Structural Design:

Guidelines to avoid collapse

The design of civilian or commercial buildings to withstand the effects of a terrorist blast is unlike the design of military installations or the design of embassies for the Department of State, Office of Foreign Building Operations Military structures are typically associated with a specific mission that must be maintained, and they must remain operational despite the attack. The form and function of a facility is governed by the mission that it is designed to perform. Embassy structures, though less mission-oriented, must conform to a set of design guidelines that specify the terrorist threat based on location and prescribe design requirements to ensure proper structural behavior.

The objectives of the Structural Engineering Guidelines is to prevent heavy damage to components and structural collapse. Adherence to the provisions of the guidelines will minimize injuries and loss of life and facilitate the evacuation and rescue of survivors. The blast-protection objective of any commercial or public building must be similar to those of embassy structures, that is, to prevent structural collapse, to save lives, and to evacuate victims. Embassies and military structures occupy secure sites with substantial keep-out distances surrounding the assets; unfortunately, this is not possible for most civilian structures. Civilian real-estate owners typically want to attract the public to keep the property profitable and can rarely afford the real estate necessary to secure the site.

This keep-out distance is vital in the design of blast resistant structures since it is the key parameter that determines, for a given charge weight, the blast overpressures that load the building and its structural elements. The degree of fenestration is another key parameter as it determines the pressures that enter the structure. The smaller the door and window openings the better protected the occupants are within the structure. Following these key parameters, architectural and structural features play a significant role in determining how the building will respond to the blast loading. These features can include adjacent or underground parking, atriums, transfer girders, slab configurations, and structural-frame systems.

Designers of civilian structures are caught in a dilemma. Many of the features that make the structures desirable work spaces are the same features that make them more vulnerable to attack. Situated on urban sites, civilian structures are limited in their ability to restrict terrorist access to a prescribed keep-out distance. Windows and atriums enhance the work space by providing openness and natural light. Hence, the role of the blast engineer is further complicated by architectural criteria that directly contradict the blast-mitigation objectives. Within these constraints the blast engineer is unable to make a building blast resistant. The objectives are therefore more modestly defined to permit significant localized damage while preventing catastrophic collapse. The casualties that will occur to occupants in the immediate vicinity of the explosion may be unavoidable, but by preventing progressive collapse, the remaining occupants may be spared injury or death. The means to achieve these objectives requires a thorough review of the design, to identify weaknesses that may put the occupants at risk. Attention must be given to the behavior of the structural elements to improve their redundancy, toughness, and ductility, and to provide adequate means to guarantee the keep-out distance available to the site.

The first part of the present study will summarize the blast loading requirements that might be considered by a design engineer. A typical commercial building, which is not initially designed for blast resistance, will be introduced next. This building represents any private- or public-sector structure that, because of its occupants or visibility, is a potential target for terrorist attack. The main parts of this building, which is typical of commercial construction, will be discussed, such as, floor slabs, columns, transfer girders, glazing, atrium, lateral building resistance, internal explosion threat, and external treatments. For each item we will discuss the vulnerability of the original design to blast loading. A set of recommendations to improve the blast resistance of that particular item will then be introduced. The total set of recommendations should be helpful to the design engineer in producing a blast-worthy structure.

GENERAL DESCRIPTION OF LOADING REQUIREMENTS

To resist blast loads, the first requirement in the assessment of a structure is to determine the threat. While numerous threats exist, let's consider only the intentional explosions, such as those caused by terrorist bombings. In recent terrorist attacks, the explosive device was a mixture of Ammonium Nitrate and Fuel Oil (ANFO). These ingredients and detonating devices can be purchased relatively easily; however there are many other types of explosive devices, including TNT, C-4, and Semtex, which are more efficient and must be considered. To standardize the criteria, the industry refers to the charge weight of an explosive device in terms of equivalent TNT weight. The relative effect on pressure and impulse can be scaled to an equivalent amount of TNT.

The threat for a conventional bomb is defined by two equally important elements, the bomb size, or charge weight, and the standoff distance, the minimum guaranteed distance between the blast source and the target.

