Architectural And Structural Design Of Blast Resistant Buildings

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The objective of this study is to shed light on blast resistant building theories, the enhancement of building security against the effect of explosives in both architectural and structural design …

The objective of this study is to shed light on blast resistant building theories, the enhancement of building security against the effect of explosives in both architectural and structural design process and the design techniques that should be carried out. Firstly, explosives and explosion type have been explained briefly. In addition, the general aspects of explosion process have been presented to clarify the effect of explosives on buildings. To have a better understanding of explosives and characteristics of explosions will enable us to make blast resistant building design much more efficiently. Essential techniques for increasing the capacity of a building to provide protection against explosive effects is discussed both with an architectural and structural approach.

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  • 1. BLAST RESISTANT BUILDINGS A seminar report submitted in partial fulfillment of the requirements for the award of the degree of Bachelor of Technology in Civil Engineering PAUL JOMY (SYAKECE033) Eighth Semester 2010 Admission Sreepathy Institute of Management & Technology Vavanoor, Palakkad-679533 Affiliated to University Of Calicut
  • 2. Department of Civil Engineering Sreepathy Institute of Management & Technology Vavanoor, Palakkad-679533 CERTIFICATE This is to certify that the seminar entitled ”BLAST RESISTANT BUILDINGS” is a bonafide record of the seminar presented by PAUL JOMY (Reg No. SYAKECE033) under our supervision and guidance. The seminar report has been submitted to the Department of Civil Engineering of SIMAT Vavanoor, Palakkad679533 in partial fulfillment of the award of the Degree of Bachelor of Technology in Civil Engineering, during the year 2013-2014. Mr.ARUN ASHOK Guide Asst. Professor Civil Engg SIMAT, Vavanoor Mr.SUDHEER.K.V Head of the Dept Civil Engg SIMAT, Vavanoor Palakkad
  • 3. ABSTRACT The increase in the number of terrorist attacks especially in the last few years has shown that the effect of blast loads on buildings is a serious matter that should be taken into consideration in the design process. Although these kinds of attacks are exceptional cases, man-made disasters; blast loads are in fact dynamic loads that need to be carefully calculated just like earthquake and wind loads. The objective of this study is to shed light on blast resistant building design theories, the enhancement of building security against the effects of explosives in both architectural and structural design process and the design techniques that should be carried out. Firstly, explosives and explosion types have been explained briefly. In addition, the general aspects of explosion process have been presented to clarify the effects of explosives on buildings. To have a better understanding of explosives and characteristics of explosions will enable us to make blast resistant building design much more efficiently. Essential techniques for increasing the capacity of a building to provide protection against explosive effects is discussed both with an architectural and structural approach.
  • 4. ACKNOWLEDGEMENT I am extremely thankful to our Principal Dr.S.P. SUBRAMANIAN for giving his consent for this seminar. And also i’m thankful to Mr.SUDHEER.K.V, Head of the Department of Civil engineering, for his valuable suggestions and support. The valuable help and encouragement rended in this endeavour by my guide Mr.ARUN ASHOK, Asst.Professors, Dept.of Civil Engineering for his constant help and support throughout the presentation of the seminar by providing timely advices and guidance. I thank God almighty for all the blessing received during this endeavor. Last, but not least I thank all my friends for the support and encouragement they have given me during the course of my work. PAUL JOMY (SYAKECE033) Eight Semester 2010 Admission Dept. of Civil Engg. SIMAT, Vavannoor, Palakkad
  • 5. Contents List of Figures iii 1 Introduction 1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Objective Of The Blast Design . . . . . . . . . . . . . . . . . . . . . . 1 1 2 2 Literature survey 2.1 General . . . . . . . . . . . . . . . . . . . . . . . 2.2 Explosion - Major of All Terrorist Activities . . . 2.2.1 Expected Terrorist Blast On Structures . . 2.2.2 Major Cause of Life Loss After The Blast 2.3 Goals of Blast Resistant Design . . . . . . . . . . 2.4 Basic Requirements To Resist Blast Loads . . . . 2.4.1 Mechanics of a Conventional Explosion . . 2.5 Types of Explosions . . . . . . . . . . . . . . . . . 2.5.1 Unconfined Explosion . . . . . . . . . . . . 2.5.2 Confined Explosions . . . . . . . . . . . . 2.6 Explosion Process For High Explosive . . . . . . . . . . . . . . . . . . 3 3 3 3 3 4 4 4 5 5 6 6 . . . . . . . . . . . . . . . 8 8 8 8 10 10 10 11 11 12 12 13 13 13 14 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Architectural Aspect of Blast Resistant Building Design 3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Planning And Layout . . . . . . . . . . . . . . . . . . . . . 3.3 Structural Form and Internal Layout . . . . . . . . . . . . 3.4 Bomb Shelter Areas . . . . . . . . . . . . . . . . . . . . . . 3.5 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Glazing And Cladding . . . . . . . . . . . . . . . . . . . . 3.7 Floor Slabs . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Transfer Girders . . . . . . . . . . . . . . . . . . . . . . . . 3.10 External Treatments . . . . . . . . . . . . . . . . . . . . . 3.11 Facade And Atrium . . . . . . . . . . . . . . . . . . . . . . 3.12 Overall Lateral Building Resistance, Shear Walls . . . . . . 3.13 Lower Floor Exterior . . . . . . . . . . . . . . . . . . . . . 3.14 Stand Off Distance . . . . . . . . . . . . . . . . . . . . . . 3.15 Internal Explosion Threat . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
  • 6. 4 Structural Aspect of Blast Resistant Building 4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Structural Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Comparison of Blast And Seismic Loading . . . . . . . . . . . . . . . 4.4 Damage Evaluation Procedure For Building Subjected To Blast Impact 15 15 17 18 19 5 Case Study 5.1 World Trade Center Collapse . . . . . . . . . . . . . . . . . . . . . 5.1.1 The Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2 The Details of The Impact . . . . . . . . . . . . . . . . . . . 5.1.2.1 The Airplane Impact . . . . . . . . . . . . . . . . . 5.1.2.2 The Collapse . . . . . . . . . . . . . . . . . . . . . 5.1.3 Can Building Resist Direct Airplane Hits . . . . . . . . . . . 5.1.4 How Can We Minimize The Chance of Progressive Collapse 5.2 Israel as a Case Study And Paradigm . . . . . . . . . . . . . . . . . . . . . . . . . 20 20 20 21 21 24 25 26 27 . . . . 31 31 31 32 33 6 Design Principles for Protection of Structures 6.1 General . . . . . . . . . . . . . . . . . . . . . . 6.2 Preventative Measures . . . . . . . . . . . . . . 6.3 Hardening of the structure . . . . . . . . . . . . 6.4 Hardening of the structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Conclusion 35 7.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 References 37 ii
