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All western democracies are now acutely aware of the apocalyptic consequences of a well-orchestrated attack on high-profile government facilities and other related targets. Many of these buildings are historical, ornate, listed and constructed using traditional techniques. Many of the modern retrofitted reinforcement techniques used to protect these structures against terrorist attacks are unsightly, intrusive or inappropriate. However, security specialists are well aware that while there might be little that can be done to defend a building against an aircraft attack, much can be done to defeat the more traditional car bomb and bullet. The methods available to the structural engineer to strengthen existing structures and provide resistance to the effects of a blast attacks are discussed in this paper.
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 embassy buildings. The objectives of the “Structural Engineering Guidelines” for the Design of New Embassy Buildings are 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.
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. The keep-out distance is vital in the design of blast resistant structures since it is the key parameter that determines 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 Embassies and military structures occupy secure sites with substantial keep-out distances better protected the occupants are within the structure. Following these key parameters,
The probability that any single building will sustain damage from accidental or deliberate explosion is very low, but the cost for those who are unprepared is very high.
2.1 EXPECTED TERRORIST BLASTS ON STRUCTURES.
• External car bomb
• Internal car bomb
• Internal package
• Suicidal car bombs
2.2 MAJOR CAUSES OF LIFE LOSS AFTER THE BLAST.
• Flying debris
• Broken glass
• Smoke and fire
• Blocked glass
• Power loss
• Communications breakdown
• Progressive collapse of structure
The goals of blast-resistant design are to:
• Reduce the severity of injury
• Facilitate rescue
• Expedite repair
• Accelerate the speed of return to full operations.
To resist blast loads,
– The first requirement is to determine the threat. The major threat is caused by terrorist bombings. 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
– Another requirement 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.
– Structural hardening should actually be the last resort in protecting a structure; detection and prevention must remain the first line of defense. 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.
4.1. MECHANICS OF A CONVENTIONAL EXPLOSION
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 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 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
5.1 FLOOR SLABS
Treatments for conventional flat slab design are as follows:
1. More attention must be paid to the design and detailing of exterior bays and lower floors, which are the most susceptible to blast loads.
2. In exterior bays/lower floors, drop panels and column capitols are required to shorten the effective slab length and improve the punching shear resistance.
3. If vertical clearance is a problem, shear heads embedded in the slab will improve the shear resistance and improve the ability of the slab to transfer moments to the columns.
4. The slab-column interface should contain closed-hoop stirrup reinforcement properly anchored around flexural bars within a prescribed distance from the column face.
5. Bottom reinforcement must be provided continuous through the column. This reinforcement serves to prevent brittle failure at the connection and provides an alternate mechanism for developing shear transfer once the concrete has punched through.
6. The development of membrane action in the slab, once the concrete has failed at the column interface, provides a safety net for the post damaged structure. Continuously tied reinforcement, spanning both directions, must be detailed properly to ensure that the tensile forces can be developed at the lapped splices. Anchorage of the reinforcement at the edge of the slab is required to guarantee the development of the tensile forces.
Treatment for conventionally designed columns to improve blast resisting mechanism:
1. The potential for direct lateral loading on the face of the columns, resulting from the blast pressure and impact of explosive debris, requires that the lower-floor columns be designed with adequate ductility and strength
2. The perimeter columns supporting the lower floors must also be designed to resist this extreme blast effect
3. Encasing these lower-floor columns in a steel jacket will provide confinement, increase shear capacity, and improve the columns’ ductility and strength. An alternative, which provides similar benefits, is to embed a steel column within the perimeter concrete columns or wall section.
4. The possibility of uplift must be considered, and, if deemed likely, the columns must be reinforced to withstand a transient tensile force.
5. For smaller charge weights, spiral reinforcement provides a measure of core confinement that greatly improves the capacity and the behavior of the reinforced concrete columns under extreme load.
5.3 TRANSFER GIRDERS
The building relies on transfer girders at the top of the atrium to distribute the loads of the columns above the atrium to the adjacent columns outside the atrium. The transfer girder spans the width of the atrium, which insures a column-free architectural space for the entrance to the building.
Transfer girders typically concentrate the load-bearing system into a smaller number of structural elements. This load-transfer system runs contrary to the concept of redundancy desired in a blast environment. The column connections, which support the transfer girders, are to provide sustained strength despite inelastic deformations. The following recommendations must be met for transfer girders:
1. The transfer girder and the column connections must be properly designed and detailed, using an adequate blast loading description.