As terrorist attacks range from the small letter bomb to the gigantic truck bomb as experienced in Oklahoma City, the mechanics of a conventional explosion and their effects on a target must be addressed. With the detonation of a mass of TNT at or near the ground surface, the peak blast pressures resulting from this hemispherical explosion decay as a function of the distance from the source as the ever-expanding shock front dissipates with range. The incident peak pressures are amplified by a reflection factor as the shock wave encounters an object or structure in its path. Except for specific focusing of high intensity shock waves at near 45° incidence, these reflection factors are typically greatest for normal incidence (a surface adjacent and perpendicular to the source) and diminish with the angle of obliquity or angular position relative to the source. Reflection factors depend on the intensity of the shock wave, and for large explosives at normal incidence these reflection factors may enhance the incident pressures by as much as an order of magnitude.

The duration of the positive-phase blast wave increases with range, resulting in a lower-amplitude, longer-duration shock pulse the further a target structure is situated from the burst. Charges situated extremely close to a target structure impose a highly impulsive, high intensity pressure load over a localized region of the structure; charges situated further away produce a lower-intensity, longer-duration uniform pressure distribution over the entire structure. In short, by purely geometrical relations, the larger the standoff, the more uniform the pressure distribution over the surface of the target. Eventually, the entire structure is engulfed in the shock wave, with reflection and diffraction effects creating focusing and shadow zones in a complex pattern around the structure. Following the initial blast wave, the structure is subjected to a negative pressure, suction phase and eventually to the quasi-static blast wind. During this phase, the weakened structure may be subjected to impact by debris that may cause additional damage.

While it may be possible to predict effects of a certain charge weight at a specified standoff distance, the actual charge weight of explosive used by the terrorist, the efficiency of the chemical reaction and the source location are not reliably predictable. The most significant observation that one draws from blast-pressure phenomenology is that the most effective means of protecting a structure is to keep the bomb as far away as possible, by maximizing the keepout distance. No matter what size the bomb, the damage will be less severe the further the target is from the source. The external explosive threat is by no means the only type of terrorist attack, but for the purpose of the present paper an uncased bomb at street level is assumed.

Structural hardening should actually be the last resort in protecting a structure; detection and prevention must remain the first line of defense. As the cost of protection increases dramatically with the assumed charge weight, to the point at which the cost of protection becomes untenable, and since the size of the potential threat is such an unknown quantity, the structural engineer is put in a very uncharacteristic role. Rather than designing to a specific charge weight, as one would a live load or 50-year wind load that has a presumed return period, the structural engineer must design a structure to exhibit its best behavior in the presence of a blast loading. The blast loading may originate from any point around the perimeter of the structure-within the loading dock, the mail room, or the lobby. The engineer must design and detail specific components to withstand the various threats such that catastrophic failure and progressive collapse is avoided and the rescue of victims may proceed unhindered. The recognition of the localized intensity of the close-in blast and the inability to design the entire structure to withstand this type of loading is the first step in prescribing the design forces to be withstood.

THE TYPICAL OFFICE BUILDING STRUCTURE

Throughout the world, there are innumerable commercial office and retail buildings that are vulnerable to a terrorist attack. This article examines a prototypical office building, critique its deficiencies and vulnerabilities to a blast attack, and recommend design modifications that would improve the structure’s response. The building studies is an eight-story, cast-in-place reinforced-concrete structure with flat-plate construction. As unremarkable as this conventional building is, it is remarkably vulnerable to blast attacks. In fact, the building and many of its amenities have been chosen to highlight their vulnerabilities. Many other structural systems perform inherently better under blast loading conditions, but this system was chosen to illustrate typical structural shortcomings. The reader is cautioned that every building and every site is unique and that the recommendations provided are not intended to fit all structures; each structure must be properly evaluated and analyzed in response to the associated threat.

The structural system for all the floors and the roof is typical of reinforced-concrete flat-slab construction. Columns are spaced 9 m on center, the typical story height is 3.9 m, and the first floor is 6 m high. Spandrel beams are provided around the perimeter of the building to support the facade. The lateral loads are resisted by shear walls located at the elevator core. The building is designed to resist lateral loads due to wind and seismic base motion.

The slab, columns and shear walls are all cast-in-place concrete.

The building occupies a full city block, such that the site has pubic access on all sides, including a surface parking lot in the rear. The building is serviced by a loading dock, also located in the rear of the building. A distinctive feature of this prestigious office building is a two-story entrance atrium, faced with exterior glass along the building perimeter and open in the interior. The perimeter of the atrium is ringed by a spandrel beam that supports the outer edge of the slab. Above the atrium, transfer girders restore the column spacing to the 9-m on-center grid.

