need to be designed for the actual air-blast pressure levels, if these are occupied areas. If these areas are unsecured, such as a garage, consider letting the roof fail if adequate egress routes are available on other sides of the building away from the failed plaza level.
Another consideration for below ground portions of the building is the design of the perimeter security barriers. The perimeter barriers often require deep foundations which may interfere with the underground structure.
It is preferable to place underground garages adjacent to the main structure rather than directly underneath the building, to protect against the effects of an internal weapon. Foundation walls are generally not a major concern from the effects of an internal weapon. The soil on the other side of the wall provides a buffer which mitigates the response. One exception to this would be a situation where the foundation is below the water table where even a localized breach of the wall may cause extensive collateral damage.
Vaults for transformers placed beneath the ground which are close to public streets are of concern. If possible, place away from public streets. If they are close are in driveway, the vault lid needs to be designed to resist the downward pressure.
In high seismic regions, seismic isolators may be used at the base of a building. In this case, the response of the building globally should be checked for the total air-blast loading acting on the side facing the explosion. Preliminary studies investigating this issue have shown that seismic response governs.
The following details are from TM 5-1300 and can be viewed online in Adobe PDF (Portable Document Format) by clicking on the "PDF" to the right of the drawing title. Download Adobe Reader.
Figure 4-84 Floor slab-wall intersections PDF
Figure 4-85 Typical horizontal corner details of conventionally reinforced concrete walls PDF
Figure 4-87 Splice locations for multi-span slab PDF
Figure 4-101 Element reinforced with single leg stirrups PDF
Figure 5-27 Typical connections for cold-formed steel panels PDF
Figure 5-33 Typical framing detail at end Column 1-C PDF
Figure 6-2 Masonry wall with rigid support PDF
Figure 6-5 Special masonry unit for use with reinforcing bars PDF
Figure 4-62 Typical interior column sections PDF
Blast mitigating design of civilian buildings is a rapidly evolving technology where new information about innovative structural materials and systems is becoming available almost daily. 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 provide verification that the system works. If the site specific application is significantly different from the tested specimen, then it may be necessary to perform a computational analysis to verify results. It is also recommended in this situation to explosively test the system as part of the design effort. Air-blast testing can be done in open air experiments or by performing shock tube tests. Shock tube tests are typically more cost effective.
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 in particular have been active in this area. An internet search is recommended to locate the latest information that has been made available.
Another area which is evolving is the development of a multi-hazard approach to blast mitigation technologies. For instance, blast resistant windows may make it more difficult for firefighters to get inside buildings in the event of an explosion. Testing conducted by GSA is currently underway to explore this issue. Another example is the use of elastomeric materials for the walls of primary emergency egress routes. These materials show promise for blast resistance but not necessarily for fire protection. Other issues with these materials which need clarification from the vendors are their effectiveness with regards to their ability to resist moisture.
Also, sustainability and blast mitigation is another area where there is some evidence of compatibility.
Finally, there is sometimes a misunderstanding about the blast resistance that is provided by a building designed to resist earthquakes. Although there is some overlap between the disciplines (see Figure 14a), mostly in the area of progressive collapse prevention, earthquake resistant buildings are unlikely to meet the direct effects of an air-blast loading acting on the exterior skin of a building. The reasons for the differences between these loading are as follows:
- 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 14b)
Left: Figure 14a. Seismic versus blast overlap; Right: Figure 14b. Seismic versus blast loading type.
- 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 14c)
- Explosion loads generally cause localized damage whereas seismic loads cause global response (see Figure 14d)
- Mass helps resist explosion loads whereas mass worsens earthquake response
Left: Figure 14c. Seismic versus blast loading time histories; Right: Figure 14d. Seismic versus blast response
Relevant Codes and Standards
- 2010/Physical Security Criteria for Federal Facilities (FOUO): Establishes a baseline set of physical security measures to be applied to all federal facilities and provides a framework for the customization of security measures to address unique risks at a facility. These baseline measures provide comprehensive solutions in all five areas of physical security, including site, structural, facility entrance, interior, security systems, and security operations and administration.
- Interim Antiterrorism/Force Protection Construction Standards—Progressive Collapse Guidance, April 4, 2000 Contact US Army Corps of Engineers Protective Design Center, ATTN: CENWO-ED-ST, 215 N. 17th Street, Omaha, Nebraska, 68102-4978, phone: (402) 221-4918.
- U.S. Department of Defense UFC 4-010-01 DoD Minimum Antiterrorism Standards for Buildings.
- U.S. Department of State, Bureau of Diplomatic Security, Architectural Engineering Design Guidelines (5 Volumes) [For Official Use Only]
- American Society of Civil Engineers, Design of Blast Resistant Buildings in Petrochemical Facilities. American Society of Civil Engineers, 1997.
- Council on Tall Buildings and Urban Habitat, Building Safety Enhancement Guidebook, 2002.
- Department of Housing and Urban Development, The Avoidance of Progressive Collapse, Regulatory approaches to the problem, PB-248 781. Division of Energy, Building Technology and Standards, Office of Policy Development and Research, Washington D.C. 20410, October 1975.
- Department of Homeland Security, BIPS 05 Preventing Structures from Collapsing, 2011.
- Department of Homeland Security, BIPS 06 / FEMA 426 Reference Manual to Mitigate Potential Terrorist Attacks Against Buildings
- Federal Emergency Management Agency, FEMA 427 Primer for Design of Buildings to Mitigate Terrorist Attack, 2004.
- General Services Administration Standard Test Method for Glazing or Window Systems Subject to Dynamic Overpressure Loadings (PDF 274 KB)
- National Capital Planning Commission. National Capital Urban Design and Security Plan (PDF 3.1 MB)
- Technical Support Working Group (TSWG), Bomb Threat Standoff Card Set.
- The American Institute of Architects. Building Security through Design: A Primer for Architects, Design Professionals, and their Clients, November 2001.
- U.S. Army Corps of Engineers, TM 5-1300, Structures to Resist Accidental Explosions, November 19, 1990, (also Navy NAVFAC (Naval Facilities) P-397, Air Force Regulation 88-2); Contact David Hyde, U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180 or via e-mail to email@example.com