2. A progressive-collapse analysis must be performed, particularly if the blast loading exceeds the capacity of the girder
Typical annealed plate glass is only capable of resisting, at most, 14 kPa of blast pressure and it behaves poorly on explosion. On failure annealed glass creates large sharp edged shards, resembling knives and daggers which cause injuries and casualties..
There exist better types of glazing that can resist some modest blast pressures. Thermally Tempered Glass (TTG) and Polycarbonate layups can be made in sheets up to about l-in. thick and can resist pressures up to about 200 to 275KPa. The greatest benefit of TTG is unlike annealed glass; TTG breaks into rock-salt sized pieces that will inflict less injury on the occupants. The failed Polycarbonate glass unfortunately remains in one piece,
5.5 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.
5.6 FACADE AND ATRIUM
The facade is comprised of the glazing and the exterior wall. Better glazing has already been discussed above and wall obviously should be hardened to resist the loading
Presence of an atrium along the face of the structure will require two protective measures. On the outside of the structure, the glass and glass framing must be strengthened to withstand the loads. On the inside, the balcony parapets, spandrel beams, and exposed slabs must be strengthened to withstand the loads that enter through the shattered glass.
5.7 OVERALL LATERAL BUILDING RESISTANCE, SHEAR WALLS
The ability of structures to resist a highly impulsive blast loading depends on the ductility of the load-resisting system. This means that the structure has to be able to deform in elastically under extreme overload, thereby dissipating large amounts of energy, prior to failure..In addition to providing ductile behavior for the structure, the following provisions would improve the blast protection capability of the building:
1. Use a well-distributed lateral-load resisting mechanism in the horizontal floor plan. This can be accomplished by using several shear walls around the plan of the building this will improve the overall seismic as well as the blast behavior of the building.
2. If adding more shear walls is not architecturally feasible, a combined lateral-load resisting mechanism can also be used. A central shear wall and a perimeter moment-resisting frame will provide for a balanced solution. The perimeter moment-resisting frame will require strengthening the spandrel beams and the connections to the outside columns. This will also result in better protection of the outside columns.
Several recommendations were presented for each of the identified features. The implementation of these recommendations will greatly improve the blast-resisting capability of the building under consideration.
5.8 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. In general, two 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.
5.9 Stand Off DISTANCE
The keep out 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 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. 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. 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
4. 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 keep out distance, it could eliminate the “parked” bomb, thereby limiting bombings to “Park and run”
5.10 INTERNAL EXPLOSION THREATS
The blast environment could be introduced into the interior of the structure in four vulnerable locations:
The entrance lobby, the basement mechanical rooms, the loading dock, and the primary mail rooms. Specific modifications to the features of these vulnerable spaces can prevent an internal explosion from causing extensive damage and injury inside the building.
1. Walls and slabs adjacent to the lobby, loading dock, and mail rooms must be hardened to protect against the hand delivered package bomb, nominally a 10-20 kg explosive. This hardening can be achieved by redesigning the slabs and erecting cast-in-place reinforced-concrete walls, with the thickness and reinforcement determined relative to the appropriate threats.
2. The basement must be similarly isolated from all adjacent occupied office space, including the floor above, from the threat of a small package bomb.
The collapse of the World Trade Center (WTC) towers on September 11, 2001, was as sudden as it was dramatic; the complete destruction of such massive buildings shocked nearly everyone. Immediately afterward and even today, there is widespread speculation that the buildings were structurally deficient, that the steel columns melted, or that the fire suppression equipment failed to operate. In order to separate the fact from the fiction, I have attempted to quantify various details of the collapse.
The major events include the following:
The airplane impact with damage to the columns.
The ensuing fire with loss of steel strength and distortion (figure 6.3)
The collapse, which generally occurred inward without significant tipping. (figure6.4)
Before going to the details it is useful to review the overall design of the towers.
6.1 THE DESIGN
The towers were designed and built in the mid-1960s through the early 1970s each tower was 64 m square, standing 411 m above street level and 21 m below grade. This produces a height-to-width ratio of 6.8. The total weight of the structure was roughly 500,000 t. The building is a huge sail that must resist a 225 km/h hurricane. It was designed to resist a wind load of 2 KPa—a total of lateral load of 5,000 t.
In order to make each tower capable of withstanding this wind load, the architects selected a lightweight “perimeter tube” design consisting of 244 exterior columns of 36 cm square steel box section on 100 cm centers(figure 3). This permitted windows more than one-half meter wide. Inside this outer tube there was a 27 m × 40 m core, which was designed to support
Figure 6.1 a cutaway view of WTC structure.