EXTERNAL TREATMENTS

The two parameters that most directly influence the blast environment that the structure will be subjected to are the bomb’s charge weight and the standoff distance. Of these two, the only parameter that anyone has any control over is the standoff distance, and this is primarily dictated by the site. Regardless of the selected charge weight, the maximum attainable standoff or keepout distance must be secured around the entire perimeter of the building.

For the building under consideration, it is clear that only the pubic sidewalk around the building can be controlled to limit the standoff distance. Thus the building is extremely vulnerable to unimpeded hand-delivered or car-bomb attacks. The most directly affected building elements are the lower-floor facade and structural members. Therefore, the site parameter and the exterior elements at the lower floors require special attention.

Stand Off

The keepout distance, within which explosives-laden vehicles may not penetrate, must be maximized and guaranteed. As we all know, the greater the standoff distance, the more the blast forces will dissipate resulting in reduced pressures on, and impulse imparted to, the building. Several recommendations can be made to maintain and improve the standoff distance for the building under consideration:

1. Use anti-ram bollards or large planters, placed around the entire perimeter. These barriers must be designed to resist the maximum vehicular impact load that could be imposed. The site conditions will determine the maximum speeds attainable, thus the kinetic energy resulting from an impact that must be resisted. The bollard’s slab connection must be designed to resist the impact loading at the maximum speed attainable. Conversely, if design restrictions limit the capacity of the bollard’s slab connection, then site restrictions will be required to limit the maximum speed attainable.

For maximum effectiveness, the barriers-bollards or planters-must be placed at the curb.

2. The public parking lot at the corner of the building must be secured to guarantee the prescribed keepout distance from the face of the structure. Securing this parking lot means that all vehicles must be cleared, i.e., employee owned or visually inspected, such as delivery trucks. Preferably, the parking lot should be eliminated.

3. Street parking should not be permitted on the near side of the street, adjacent to the building. However, the city typically gains large revenue from street parking and might require annual fees from the owner to compensate for the losses.

4. Additional standoff distance can be gained by removing one lane of traffic and turning it into an extended sidewalk or plaza. Again, while this will increase the standoff distance, it will require the approval of city officials.

5. An additional measure to reduce the chances of an attack would be to prevent parking on the opposite side of the street. While this does not improve the keepout distance, it could eliminate the "parked" bomb, thereby limiting bombings to "park and run," drive-by, and suicide bombers. Unfortunately, as was the case with the Oklahoma
City Bombing, the truck laden with explosives was only parked for approximately two minutes. Even in the most security-aware environments, this may not be long enough to draw the attention of security officers.

Note that the practical benefit of increasing the standoff depends on the charge weight. If the charge weight is small, this measure will significantly reduce the forces to a more manageable level. If the threat is a large charge weight, the blast forces may overwhelm the structure despite the addition of nine or ten feet to the standoff distance, and the measure may not significantly improve the survivability of the occupants or the structure.

Lower Floor Exterior

The architectural design of the building of interest currently calls for window glass around the first floor. Unless this area is constructed in reinforced concrete, the damage to the lower floor structural elements and their connections will be quite severe. Consequently, the injury to the lower floor inhabitants will be equally severe, especially at these short standoffs. In general, three sizes of charges can be discussed.

1. To protect against a small charge weight, a nominal 300 mm (12 in.) thick wall with 0.3% steel doubly reinforced in both directions might be required.

2. For intermediate charge weight protection, a 460 mm (18 in.) thick wall with 0.5% steel might be needed.

3. Finally, a large charge weight at these small standoffs will likely breach any reasonably sized wall at the lower levels. Therefore, precautions have to be taken and adjustments made for the design of the entire structure.

GLAZING

Glazing has been described as the first weak link. It should be assumed that all glazing on the target structure will fail for most realistic car bomb threats, particularly on the side of the building facing the bomb. Commonly used annealed glass behaves
poorly when loaded dynamically.
The failure mode for annealed glass creates large sharp edged shards, resembling knives and daggers. Historically, failed window glazing
due to the direct pressures produced by an explosion has resulted in a considerable proportion of the injuries and casualties.