Architectural And Structural Design Of Blast Resistant Buildings

BLAST RESISTANT BUILDINGS

A seminar report submitted in partial fulfillment of the requirements for the award of the degree degree of

Bachelor of Technology in

Civil Civil Engine Engineerin ering g

PAUL JOMY (SYAKECE033)

Eighth Semester 2010 Admission

Sreepathy Institute of Management & Technology Vavanoor, Palakkad-679533 Affiliated to University Of Calicut

Depart Departmen mentt of Civil Engine Engineeri ering ng Sreepathy Institute of Management & Technology Vavanoor, avanoor, Palakkad-679533 Palakkad-679533

CERTIFICATE

This is to certify that the seminar entitled ”BLAST ”BLAST RESIST RESISTANT ANT BUILDBUILDINGS” is a bonafide record of the seminar presented by PAUL JOMY (Reg No. SYAKECE03 SY AKECE033) 3) under our supervision and guidance. The seminar report has been submitted submitted to the Department of Civil Engineering Engineering of SIMAT SIMAT Vav Vavanoor, PalakkadPalakkad679533 in partial fulfillment of the award of the Degree of Bachelor of Technology in Civil Engineering, Engineering, during during the year 2013-2014. 2013-2014.

Mr.ARUN ASHOK Guide Asst. Professor Professor Civi Civill Engg Engg SIMAT, Vavanoor

Mr.SUDHEER.K.V Head of the Dept Civi Civill Engg Engg SIMAT, Vavanoor Palakkad

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 Firstly, explosives explosives and explosion explosion types have been explained explained briefly. 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 explosive effects is discussed both with an architectural architectural and structural approach.

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. guidance. I thank God Go d almighty almighty for all the blessing received during this endeavor. 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. Dept. of Civil Civil Engg. Engg. SIMAT, Vavannoor, Palakkad

Contents

List of Figures

iii

1 Intro duction

1

1.1 1.2

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Obje bjective Of The Blast Design . . . . . . . . . . . . . . . . . . . . . .

2 Literature survey

2.1 2.2 2.2

2.3 2.4 2.4 2.5

2.6 2.6

3

General . . . . . . . . . . . . . . . . . . . . . . . Expl Explos osiion - Maj Major of All Terrori rorisst Activ tivities . . . 2.2. 2.2.11 Expect pected ed Terror rroriist Blast ast On Struct ructu ures res . . 2.2. 2.2.22 Majo ajor Cause use of Life Loss oss After The Blast ast Goals of Blast Resistant Design . . . . . . . . . . Basic Requi equire rem ments ents To Resi esist Blast Loa Loads . . . . 2.4. 2.4.11 Mech echanics of a Conventional Explo plosion . . Types of Explosions . . . . . . . . . . . . . . . . . 2.5.1 Unconfined Explosion . . . . . . . . . . . . 2.5.2 Confined Explosions . . . . . . . . . . . . Expl Explos osiion Proc Proces esss For High Explosive . . . . . . .

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3 Architectural Aspec pect of Blast Resistant Building Design

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.12 3.13 3.14 3.15

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General . . . . . . . . . . . . . . . . . . . . . . . Planning And Layout . . . . . . . . . . . . . . . . Structural Form and Internal Layout . . . . . . . Bomb Shelter Areas . . . . . . . . . . . . . . . . . Installation . . . . . . . . . . . . . . . . . . . . . Glazing And Cladding . . . . . . . . . . . . . . . Flo or Slabs . . . . . . . . . . . . . . . . . . . . . Columns . . . . . . . . . . . . . . . . . . . . . . . Transfer Girders . . . . . . . . . . . . . . . . . . . Ext External Treatments . . . . . . . . . . . . . . . . Facade And Atrium . . . . . . . . . . . . . . . . . Over Overal alll Lat Later eral al Bu Buil ildi ding ng Resi Resist stan ance ce,, She Shear ar Walls alls . Lower Floor oor Exterior . . . . . . . . . . . . . . . . Sta Stand Off Distance . . . . . . . . . . . . . . . . . Internal Explosion Threat . . . . . . . . . . . . .

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4 Structural Aspect of Blast Resistant Building

4.1 4.2 4.3 4.3 4.4

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General . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural Failure . . . . . . . . . . . . . . . . . . . . . . Comp omparis rison of Blast And Seismic Load adiing . . . . . . . . Damage Evaluatio Evaluation n Procedure For Building Building Subjected To To

. . . . . . . 15 . . . . . . . 17 . . . . . . . 18 Blast Blast Impact Impact 19

5 Case Study

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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. 5.1.33 Ca Can n Bu Buil ildi ding ng Resi Resist st Dire Direct ct Airpl irplan anee Hits its . . . . . . . . . . . 5.1. 5.1.44 How How Can Can We Min Minim imiz izee The The Ch Chanc ancee of of Pro Progre gress ssiv ivee Col Colla laps psee 5.2 Israel as a Case Study And Paradigm . . . . . . . . . . . . . . . . .