The weight of the tower It also housed the elevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tall connected the core to the perimeter at each story. Concrete slabs were poured over these joists to form the floors. In essence, the building is an egg-crate construction, i.e. 95 percent air.
The egg-crate construction made a redundant structure (i.e., if one or two columns were lost, the loads would shift into adjacent columns and the building would remain standing). The WTC was primarily a lightweight steel structure; however, its 244 perimeter columns made it “one of the most redundant and one of the most resilient” skyscrapers.
6.2 DETAILS OF THE COLLAPSE
6.2.1 THE AIRPLANE IMPACT
The early news reports noted how well the towers withstood the initial impact of the aircraft; however, when one recognizes that the buildings had more than 1,000 times the mass of the aircraft and had been designed to resist steady wind loads of 30 times the weight of the aircraft, this ability to withstand the initial impact is hardly surprising. Furthermore, since there was no significant wind on September 11, the outer perimeter columns were only stressed before the impact to around 1/3 of their 200 MPa design allowable.
Figure 6.2 A graphic illustration, from the USA Today newspaper web site, of the World Trade Center points of impact.
The only individual metal component of the aircraft that is comparable in strength to the box perimeter columns of the WTC is the keel beam at the bottom of the aircraft fuselage. While the aircraft impact undoubtedly destroyed several columns in the WTC perimeter wall, the number of columns lost on the initial impact was not large and the loads were shifted to remaining columns in this highly redundant structure. Of equal or even greater significance during this initial impact was the explosion when 90,000 L gallons of jet fuel, comprising nearly 1/3 of the aircraft’s weight, ignited. The ensuing fire was clearly the principal cause of the collapse (see figure 6.2)
Figure 6.3 Flames and debris exploded from the World Trade Center south tower immediately after the airplane’s impact. The black smoke indicates a fuel-rich fire
The fire is the most misunderstood part of the WTC collapse. Even today, the media report (and many scientists believe) that the steel melted. It is argued that the jet fuel burns very hot, especially with so much fuel present. This is not true.
Part of the problem is that people often confuse temperature and heat. While they are related, they are not the same. Thermodynamically, the heat contained in a material is related to the temperature through the heat capacity and the mass. Temperature is defined as an intensive property, meaning that it does not vary with the quantity of material, while the heat is an extensive property, which does vary with the amount of material. One way to distinguish the two is to note that if a second log is added to the fireplace, the temperature does not double; it stays roughly the same, but the length of time the fire burns, doubles and the heat so produced is doubled. Thus, the fact that there were 90,000 L of jet fuel on a few floors of the WTC does not mean that this was an unusually hot fire. The temperature of the fire at the WTC was not unusual, and it was most definitely not capable of melting steel.
In combustion science, there are three basic types of flames, namely, a jet burner, a pre-mixed flame, and a diffuse flame. A jet burner generally involves mixing the fuel and the oxidant in nearly stoichiometric proportions and igniting the mixture in a constant-volume chamber. Since the combustion products cannot expand in the constant-volume chamber, they exit the chamber as a very high velocity, fully combusted, jet. This is what occurs in a jet engine, and this is the flame type that generates the most intense heat.
In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exits a nozzle, under constant pressure conditions. It does not attain the flame velocities of a jet burner. An oxyacetylene torch or a Bunsen burner is a pre-mixed flame.
In a diffuse flame, the fuel and the oxidant are not mixed before ignition, but flow together in an uncontrolled manner and combust when the fuel/oxidant ratios reach values within the flammable range. A fireplace flame is a diffuse flame burning in air, as was the WTC fire. Diffuse flames generate the lowest heat intensities of the three flame types.
If the fuel and the oxidant start at ambient temperature, a maximum flame temperature can be defined. For carbon burning in pure oxygen, the maximum is 3,200°C; for hydrogen it is 2,750°C. Thus, for virtually any hydrocarbons, the maximum flame temperature, starting at ambient temperature and using pure oxygen, is approximately 3,000°C.
This maximum flame temperature is reduced by two-thirds if air is used rather than pure oxygen. The reason is that every molecule of oxygen releases the heat of formation of a molecule of carbon monoxide and a molecule of water. If pure oxygen is used, this heat only needs
To heat two molecules (carbon monoxide and water), while with air, these two molecules must be heated plus four molecules of nitrogen. Thus, burning hydrocarbons in air produces only one-third the temperature increase as burning in pure oxygen because three times as many molecules must be heated when air is used. The maximum flame temperature increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000°C—hardly sufficient to melt steel at 1,500°C.