  • 7. List of Figures 2.1 2.2 2.3 2.4 Air burst with ground reflections . . . . . . . . . . . . . . Surface burst . . . . . . . . . . . . . . . . . . . . . . . . . Fully vented, partially vented and fully confined explosions Blast wave pressures plotted against time . . . . . . . . . . . . . . 5 6 6 7 3.1 3.2 Schematic layout of site for protection against bombs . . . . . . . . . Internal planning of a building . . . . . . . . . . . . . . . . . . . . . . 9 9 4.1 4.2 4.3 4.4 Sequence of air-blast effects . . . . . . . . . . . Enhanced beam-to-column connection details for forced concrete . . . . . . . . . . . . . . . . . . Shock Front from Air Burst . . . . . . . . . . . Shock Front from Surface Burst . . . . . . . . . 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 A cutaway view of WTC structure . . . . . . . . . . . . . A graphic illustration of WTC . . . . . . . . . . . . . . . . Airplane’s impact on WTC . . . . . . . . . . . . . . . . . . Collapse of WTC . . . . . . . . . . . . . . . . . . . . . . . Entrance to an underground shelter in Israel . . . . . . . . Shelter used as a playroom . . . . . . . . . . . . . . . . . . Shelter used as a playroom . . . . . . . . . . . . . . . . . . The change from underground shelters to protected spaces Example of Israeli structural blast desing . . . . . . . . . . Example of Israeli structural blast desing . . . . . . . . . . Example of traditional American structual blast desing . . iii . . . . . . . . . . . . . . . . steelwork and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . rein. . . . . . . . . . 16 . 18 . 18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 21 22 23 24 27 28 28 29 29 30 30
  • 8. Chapter 1 Introduction 1.1 General The increase in the number of terrorist attacks especially in the last few years has shown that the effect of blast loads on buildings is a serious matter that should be taken into consideration in the design process. Although these kinds of attacks are exceptional cases, man-made disasters; blast loads are in fact dynamic loads that need to be carefully calculated just like earthquake and wind loads. The objective of this study is to shed light on blast resistant building design theories, the enhancement of building security against the effects of explosives in both architectural and structural design process and the design techniques that should be carried out. Firstly, explosives and explosion types have been explained briefly. In addition, the general aspects of explosion process have been presented to clarify the effects of explosives on buildings. To have a better understanding of explosives and characteristics of explosions will enable us to make blast resistant building design much more efficiently. Essential techniques for increasing the capacity of a building to provide protection against explosive effects is discussed both with an architectural and structural approach. Damage to the assets, loss of life and social panic are factors that have to be minimized if the threat of terrorist action cannot be stopped. Designing the structures to be fully blast resistant is not an realistic and economical option, however current engineering and architectural knowledge can enhance the new and existing buildings to mitigate the effects of an explosion. 1
  • 9. B LAST RESISTANT BUILDINGS 1.2 Objective Of The Blast Design The primary objectives for providing blast resistant design for buildings are: -Personnel safety -Controlled shutdown -Financial consideration Blast resistant design should provide a level of safety for persons in the building that is no less than that for persons outside the buildings in the event of an explosion. Evidence from past incidents has shown that many of the fatalities and serious injuries were due to collapse of buildings onto the persons inside the building. This objective is to reduce the probability that the building itself becomes a hazard in an explosion. Preventing cascading events due to loss of control of process units not involved in the event is another objective of blast resistant design. An incident in one unit should not affect the continued safe operation or orderly shutdown of other units. Preventing or minimizing financial losses is another objective of blast resistant design. Buildings containing business information, critical or essential equipment, expensive and long lead time equipment, or equipment which if destroyed, would constitute significant interruption or financial loss to the owner should be protected. Dept. of Civil Engg. 2 SIMAT, Vavanoor
  • 10. Chapter 2 Literature survey 2.1 General The need and requirements for blast resistance in buildings have evolved over recent years. Buildings have become more complex and have increased in size thus increasing the risk of accidental explosions. Such explosions have demolished the buildings, in some cases resulting in substantial personnel causalities and business losses. Such events have heightened the concerns of the industry, plant management, and regulatory agencies about the issues of blast protection in buildings have the potential for explosions. Generally, these issues relate to plant building safety and risk management to prevent or minimize the occurrence of such incidents and to siting, design, and operations. 2.2 Explosion - Major of All Terrorist Activities The probability that any single building will sustain damage from accidental or deliberate explosion is very low, but thecost for those who are unprepared is very high. 2.2.1 Expected Terrorist Blast On Structures -External car bomb -Internal car bomb -Internal package -Suicidal car bombs 2.2.2 Major Cause of Life Loss After The Blast -Flying debris -Broken glass -Smoke and fire -Blocked glass -Power loss -Communications breakdown -Progressive collapse of structure 3
  • 11. B LAST RESISTANT BUILDINGS 2.3 Goals of Blast Resistant Design The goals of blast-resistant design are to : -Reduce the severity of injury -Facilitate rescue -Expedite repair -Accelerate the speed of return to full operation. 2.4 Basic Requirements To Resist Blast Loads 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. 2.4.1 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 Dept. of Civil Engg. 4 SIMAT, Vavanoor