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6 Design Principles for Protection of Structures

6.1 6.2 6.3 6.4

General . . . . . . . . . . Preventative Measures . . Hardening of the structure Hardening of the structure

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7 Conclusion

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General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

References

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List of Figures

2.1 2.2 2.3 2.3 2.4 2.4

Air burst with ground reflections . . . . . . . . . . . . . . Surface burst . . . . . . . . . . . . . . . . . . . . . . . . . Fully ully vented ented,, parti partial ally ly vented ented and full fully y confin confined ed explo explosi sions ons Blast wave pres ressures res plotted ted agai gainst tim time . . . . . . . . . .

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5 6 6 7

3.1 3.1 3.2

Sche Schema mati ticc lay layout out of site site for for prot protec ecti tion on agai agains nstt bombs bombs . . . . . . . . . Internal planning of a building . . . . . . . . . . . . . . . . . . . . . .

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4.1 Sequence of air-blast effects . . . . . . . . . . . . . . . . . . . . . . . 4.2 Enhanced beam-to-column beam-to-column connection connection details details for for steelwork steelwork and reinreinforced concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Shock ock Front from Air Burst . . . . . . . . . . . . . . . . . . . . . . . 4.4 Shock ock Front from Surface Burst . . . . . . . . . . . . . . . . . . . . .

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5.1 5.2 5.3 5.4 5.5 5.5 5.6 5.7 5.8 5.8 5.9 5.9 5.10 5.10 5.11 5.11

21 22 23 24 27 28 28 29 29 30 30

A cutaway view of WTC structure . . . . . . . . . . . . . A graphic illustration of WTC . . . . . . . . . . . . . . . . Airplane’s impact on WTC . . . . . . . . . . . . . . . . . . Collapse of WTC . . . . . . . . . . . . . . . . . . . . . . . Entr En tran ance ce to an un unde derg rgro roun und d shel shelte terr in Isra Israel el . . . . . . . . Shelter used as a playroom oom . . . . . . . . . . . . . . . . . . Shelter used as a playroom oom . . . . . . . . . . . . . . . . . . Thee cha Th chang ngee fro from m und under ergr grou ound nd shel shelte ters rs to prot protec ecte ted d spa space cess Exam Exampl plee of Isra Israel elii stru struct ctur ural al blas blastt desi desing ng . . . . . . . . . . Exam Exampl plee of of Isra Israel elii str struc uctu tura rall blas blastt des desin ingg . . . . . . . . . . Examp Example le of tradi traditi tion onal al Ameri American can struct structua uall bla blast st desi desing ng . .

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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 Firstly, explosives explosives and explosion explosion types have been explained explained briefly. 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 explosive effects is discussed both with an architectural architectural and structural approach. Damage to the assets, loss of life and social panic are factors that have to be minimi minimized zed if the threat of terroris terroristt action action cannot be stopped. stopped. Design Designing ing the strucstructures 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

B LAST RESISTANT BUILDINGS

1.2

Objective Objective Of Of The The Blast Blast Design 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 Evidence from past incidents 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 even eventt is another another objective objective of blast blast resistant resistant design. design. An incident 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 Buildings containing containing business business information, critical or essential essential equipment, 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.

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Chapter 2 Literature survey

2.1

General

The need and requirements for blast resistance in buildings have evolved over recent cent years. years. Buildi Buildings ngs have have become become more complex complex and have have increase increased d in size size thus thus increas increasing ing the risk risk of acciden accidental tal explosi explosions. ons. Such Such explosi explosions ons have have demolishe demolished d 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 potential for explosions. explosions. Generally Generally,, these issues relate to plant plant building safety and risk management to prevent or minimize the occurrence of such incidents and to siting, design, and operations.

