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Blast Safety of the Building Envelopeby Eve Hinman, PEHinman Consulting Engineers, Inc.
Last updated: 05-06-2008
Introduction
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 sameconcepts 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 buildingperformance 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. Finally, we seek to
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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 against 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 minimizing the impact to other design considerations.
Description
In this sub-section the threat, loads and damages resulting from explosions are explained.
Threat Definition
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.
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
Figure 1. Vehicle Weapon Threat
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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.
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
Left:Figure 2. Anti-ram bollards; Right:Figure 3. Anti-ram knee wall
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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 isnot 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 pressurephase. 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 acton 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 isacting.
Rebound of the exterior envelope components following the explosion can pull the faade
members off the building exterior. Note that this effect is different from the negative pressure
Figure 5. Air-blast as a function of time
Figure 6. Sequence of air-blast effects
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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 undergroundportions of the building which 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 theweapon 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.
Building Damages
Damage due to explosions may be divided into direct air-blasteffects 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).
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
Figure 7. Khobar Towers Bombing, 1996
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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 designby providing prescriptive design measures such as requiring special seismicdetailing at column-spandrel connections, design of the roof for a prescribed static loading, or
using two-way systems to improve redundancy.
direct loaddesign of the member to resist the explosive loads generated by design threat. Forinstance the design of a column to resist the effects of a hand carried weapon placed directly
against it.
alternate load pathdesign 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 thegreatest 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
Fundamentals
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.
Building (Oklahoma City Bombing, 1995)
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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 are 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 willbe 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. 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.
Design Philosophy
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
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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.
Design Methods
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
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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) toempirically 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.
Applications
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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 areadjacent 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.
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
Figure 9. Affect of building shape on air-blast loading
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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.
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 suchas 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.
Walls
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.
Figure 10. U.S. Courthouse, Seattle,Washington
(photo courtesy of Frank Ooms/NBBJ)
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POURED-IN-PLACE REINFORCED CONCRETE
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 theconstruction 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 thicknessapart.
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 includingconnections with a minimum bend angle of 135 degrees and a spacing not exceeding d/2.
PRE-CAST CONCRETE
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
Figure 11. U.S. Embassy, Kampala,
Uganda
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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.
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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. Connectionsneed 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.
MASONRY
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.
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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 is 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 ga 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 reachingits 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
Timber should not be used for blast mitigating solutions. Timber is too light and fragile to resist the types
of loads discussed here.
NON-STRUCTURAL ELEMENTS
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.
Exterior Frame
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
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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 faade 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 improveconfinement, 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 reducingthe 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 explosivepressures 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 extendseveral 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
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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.
Fenestration
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 theweakest 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.
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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 DESIGN
Glass is often the weakest part of a building, breaking at low pressures compared to other components
Figure 12. Preferred failure modes for windows
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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
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 from1 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.
Figure 13. Illustration showing performance level
conditions for windows
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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 causelaceration 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 faade. 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 providedbehind the spandrel panel, the connections need to be designed to resist the capacity of the box
structure.
MULLION DESIGN
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
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.
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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 unitof 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 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 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 orbuilt-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.
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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. Thesewindows 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.
Other Openings
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 pressureloading 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.
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Air intakes that are at or close to the ground level should always have grates so that weapons can not be
lobed into them. Also consider using a sloped grating for horizontal air intakes so that a potential weapon
can roll off prior to detonation.
Roof
The primary loading on the roof is the downward air-blast pressure. The exterior bay roof system on theside(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 withtension 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.
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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 flyinghazard, in the context of an explosion event this hazard is not significant enough to warrant much
concern.
Below Grade
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.
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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.
Details
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
Emerging Issues
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
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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 clarified 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 inplane response for seismic loads (see Figure 14b)
Explosion loads are characterized by a single high pressure impulsive pulse acting overmilliseconds 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
Left:Figure 14a. Seismic versus blast overlap; Right:Figure 14b. Seismic versus blast
loading type.
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(see Figure 14d)
Mass helps resist explosion loads whereas mass worsens earthquake response
Relevant Codes and Standards
Interagency Security Committee ISC (executive agentGSA), Security Design Criteria for NewFederal Office Buildings and Major Modernization Projects, Washington DC, May 28, 2001 [ForOfficial Use Only]
Interim Antiterrorism/Force Protection Construction StandardsProgressive 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.
Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and MajorModernization Projects, November 2000
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 DesignGuidelines(5 Volumes) [For Official Use Only]
Additional Resources
American Society of Civil Engineers, Design of Blast Resistant Buildings in Petrochemical Facilities.
American Society of Civil Engineers, 1997.
American Society of Civil Engineers, Vulnerability and Protection of Infrastructure Systems: TheState of the Art. American Society of Civil Engineers, 2002. An ASCE Journals Special Publicationcompiling articles from 2002 and earlier available online.
Council on Tall Buildings an 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 andStandards, Office of Policy Development and Research, Washington D.C. 20410, October 197