Architectural Considerations in Blast Mitigation

With the increase in public awareness of possible terrorist attacks worldwide, many organizations & agencies are trying to secure methods of constructing facilities that will survive blast loads due to explosions.

A lot can be done architecturally to mitigate the effects of an Improvised Explosive Device (IED) attack on a building. These measures often cost nothing or very little if implemented early in the design process. Architectural considerations include building configuration, space design & building detailing. Designing buildings to resist failure due to blast loads is an extremely complex procedure.

When considering mitigation measures for explosive blast threats, the primary strategy is to maximize stand-off distance. This is usually the easiest & least costly way to achieve a desired level of protection. In cases where sufficient stand-off distance is not available to protect the building, hardening of the building’s structural systems may be required, as well as design to prevent progressive collapse & the accompanying casualties. This can also be achieved by strategic location of hostile vehicle mitigation measures (bollards, trees & street furniture). Designing security into a building requires a complex series of trade-offs.

Security concerns need to be balanced with other constraints such as accessibility, initial & life-cycle costs, natural hazard mitigation, fire protection, energy efficiency & aesthetics. The measures incorporated should be as unobtrusive as possible to provide an inviting, efficient environment & not attract the 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.

However, as the probability of a terrorist attack is usually assessed to be very low, 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 & minimize business interruption in the unlikely event of an attack.

In blast-resistant design, the foremost concern is structural collapse. The sources of dynamic excitation in a building under blast & earthquake loads are totally different in nature because blast loading is fast, localized & occurs at a much greater frequency than earthquake loading. Very often, there are also secondary concerns which are to maintain critical functions & minimizing business interruption.

However, before designing blast resistance into a structure, some evaluation needs to be done to determine the feasibility of such a design. This includes a realistic assessment of the threats posed, the possible attackers & their likely tactics. Once this has been established, the associated structural loads of such an attack can be calculated.

Planning & Layout

The building envelope is the most vulnerable to an exterior explosive threat because it the part of the building that is closest to the external IED & is the critical line of defense for protecting the occupants of the building.

Much can be done at the planning stage of a new building to reduce potential threats & the associated risks of injury & damage. Typically, explosive blasts lose their intensity as they move away from their source. An explosive detonation within or immediately nearby a building can cause catastrophic damage on the building’s external & internal structural frames, collapsing of walls, blowing out of large expanses of windows, & shutting down of critical life-safety systems.

The overlying design characteristic that prevents progressive collapse is structural redundancy which implies that the structure has more than one alternative load path for the transfer of gravity loads. In the event of the loss of a column or girder, the redundant elements will effectively carry the redistributed load. This is achieved by increasing the number of members, thereby reducing beam spans & column spacing, or the size of the members, or by enhancing the detailing of reinforcement.

There are 2 primary considerations for the exterior frame - the exterior columns should resist the direct effects of the IED threat & it has sufficient structural integrity to accept localized failure without initiating progressive collapse.

Air-blast loads on exposed stand alone columns (which do not have much surface area) that are not supporting adjacent wall systems tend to be mitigated by pressure wave washing around these slender tall members & consequently the entire duration of the pressure wave does not act upon them. However, for columns subject to an IED threat on an adjacent street, buckling & shear are the primary effects to be considered. For a very large IED detonation close to a column, shattering of the concrete due to multiple tensile reflections within the concrete section will destroy its integrity. It is noted that circular columns shed load more rapidly than rectangular columns. 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 2 or more stories without buckling.

Buildings can resist blasts better with more mass as the energy of a blast is more easily absorbed by a more massive structure which leads to the use of reinforced concrete (RC) as the principal material of choice in blast-resistant design.

The vertical or horizontal profile of a building has serious implications for its protection. The shape of the building can contribute to the overall damage to the structure. Reentrant corners (any inside corner that forms an angle of 180° or less) & overhangs are likely to trap the shock wave, which may amplify the effect of the air-blast. However, large or gradual reentrant corners have less effect than small or sharp reentrant corners & overhangs. Convex rather than concave shapes are preferred for the exterior of the building because the reflected pressure on the surface of a circular building decay more rapidly than on a flat building or "U" shaped building. Circular buildings act to reduce the air-blast pressures because the angle of incidence of the shock wave increases more rapidly than in a rectangular building.

The protection of the building interior can be divided into 2 categories:

• Functional layout - Public areas such as the lobby, loading dock, mail room, garage & retail areas which are vulnerable to attack are separated from the more secured areas of the facility. This can be achieved by creating internal buffer zones, using secondary stairwells, elevator shafts, corridors & storage areas between public & secured areas.

A building where the lobby is separate from the main structure is the US Courthouse in Seattle.