  • 12. B LAST RESISTANT BUILDINGS 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 2.5 Types of Explosions Mainly there are two types of explosions 2.5.1 Unconfined Explosion Unconfined explosions can occur as an air-burst or a surface burst. In an air burst Figure 2.1: Air burst with ground reflections explosion, the detonation of the high explosive occurs above the ground level and intermediate amplification of the wave caused by ground reflections occurs prior to the arrival of the initial blast wave at a building Figure 2.1. As the shock wave continues to propagate outwards along the ground surface, a front commonly called a Mach stem is formed by the interaction of the initial wave and the reflected wave. However a surface burst explosion occurs when the detonation occurs close to or on the ground surface. The initial shock wave is reflected and amplified by the ground surface to produce a reflected wave. Figure 2.2. Unlike the air burst, the reflected wave merges with the incident wave at the point of detonation and forms a single wave. In the majority of cases, terrorist activity occurres in built-up areas of cities, where devices are placed on or very near the ground surface. Dept. of Civil Engg. 5 SIMAT, Vavanoor
  • 13. B LAST RESISTANT BUILDINGS Figure 2.2: Surface burst 2.5.2 Confined Explosions When an explosion occurs within a building, the pressures associated with the initial shock front will be high and therefore will be amplified by their reflections within the building. Figure 2.3: Fully vented, partially vented and fully confined explosions This type of explosion is called a confined explosion. In addition and depending on the degree of confinement, the effects of the high temperatures and accumulation of gaseous products produced by the chemical reaction involved in the explosion will cause additional pressures and increase the load duration within the structure. Depending on the extent of venting, various types of confined explosions are possible. Figure2.3 2.6 Explosion Process For High Explosive An explosion occurs when a gas, liquid or solid material goes through a rapid chemical reaction. When the explosion occurs, gas products of the reaction are formed at a very high temperature and pressure at the source. These high pressure gasses expand rapidly into the surrounding area and a blast wave is formed. Because the gases are moving, they cause the surrounding air move as well. The damage caused by explosions is produced by the passage of compressed air in the blast wave. Blast waves propagate at supersonic speeds and reflected as they meet objects. As the blast wave continues to expand away from the source of the explosion its intensity Dept. of Civil Engg. 6 SIMAT, Vavanoor
  • 14. B LAST RESISTANT BUILDINGS diminishes and its effect on the objects is also reduced. However, within tunnels or enclosed passages, the blast wave will travel with very little diminution. Close to the source of explosion the blast wave is formed and violently hot and expanding gases will exert intense loads which are difficult to quantify precisely. Once the blast wave has formed and propagating away from the source, it is convenient to separate out the different types of loading experienced by the surrounding objects. Three effects have been identified in three categories. The effect rapidly compressing the surrounding air is called air shock wave. The air pressure and air movement effect due to the accumulation of gases from the explosion chemical reactions is called dynamic pressure and the effect rapidly compressing the ground is called ground shock wave. Figure 2.4: Blast wave pressures plotted against time The air shock wave produces an instantaneous increase in pressure above the ambient atmospheric pressure at a point some distance from the source. This is commonly referred to as overpressure. As a consequence, a pressure differential is generated between the combustion gases and the atmosphere, causing a reversal in the direction of flow, back towards the center of the explosion, known as a negative pressure phase. This is a negative pressure relative to atmospheric , rather than absolute negative pressure Figure 2.4. Equilibrium is reached when the air is returned to its original state. As a rough approximation, 1kg of explosive produces about 1m3 of gas. As this gas expands, its act on the air surrounding the source of the explosion causes it to move and increase in pressure. The movement of the displaced air may affect nearby objects and cause damage. Except for a confinement case, the effects of the dynamic pressure diminish rapidly with distance from source. The ground shock leaving the site of an explosion consists of three principal components . A compression wave which travels radially from the source; a shear wave which travels radially and comprises particle movements in a plane normal to the radial direction where the ground shock wave intersects with the surface and a surface or Raleigh wave. These waves propagate at different velocities and alternate at different frequencies. Dept. of Civil Engg. 7 SIMAT, Vavanoor
  • 15. Chapter 3 Architectural Aspect of Blast Resistant Building Design 3.1 General The target of blast resistant building design philosophy is minimizing the consequences to the structure and its inhabitants in the event of an explosion. A primary requirement is the prevention of catastrophic failure of the entire structure or large portions of it. It is also necessary to minimize the effects of blast waves transmitted into the building through openings and to minimize the effects of projectiles on the inhabitants of a building. However, in some cases blast resistant building design methods, conflicts with aesthetical concerns, accessibility variations, fire fighting regulations and the construction budget restrictions. 3.2 Planning And Layout Much can be done at the planning stage of a new building to reduce potential threats and the associated risks of injury and damage. The risk of a terrorist attack, necessity of blast protection for structural and non-structural members, adequate placing of shelter areas within a building should be considered for instance. In relation to an external threat, the priority should be to create as much stand-off distance between an external bomb and the building as possible. On congested city centers there may be little or no scope for repositioning the building, but what small stand-off there is should be secured where possible. This can be achieved by strategic location of obstructions such as bollards, trees and street furniture. Figure 4.1 shows a possible external layout for blast safe planning. 3.3 Structural Form and Internal Layout Structural form is a parameter that greatly affects the blast loads on the building. Arches and domes are the types of structural forms that reduce the blast effects on the building compared with a cubicle form. The plan-shape of a building also has a significant influence on the magnitude of the blast load it is likely to experience. Complex shapes that cause multiple reflections of the blast wave should be discouraged. Projecting roofs or floors, and buildings that are U-shaped on plan are 8