6.2.3 THE COLLAPSE
Figure 6.4 as the heat of the fire intensified, the joints on the most severely burned floors gave way, causing the perimeter wall columns to bow outward and the floors above them to fall. The buildings collapsed within ten seconds, hitting bottom with an estimated speed of 200 km/hr.
Nearly every large building has a redundant design that allows for loss of one primary structural member, such as a column. However, when multiple members fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down.
The perimeter tube design of the WTC was highly redundant. It survived the loss of several exterior columns due to aircraft impact, but the ensuing fire led to other steel failures. Many structural engineers believe that the weak points—were the angle clips that held the floor joists between the columns on the perimeter wall and the core structure .With a 700 Pa floor design allowable, each floor should have been able to support approximately 1,300 t beyond its own weight. The total weight of each tower was about 500,000 t.
As the joists on one or two of the most heavily burned floors gave way and the outer box columns began to bow outward, the floors above them also fell. The floor below (with its 1,300 t design capacity) could not support the roughly 45,000 t of ten floors (or more) above crashing down on these angle clips. This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h.
Can buildings resist direct airplane hits?
If the “design” terrorist attack is similar to that of Sept. 11, can buildings be given the capacity to meet this demand? To answer this question, it is important to understand the physics at work when a plane in flight is stopped by a building.
If the performance objective is to “resist” a direct airplane hit and protect people inside the building, the plane cannot be allowed to penetrate the exterior wall. To stop a Boeing 767 traveling in excess of 500 miles per hour in a distance of a few feet would take a deceleration force in excess of 400 million pounds.
Each tower of the World Trade Center was designed for a total horizontal force (or “design wind load”) of about 15 million pounds. The total design wind load for a more commonly sized high-rise, say, 40 stories tall, would be about 4 million pounds. In other words, to resist the amount of force generated by a direct 767 hit, today’s buildings would need to be 100 times stronger than dictated by code, which is both physically and economically impossible.
So why did the World Trade Center Towers not collapse immediately due to the impact load on the system? The planes did not stop in a few feet, but had an effective stopping distance of over 100 feet. This would drop the deceleration force down to something close to the capacity of the building.
Another part of the answer to this question lies in the way that the exterior of the building was structured. The exterior columns were 14-inch square welded steel box columns spaced at 40 inches on center. This means that there was only 26 inches clear between each column. The columns were integral with the steel spandrels beams and formed essentially a solid wall of steel with perforations for windows. This wall construction was able to form a Vierendeel “bridge” over the hole created in one side of each of the towers.
Both of these facts — that the plane was not stopped at the exterior and that the columns and spandrels were extremely dense — were necessary to prevent the building from collapsing immediately upon impact.
Can buildings be designed for direct airplane hits? Yes and no. Yes, for small aircraft. A definite no, for large commercial aircraft.
How can we minimize the chance of progressive collapse?
This is still one more question that some people are asking. Because the towers ultimately collapsed with one floor crashing down upon the next, it has been called a progressive collapse.
Again, it is important to think carefully about the question. Aren’t all collapses progressive? Something breaks, and then something else breaks, and so on. Normally, when the term progressive collapse is used, it specifically refers to the loss of one or two columns or bearing walls that cause a collapse to propagate vertically.
In the case of the World Trade Center there were about 40 columns lost on one face of each of the towers and there was no propagation of collapse from this loss. So did the World Trade Center have good resistance to progressive collapse? By normal use of the term “progressive collapse” it did. The collapse that did ultimately occur was progressive, like all collapses, but was not “progressive collapse” that some international codes address.
The difficulty in understanding this concept is illustrated with the following story.
A New York fire chief wrote that experienced firefighters know that the buildings that are most susceptible to progressive collapse are buildings that are well-tied together (i.e., able to transfer building loads from one element to another, such as a column). Yet, virtually every structural engineer will advise that one of the best ways to prevent progressive collapse is to tie the building together. How can there be this kind of a contradiction?
The difference is that the engineer is thinking about losing a column or two and the fire chief is talking about losing a whole part of a building. As the event that initiates the progressive collapse becomes larger than losing a column, the risk becomes that the strong horizontal ties of a building will cause the collapse to propagate horizontally.
Any discussion of code provisions with respect to progressive collapse must recognize that both the engineer and the fire chief are right depending on the kind of hazard that is defined.
At least six safety systems present in the World Trade Center towers were completely and immediately disabled or destroyed upon impact: fireproofing, automatic sprinklers, compartmentalization and pressurization, lighting, structure and exit stairs.
There are structural techniques that can increase the capacity of building structures to resist certain kinds of terrorist attacks. However, there is absolutely no reliable way to design for the impact of a large scale commercial airliner.
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