• Structural layout - The performance of building systems in response to explosive loading is highly dynamic, inelastic & interactive. By controlling the flexibility & resulting deformations, structural or fa?ade components may be designed to dissipate considerable amounts of blast energy. Through effective structural design, the overall damage levels may be reduced to make it easier for people to get out safely & allow emergency responders to enter safely. The phasing of the different responses & the energy that is dissipated through inelastic deformation must be carefully represented in order to accurately determine behavior. The SDOF approach commonly used to analyze individual components is likely to produce overly conservative designs. An accurate representation of the structural system actually requires a multi-degree-of-freedom (MDOF) model.?It is noted that transfer girders & the columns supporting transfer girders are particularly vulnerable to blast loading. Unfortunately, transfer girders concentrate the load-bearing system to a fewer number of structural elements, which contradicts the concept of redundancy in a blast mitigation. Transfer girders typically span large openings such as a loading dock. Damage to the girder may leave several lines of columns totally unsupported which may result in progressive collapse.?So if a transfer girder is required but vulnerable to blast loading, then the girder should be designed to be continuous over several supports. There should be substantial structure framing into the transfer girder to create 2-way redundancy in the event of failure.

Windows are typically the most vulnerable of any building. Designing windows to provide protection against the effects of explosions can be effective in reducing glass laceration injuries. For a large IED detonation, the 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. If blast resistant walls are used, fewer &/or smaller windows will cause less air-blast to enter the building thus reducing the interior damage & injuries. Limit the number of windows on the lower floors where the pressures are higher due to an external explosive threat. The use of an internal atrium design with windows facing inward, not outward & angling the windows away from the curb can help to reduce pressure levels.

For buildings that are very close to the secured perimeter, there is the possibility of the foundations becoming undermined by cratering effects. This will usually be accompanied by heavy damage to the superstructure. If the crater reaches the building, the most cost effective option may be to increase the building setback.

Soil can be highly effective in reducing the impact of a major IED attack. There are significant benefits to placing secured areas below grade to mitigate explosion effects from an exterior IED. The massiveness & softness of the soil provides a protective layer than significantly reduces the impact on the structural systems below grade. Bermed walls are highly effective for military applications & can be effectively extended to conventional construction. Berms can also be effectively employed for above ground portions of the building. However, if this approach is taken, no parking is to be permitted over the building.

Ground shock effects are generally a secondary effect since most of the energy of a VBIED is transmitted to the air rather than the soil. Therefore, 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 is where the foundation is below the water table where even a localized breach of the wall may cause extensive collateral damage. 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 underground structures.

One of the last frontiers in structural dynamics is the behavior of structures under the effects of impact & explosions. Many serious difficulties exist in material modeling, load definition, & selecting effective approaches for addressing the behavior of complicated geometrical domains in a structure. There are a variety of materials not normally used in building construction that may also provide blast hardening solutions such as shock attenuating chemically bonded ceramics (SA/CBC) & composite systems comprised of carbon, aramid & polyethylene fibers & resin.?

Simulation-based engineering science (SBES) allows researchers to predict the effects of building explosions & analyze the response of building materials to those threats.

In the 1990's, a computer software named BombCAD permitted the user to model a building or complex of buildings & then expose the computer model to a specific interior or exterior detonation. BombCAD made detailed predictions of blast damage & human injury. These estimates could be graphically presented on a window-by-window, wall-by-wall, space-by-space basis. Special computation routines concentrate on specific damage aspects such as glass damage from exterior bombs, the dominant source of damage & injury from IEDs. Analyses made with BombCAD defined the precise circumstances (explosive type, weight & location) that result in no damage, specific glass damage, specific surface damage (wall, floor, or roof), or potential structural collapse from a specific IED detonation. BombCAD has since been superseded by newer & more powerful blast prediction software like LS-DYNA, BLASTX, CTH, FEFLO, FOIL, SHARC, DYNA3D, Air3D, CONWEB, AUTODYN & ABAQUS.

Software used for the prediction of blast effects are grouped into various categories depending upon how the calculations are made, how the blast & structural calculations are combined, & the relative distance of the explosive from the target. Blast prediction & structural response calculations are based on basic principles of physics & mechanics, or semi-empirical solutions. This software can predict the dynamic response of structural elements, provided the load deformation characteristics are defined. The software has built-in capabilities to generate the load deformation characteristics of common structural (RC columns, one-way walls, beams & slabs) & non-structural members. The input consists of member geometry, boundary conditions, dynamic material properties, explosive threat parameters & desired performance levels.

Full scale blast tests are expensive & time consuming but by using computational based numerical simulations can virtually predict these wave propagations & minimize the need of experimental testing. Computational fluid dynamics (CFD) is a common tool to do an analysis of free-field blast wave & against structure.

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Endro Sunarso is an expert in Security Management, Physical Security & Counter Terrorism. He is regularly consulted on matters pertaining to transportation security, off-shore security, critical infrastructure protection, security & threat assessments, & blast mitigation.

Besides being a Certified Protection Professional (CPP?), a Certified Identity & Access Manager (CIAM?), a Project Management Professional (PMP?) & a Certified Scrum Master (CSM?), Endro is also a Fellow of the Security Institute (FSyl) & the Institute of Strategic Risk Management (F.ISRM).

Endro has spent about 2 decades in Corporate Security (executive protection, crisis management, critical infrastructure protection, governance, business continuity, loss mitigation, due diligence, counter corporate espionage, etc). He also has more than a decade of experience in Security & Blast Consultancy work, initially in the Gulf Region & later in South East Asia.