  • 16. B LAST RESISTANT BUILDINGS Figure 3.1: Schematic layout of site for protection against bombs undesirable for this reason. It should be noted that single story buildings are more blast resistant compared with multi-story buildings if applicable. Figure 3.2: Internal planning of a building Partially or fully embed buildings are quite blast resistant. These kinds of structures take the advantage of the shock absorbing property of the soil covered by. The soil provides protection in case of a nuclear explosion as well. The internal layout of the building is another parameter that should be undertaken with the aim of isolating the value from the threat and should be arranged so that the highest exterior threat is separated by the greatest distance from the Dept. of Civil Engg. 9 SIMAT, Vavanoor
  • 17. B LAST RESISTANT BUILDINGS highest value asset. Foyer areas should be protected with reinforced concrete walls; double-dooring should be used and the doors should be arranged eccentrically within a corridor to prevent the blast pressure entering the internals of the building. Entrance to the building should be controlled and be separated from other parts of the building by robust construction for greater physical protection. An underpass beneath or car parking below or within the building should be avoided unless access to it can be effectively controlled. A possible fire that occurs within a structure after an explosion may increase the damage catastrophically. Therefore the internal members of the building should be designed to resist the fire. 3.4 Bomb Shelter Areas The bomb shelter areas are specially designated within the building where vulnerability from the effects of the explosion is at a minimum and where personnel can retire in the event of a bomb threat warning. These areas must afford reasonable protection against explosions; ideally be large enough to accommodate the personnel involved and be located so as to facilitate continual access. For modern-framed buildings, shelter areas should be located away from windows, external doors, external walls and the top floors if the roof is weak. Areas surrounded by full-height concrete walls should be selected and underground car parks, gas storage tanks, areas light weight partition walls, e.g. internal corridors, toilet areas, or conference should be avoided while locating the shelter areas. Basements can sometimes be useful shelter areas, but it is important to ensure that the building does not collapse on top of them. The functional aspects of a bomb shelter area should accommodate all the occupants of the building; provide adequate communication with outside; provide sufficient ventilation and sanitation; limit the blast pressure to less than the ear drum rupture pressure and provide alternative means of escape. 3.5 Installation Gas, water, steam installations, electrical connections, elevators and water storage systems should be planned to resist any explosion affects. Installation connections are critical points to be considered and should be avoided to use in high-risk deformation areas. Areas with high damage receiving potential e.g. external walls, ceilings, roof slabs, car parking spaces and lobbies also should be avoided to locate the electrical and other installations. The main control units and installation feeding points should be protected from direct attacks. A reserve installation system should be provided for a potential explosion and should be located remote from the main installation system. 3.6 Glazing And Cladding Glass from broken and shattered windows could be responsible for a large number of injuries caused by an explosion in a city centre. The choice of a safer glazing maDept. of Civil Engg. 10 SIMAT, Vavanoor
  • 18. B LAST RESISTANT BUILDINGS terial is critical and it has been found out that laminated glass is the most effective in this context. On the other hand, applying transparent polyester anti-shatter film to the inner surface of the glazing is as well an effective method. For the cladding, several aspects of design should be considered to minimize the vulnerability of people within the building and damage to the building itself. The amount of glazing in the facade should be minimized. This will limit the amount of internal damage from the glazing and the amount of blast that can enter. It should also be ensured that the cladding is fixed to the structure securely with easily accessible fixings. This will allow rapid inspection after an explosion so that any failure or movement can be detected. 3.7 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 postdamaged 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. 3.8 Columns Treatment for conventionally designed columns to improve blast resisting mechanism: 1. The potential for direct lateral loading on the face of the columns, resulting Dept. of Civil Engg. 11 SIMAT, Vavanoor
  • 19. B LAST RESISTANT BUILDINGS 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. 3.9 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 loadtransfer 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. 3.10 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. Dept. of Civil Engg. 12 SIMAT, Vavanoor
  • 20. B LAST RESISTANT BUILDINGS 3.11 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. 3.12 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 lateralload resisting mechanism can also be used. A central shear wall and a perimeter moment-resisting frame will provide for a balanced solution. The perimeter momentresisting 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. 3.13 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 percent steel doubly reinforced in both directions might be required. Dept. of Civil Engg. 13 SIMAT, Vavanoor
  • 21. B LAST RESISTANT BUILDINGS 2. For intermediate charge weight protection, a 460 mm (18 in.) thick wall with 0.5 percent steel might be needed. 3.14 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 plantersmust 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. 3.15 Internal Explosion Threat 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 threat. 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. Dept. of Civil Engg. 14 SIMAT, Vavanoor
  • 22. Chapter 4 Structural Aspect of Blast Resistant Building 4.1 General The front face of a building experiences peak overpressures due to reflection of an external blast wave. Once the initial blast wave has passed the reflected surface of the building, the peak overpressure decays to zero. As the sides and the top faces of the building are exposed to overpressures (which has no reflections and are lower than the reflected overpressures on the front face), a relieving effect of blast overpressure is experienced on the front face. The rear of the structure experiences no pressure until the blast wave has traveled the length of the structure and a compression wave has begun to move towards the centre of the rear face. Therefore the pressure built up is not instantaneous. On the other hand, there will be a time lag in the development of pressures and loads on the front and back faces. This time lag causes translational forces to act on the building in the direction of the blast wave. Figure 4.1: Sequence of air-blast effects Blast loadings are extra ordinary load cases however, during structural design, this effect should be taken into account with other loads by an adequate ratio. Similar to the static loaded case design, blast resistant dynamic design also uses the limit state design techniques which are collapse limit design and functionality limit design. In collapse limit design the target is to provide enough ductility to the 15
  • 23. B LAST RESISTANT BUILDINGS building so that the explosion energy is distributed to the structure without overall collapse. For collapse limit design the behavior of structural member connections is crucial. In the case of an explosion, significant translational movement and moment occur and the loads involved should be transferred from the beams to columns. The structure doesnt collapse after the explosion however it cannot function anymore. Functionality limit design however, requires the building to continue functionality after a possible explosion occurred. Only non-structural members like windows or cladding may need maintenance after an explosion so that they should be designed ductile enough. When the positive phase of the shock wave is shorter than the natural vibration period of the structure, the explosion effect vanishes before the structure responds. This kind of blast loading is defined as impulsive loading. If the positive phase is longer than the natural vibration period of the structure, the load can be assumed constant when the structure has maximum deformation. This maximum deformation is a function of the blast loading and the structural rigidity. This kind of blast loading is defined as quasi-static loading. Finally, if the positive phase duration is similar to the natural vibration period of the structure, the behavior of the structure becomes quite complicated. This case can be defined as dynamic loading. Frame Figure 4.2: Enhanced beam-to-column connection details for steelwork and reinforced concrete buildings designed to resist gravity, wind loads and earthquake loads in the normal way have frequently been found to be deficient in two respects. When subjected to blast loading; the failure of beam-to-column connections and the inability of the structure to tolerate load reversal.Beam-to-column connections can be subjected to Dept. of Civil Engg. 16 SIMAT, Vavanoor
  • 24. B LAST RESISTANT BUILDINGS very high forces as the result of an explosion. These forces will have a horizontal component arising from the walls of the building and a vertical component from the differential loading on the upper and lower surfaces of floors. Providing additional robustness to these connections can be a significant enhancement. In the connections, normal details for static loading have been found to be inadequate for blast loading. Especially for the steelwork beam-to-column connections, it is essential for the connection to bear inelastic deformations so that the moment frames could still operate after an instantaneous explosion. Figure 2.8 shows the side-plate connection detail in question . The main features to note in the reinforced concrete connection are the use of extra links and the location of the starter bars in the connection Figure 2.8. These enhancements are intended to reduce the risk of collapse or the connection be damaged, possibly as a result of a load reversal on the beam. It is vital that in critical areas, full moment-resisting connections are made in order to ensure the load carrying capacity of structural members after an explosion. Beams acting primarily in bending may also carry significant axial load caused by the blast loading. On the contrary, columns are predominantly loaded with axial forces under normal loading conditions, however under blast loading they may be subjected to bending. Such forces can lead to loss of load-carrying capacity of a section. In the case of an explosion, columns of a reinforced concrete structure are the most important members that should be protected. Two types of wrapping can be applied to provide this. Wrapping with steel belts or wrapping with carbon fiberreinforced polymers (CFRP). Cast-insitu reinforced concrete floor slabs are the preferred option for blast resistant buildings, but it may be necessary to consider the use of precast floors in some circumstances. Precast floor units are not recommended for use at first floor where the risk from an internal explosion is greatest. Lightweight roofs and more particularly, glass roofs should be avoided and a reinforced concrete or precast concrete slab is to be preferred. 4.2 Structural Failure An explosion will create blast wave. The air-blast shock wave is the primary damage mechanism in an explosion. The pressures it exerts on building surfaces may be several orders of magnitude greater than the loads for which the building is designed. The shock wave will penetrate and surround a structure and acts in directions that the building may not have been designed for, such as upward force on the floor system. In terms of sequence of response, the air-blast first impinges on the weakest point in the vicinity of the device closest to the explosion, typically the exterior envelope of the building. The explosion pushes on the exterior walls at the lower stories and may cause wall failure and window breakage. As the shock wave continues to expand, it enters the structure, pushing both upward and downward on the floor Dept. of Civil Engg. 17 SIMAT, Vavanoor
  • 25. B LAST RESISTANT BUILDINGS slabs Figure 4.3: Shock Front from Air Burst Figure 4.4: Shock Front from Surface Burst 4.3 Comparison of Blast And Seismic Loading Blast wave and seismic loading are two different type of extreme force that may cause structural failure. However, they share some common similarities. Similarities between seismic and blast loading includes the following: 1. Dynamic loads and dynamic structural response. 2. Involve inelastic structural response. Dept. of Civil Engg. 18 SIMAT, Vavanoor
  • 26. B LAST RESISTANT BUILDINGS 3. Design considerations will focus on life safety as opposed to preventing structural damage. 4. Other considerations: Nonstructural damage and hazards. 5. Performance based design: life safety issues and progressive collapse. 6. Structural integrity: includes ductility, continuity, and redundancy; balanced design. The differences between these two types of loading include: 1. Blast loading is due to a propagating pressure wave as opposed to ground shaking. 2. Blast results in direct pressure loading to structure; pressure is in all directions, whereas a Seismic event is dominated by lateral load effects. 3. Blast loading is of higher amplitude and very short duration compared with a seismic event. 4. Magnitude of blast loading is difficult to predict and not based on geographical location. 5. Blast effects are confined to structures in the immediate vicinity of event because pressure decays rapidly with distance; local versus regional even. 6. Progressive collapse is the most serious consequence of blast loading. 4.4 Damage Evaluation Procedure For Building Subjected To Blast Impact 1.Slab failure is typical in blasts due to large surface area subjected to upward pressure not considered in gravity design. 2. Small database on blast effects on structures. 3.Seismic-resistant design is mature compared with blast-resistant design. In summary, while the effect of blast loading is localized compared with an earthquake, the ability to sustain local damage without total collapse (structural integrity) is a key similarity between seismic-resistant and blast-resistant design. In this study, the evaluation data that had been listed in inspection form is adapted and modified from inspection form for building after an earthquake. Even though, seismic loading will cause global response to building compared to blast loading which will cause localized response, but similar damage assessment procedure could be used. Dept. of Civil Engg. 19 SIMAT, Vavanoor
  • 27. Chapter 5 Case Study 5.1 World Trade Center Collapse 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 5.3) The collapse, which generally occurred inward without significant tipping.(figure 5.4) Before going to the details it is useful to review the overall design of the towers 5.1.1 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 kPaa 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 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 20
  • 28. B LAST RESISTANT BUILDINGS Figure 5.1: A cutaway view of WTC structure 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. 5.1.2 The Details of The Impact 5.1.2.1 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. 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 Lgallons of jet fuel, comprising nearly 1/3 of the aircrafts weight, ignited. The ensuing fire was clearly the principal cause of the collapse (see figure 5.2) Dept. of Civil Engg. 21 SIMAT, Vavanoor
  • 29. B LAST RESISTANT BUILDINGS Figure 5.2: A graphic illustration, from the USA Today newspaper web site, of the World Trade Center points of impact. Click on the image above to access the actual USA Today feature 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 Dept. of Civil Engg. 22 SIMAT, Vavanoor
  • 30. B LAST RESISTANT BUILDINGS Figure 5.3: Flames and debris exploded from the World Trade Center south tower immediately after the airplanes impact. The black smoke indicates a fuel-rich fire 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 premixed 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 Dept. of Civil Engg. 23 SIMAT, Vavanoor
  • 31. B LAST RESISTANT BUILDINGS 3,200C; for hydrogen it is 2,750C. Thus, for virtually any hydrocarbons, the maximum flame temperature, starting at ambient temperature and using pure oxygen, is approximately 3,000C. 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 onethird 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 1,000Chardly sufficient to melt steel at 1,500C. 5.1.2.2 The Collapse Figure 5.4: 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 Dept. of Civil Engg. 24 SIMAT, Vavanoor
  • 32. B LAST RESISTANT BUILDINGS 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 pointswere 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 1,300t 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. 5.1.3 Can Building 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, todays 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 Dept. of Civil Engg. 25 SIMAT, Vavanoor
  • 33. B LAST RESISTANT BUILDINGS 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. 5.1.4 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. Arent 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 com- Dept. of Civil Engg. 26 SIMAT, Vavanoor
  • 34. B LAST RESISTANT BUILDINGS pletely and immediately disabled or destroyed upon impact: fireproofing, automatic sprinklers, compartmentalization and pressurization, lighting, structure and exit stairs. 5.2 Israel as a Case Study And Paradigm Over the course of its history, Israel has adapted military blast design to blast design to be used as a part of civilian structures. Israels methods for integrating blast protection into its society can be used as an example for the rest of the world as it is increasingly subjected to more security threats. When the state was founded in 1948, Israel had already constructed underground shelters across the country (see Figure 5.5). Underground shelters were the first forms of civilian blast protection because one of the most effective methods of providing protection for a structure is to bury it (Smith and Hetherington, 1994). Underground bomb shelters do have some benefits; they are generally larger than what could be provided for inside of a building so they are more comfortable for long periods of time. In addition , when the shelters were not in use they could be used for recreational purposes (Einstein, 2003 ). Many shelters were turned into libraries and meeting places for youth groups (see Figures 2.10 and 2.11). These underground shelters became a part of Israeli culture. Figure 5.5: Entrance to an underground shelter in Israel In the 1970s civilians in Israel were being threatened along its border with Lebanon. Katusha rockets were being launched over the Lebanese border into the Dept. of Civil Engg. 27 SIMAT, Vavanoor
  • 35. B LAST RESISTANT BUILDINGS Figure 5.6: Shelter used as a playroom Figure 5.7: Shelter used as a playroom Israeli cities on the other side, and Israel needed to provide its citizens with protection from the attacks. Throughout northern Israel rooms designed to protect a buildings inhabitants from an explosion were included in most homes as well as schools and public buildings (Sandler, 2003). This was the beginning of the transition from underground shelters, separate from the buildings. To shelters integrated into daily structures. The biggest change in Israels policy toward protecting its citizens came in 1991 with the Gulf war. Saddam Hussein threatened Israel with Scud missiles and this not only increased the treat due to explosions, but also introduced the strong possibility of bio-chemical threats. People were now required to have protected spaces within every home, office, and public space. The windows had to be able to be sealed around the edges, and doors would have a wet towel placed at the bottom. The room also had to be blast proof so that in an attack craked walls and windows Dept. of Civil Engg. 28 SIMAT, Vavanoor
  • 36. B LAST RESISTANT BUILDINGS would not allow poisonous gas to seep in. Figure 5.8: The change from underground shelters to protected spaces New building requirements to have these protected spaces in all civilian structures, and how to design these spaces were developed and known as Haga requirements (Einstein, 2003). These regulations were fully integrated into the Israeli building code and continue to be maintained in order to protect Israeli civilians. Figure 5.9: Example of Israeli structural blast desing While the regulations being put into the building code was instigated by a need to provide protection against chemical warfare , the importance of regulating the integration of protected spaces into buildings remains and extend into blast protection. Protecting a building from explosions is now an integral part of a buildings design out security risks while preserving the essence of the design (Einstein, 2003). Israeli society cannot have all of its buildings feel like concrete fortified structures even if they rely (Figures 2.13,2.14,2.15) are examples of Israeli blast designed structures, Dept. of Civil Engg. 29 SIMAT, Vavanoor
  • 37. B LAST RESISTANT BUILDINGS Figure 5.10: Example of Israeli structural blast desing Figure 5.11: Example of traditional American structual blast desing versus the current blast designed structures in the United States. Since September 11, 2001 and the destruction of the World Trade Centre due to terrorism, it has become apparent that the U.S. must also change its approach to protecting its citizens from explosions. Israel has successfully integrated blast protection into its society and buildings as a result of years of terror and threats. By making blast protection a permanent part of the building code professionals have been forced to come up with new ways of designing building s that protect their inhabitants but still maintain peoples quality of life (Einstein, 2003). Because of the increased and continuing threat to the United States it is clear that structural engineers here too will have to make blast design an integral part of all structures. The more this mentality is put into practice the sooner blast design will be able t coexist with current structural design consideration such as architecture, sustainability, usability, and economics. Dept. of Civil Engg. 30 SIMAT, Vavanoor
  • 38. Chapter 6 Design Principles for Protection of Structures 6.1 General Designing a building to withstand blasts includes more than just hardening the structure. A lot of thought must go into the design to take into account more conceptual design aspects such as preventing an attack to begin with, maintaining a large stand-off distance in case of an attack, and designing the building so that it will remain standing in the case of localized damage. While discussing the principles of blast design I will focus on the protection of structures in the event of a close range bomb, most similar to present terrorist activities. This includes explosions due to suicide bombers near or inside a building, truck and car bombs near or driven inside a building, and package bombs. 6.2 Preventative Measures The first step in making a building blast resistant is to try to prevent a terrorist attack from occurring in the first place. This can be accomplished by making a terrorists attack from occurring in the first place. This can be accomplished by making a terrorists job as difficult as possible. There isles of a chance of a terrorist targeting a building if he feels that the chance of success is small (Mays and Smith, 1995). Preventing access into the building is the first way to deter a terrorist. Heavy security as well as physical barriers can make entering a building difficult. Also, if space allows, spreading out a complex makes an effective terrorist attack more difficult to execute. A bomb in one location will have less overall effect on a building if all of the buildings assets are spread out (Mays and smith, 1995). This strategy is only effective for building that is not set in the middle of the city and can afford to expand outwards. In addition, sites that could be possible terrorist targets such as intelligence or defence building should be kept anonymous if possible (Mays and Smith. 1995). The next thing to consider is how to disguise the critical parts of a building. If the energy from a bomb is wasted on an unimportant part of the building the consequences of an attack can be much less severe (Mays and Smith, 1995). It i important to prevent the placement of explosives near sensitive structural members. Ways of accomplishing this include hiding columns and other important structural 31
  • 39. B LAST RESISTANT BUILDINGS members, especially near the ground floors of a building where the structural members are the most critical (Eytan, 2003). By using tinted glass you can hid the exact structural system from outside viewers as well. One of the most important principles with blast design is to keep a large standoff distance between the building and the potential blast. The strength of a blast decreases in relation to the cube of the standoff distance from the explosion, this indicates that as l you get farther away from the blast the intensity of the peak pressure dies off substantially. Smith and Hetherington illustrate this by saying that keeping vehicle bombs away from your structure is probably the sing, most cost effective device you can employ. 6.3 Hardening of the structure The next principle in structural blast design is to harden the structure in the case that a blast does take place. The main way to harden a structure is to design the structure with a lot of ductility integrated thought-out the system. Explosions generate an enormous amount of energy and the role of the structures ductility i to absorb this energy. As a result steel and reinforced concrete are the bets materials to use in a blast resistant structure. Other structural concerns include how the floors are attached to the rest of the structural frame. Floors need to be securely tied to the frame and be able to withstand stresses in the direction opposite the normal gravity loads. Explosion cause a strong uplift reassure that can dislodge floors from their supports if they are not tied securely. Floors many times work as a diaphragm that carries lateral load in a structure, as a result, if the floor is removed from the rest of the structural system progressive collapse can ensue. Glazing is a major concern when hardening a building. Because normal glass is a brittle material it has almost no chance of remaining intact during an explosion. Secondary injuries and damages due to shards of glass flying at high speeds though the air can be very severe and are usually very frequent. There are several techniques for increasing the blast resistance of glazing these techniques in combination with dynamic design of the structural frame can greatly increase the performance of glazing in an explosion. These techniques include: 1.Using blast resistant glass. 2.Applying polyester anti shatter film to the inside surface of the glass. 3.Installing bomb blast net curtains inside of the glass (to prevent the shards from entering the interior of the building). 4.Installing blast resistant glazing inside the existing exterior glazing. In order to further protect the occupants of a building it is important to design the building so that it is at least three bays wide. This provides space so that in the case of an explosion people can move away from the exterior of a building. Also, Dept. of Civil Engg. 32 SIMAT, Vavanoor
  • 40. B LAST RESISTANT BUILDINGS the centre area of the structure should be designed as a concrete core. This concrete core can be designed as a hardened area that can be used as protected space for the building occupants. In addition to all of these design techniques Eytan has developed a method of hardening a structure in layers. Hardening structure inlayers is effective because it ensures that the failure of one hardening layer will not lead to the catastrophic failure of the structure due to redundancy of the protective systems. The first hardening layer is the layer furthers away from the structure. The role of this layer is to prevent a terrorists forced entry into building (like a vehicle crashing into the building), and to protect the rest of the structure from a large explosion outside of the building. The second layer is the envelope of the external structural system. The role of this layer is to prevent a terrorists forced entry further into the building. It should shield the rest of the building from the building from flying debris and shrapnel from a bomb. In addition, it should protect main structural elements from close range explosions. And, of course it should further protect the structure from the pressure wave created by a bomb outside of the building. The third layer is the layer that protects the internal structural system. This layer needs to protect the building from all of the things that the second layer is designed to protect. In addition, this layer must be able to protect the structure from explosions donated inside of the structure. 6.4 Hardening of the structure After all of these other hardening techniques are used the most important thing is that a building be designed so that progressive structural collapse does not occur in the case of severe structural damage. As we have seen in events such as the World Trade Centre bombing, as destructive as the explosion itself was, the greatest damage and loss of life was due to the eventual collapse of the structure that was as a result of structural damage. Preventing structural collapse is necessary so that as many people as possible can get out of a building safely after an attack. If progressive collapse occurs it magnifies the effect of any terrorist event and allows a terrorist to accomplish more damge than they ever could on their own. There are several guidelines that should be kept in mind in order to design a building to be protected from structural collapse: a.Create many different load paths and redundancies within a structure so that it will not collapse in the case of several columns of critical members being damaged or destroyed. b.Design floors to withstand reverse loading. c.Design connection to withstand greater loading. d.Design critical members, such as lower floor columns, to withstand a higher blast Dept. of Civil Engg. 33 SIMAT, Vavanoor
  • 41. B LAST RESISTANT BUILDINGS loading to prevent severe damage to the most important members. e.Design critical members to be surrounded by energy absorbing materials or members. Some other technique for protecting columns inside a structure include: using composite material shields around the column with an air gap between the shield and the column. Columns can be designed to be part of heavy walls so that they will not experience local failure. The strength of the columns is improved if they are designed as part of a moment frame where the connections can carry a large amount of moment and dissipate a lot of energy. Also, columns should be designed to withstand a greater buckling load in case its unsupported length is increased by damage to adjacent beams, joists, and slabs. Dept. of Civil Engg. 34 SIMAT, Vavanoor
  • 42. Chapter 7 Conclusion 7.1 General The aim in blast resistant building design is to prevent the overall collapse of the building and fatal damages. Despite the fact that, the magnitude of the explosion and the loads caused by it cannot be anticipated perfectly, the most possible scenarios will let to find the necessary engineering and architectural solutions for it. In the design process it is vital to determine the potential danger and the extent of this danger. Most importantly human safety should be provided. Moreover, to achieve functional continuity after an explosion, architectural and structural factors should be taken into account in the design process, and an optimum building plan should be put together. This study is motivated from making buildings in a blast resistant way, pioneering to put the necessary regulations into practice for preventing human and structural loss due to the blast and other human-sourced hazards and creating a common sense about the explosions that they are possible threats in daily life. In this context, architectural and structural design of buildings should be specially considered. During the architectural design, the behavior under extreme compression loading of the structural form, structural elements e.g. walls, flooring and secondary structural elements like cladding and glazing should be considered carefully. In conventional design, all structural elements are designed to resist the structural loads. But it should be remembered that, blast loads are unpredictable, instantaneous and extreme. Therefore, it is obvious that a building will receive less damage with a selected safety level and a blast resistant architectural design. On the other hand, these kinds of buildings will less attract the terrorist attacks. Structural design after an environmental and architectural blast resistant design, as well stands for a great importance to prevent the overall collapse of a building. With correct selection of the structural system, well designed beam-column connections, structural elements designed adequately, moment frames that transfer sufficient load and high quality material; its possible to build a blast resistant building. Every single member should be designed to bear the possible blast loading. For 35
  • 43. B LAST RESISTANT BUILDINGS the existing structures, retrofitting of the structural elements might be essential. Although these precautions will increase the cost of construction, to protect special buildings with terrorist attack risk like embassies, federal buildings or trade centers is unquestionable. Dept. of Civil Engg. 36 SIMAT, Vavanoor
  • 44. References [1] Koccaz Z. (2004) Blast Resistant Building Design, MSc Thesis, Istanbul Technical University, Istanbul, Turkey. [2] Smith P.D., Hetherington J.G. (1994) Blast and ballistic loading of structures. Butterworth Heinemann. [3] Yandzio E., Gough M. (1999). Protection of Buildings Against Explosions, SCI Publication, Berkshire, U.K. [4] Website : www.iitk.ac.in/nicee/wcee/article/14-05-01-0536.PDF [5] Civil engineering articles at google.com 37