2.2

Explosion Explosion - Major of All Terror Terrorist ist Activi Activities ties

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 Expected Terro Terrorist rist Blast Blast On Structures Structures

-External car bomb -Internal car bomb -Internal package -Suicidal car bombs

2.2.2

Major Cause Cause of Life Life Loss Loss After After The Blast Blast

-Flying -Flying debris -Broken glass -Smoke and fire -Blocked glass -Power loss -Communications breakdown -Progressive collapse of structure

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2.3

Goals Goals of Blast Blast Resistan Resistantt Design 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 Requi Requireme rement ntss To To Resist Resist Blast Loads Loads

To resist blast loads, - The first requirem requiremen entt is to determine determine the threat. The major threat is caused caused by terrorist bombings. bombings. The threat for a conventional 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 keepout distanc distance. e. No matter what size the bomb, the damage will be less less severe the further the target is from the source. - Structural hardening should actually be the last resort in protecting a structure; ture; detectio detection n and preven prevention tion must must remain remain the first first line line of defense . As terrorist 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 Mechanics of a Conven Conventional tional Explosion 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 extremely close to a target structure impose a highly impulsive impulsive,, high intensity intensity pressure load over over a localized region of the structure; charges situated further away produce a lower-intensity, longer-duration uniform pressure distribution over over the entire entire structure. structure. In short short by purely geometrica geometricall relatio relations, ns, the larger larger Dept. Dept. of Civil Civil Engg. Engg.

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the standoff, the more uniform the pressure distribution over over the target. Eventually 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 Explosions

Mainly there are two types of explosions

2.5.1

Unconfined Explosion

Unconfin Unconfined ed explosion explosionss can occur as an air-bu air-burst rst or a surface surface burst. In an air burst

Figure 2.1: Air burst with ground reflections explosion, explosion, the detonation detonation of the high explosive 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. surface. The initial initial shock wave wave is reflected reflected and amplifi amplified ed by the ground ground surface surface to produce produce a reflected reflected wave wave.. Figure Figure 2.2. Unlik Unlikee 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.


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Figure 2.2: Surface burst

2.5.2

Confined Explosions

When an explosion explosion occurs within a building, the pressures associated with the initial 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 Explosion Process Process For For High High Explosiv Explosive e

An explosion occurs when a gas, liquid or solid material goes through a rapid chemical reaction reaction.. When the explosion explosion occurs, gas products of the reaction reaction are formed formed at a very very high temperatu temperature re and pressur pressuree at the source. source. These These high pressure pressure gasses gasses expand rapidly into into the surrounding surrounding area and a blast wave 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 wave wa vess propagat propagatee at supersonic supersonic speeds and reflected reflected as they meet objects. As the blast wave continues to expand away from the source of the explosion its intensity Dept. Dept. of Civil Civil Engg. Engg.

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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. objects. Three Three effects have have been b een identifie identified d in three three categori categories. es. The effect rapidly rapidly compress compressing ing the surroun surroundin dingg air is called called air shock wave. wave. The air pressure 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 ambie ambient nt atmospher atmospheric ic pressure pressure at a point point some some distanc distancee from the source. source. This This is commonly commonly referred to as overpressu overpressure. re. As a consequence, a pressure differential 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 move and increase in pressure. The movement movement of the displaced 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 componen components ts . A compress compression ion wave wave which which travels travels radially radially from the source; a shear 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 Raleigh wave. These waves waves propagate at different different velocities and alternate alternate at different frequencies. Dept. Dept. of Civil Civil Engg. Engg.

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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 inhabit inhabitan ants ts of a buildi building. ng. Howe Howeve ver, r, in some cases blast blast resistan resistantt buildi building ng design design methods, conflicts with aesthetical aesthetical concerns, accessibilit accessibility y variations, ariations, fire fighting fighting regulations and the construction budget restrictions.

3.2

Planning And Layout 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 placi placing ng of shel shelter ter areas within within a build buildin ingg shoul should d be consi consider dered ed for instan instance. ce. In relation to an external threat, the priority should be to create as much stand-off dista distance nce between between an exter externa nall bomb bomb and the bu buil ildi ding ng as possi possibl ble. e. On conge congeste sted d city centers there may be little or no scope for repositioning the building, but what small small stand-off stand-off there is should should be secured secured where possible. possible. This This can be achiev achieved ed 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 Internal Layout 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 buildin buildingg compared compared with a cubicle cubicle form. The plan-sha plan-shape pe of a buildi building ng also has a significant influence on the magnitude of the blast load it is likely to experience. ence. Complex Complex shapes that cause cause multi multiple ple reflections reflections of the blast wave wave should should be discouraged. Projecting roofs or floors, and buildings that are U-shaped on plan are 8


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 advantage of the shock absorbing property of the soil covered covered by. 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. Dept. of Civil Civil Engg. Engg.

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highest value asset. Foyer areas should be protected with reinforced concrete walls; double-dooring double-dooring should be used and the doors should be arranged eccentricall eccentrically y within a corrido corridorr to preven preventt the blast pressure pressure enteri entering ng the internals internals of the building building.. Entrance to the building should be controlled and be separated from other parts of the building by robust construction construction for greater physical 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 Bomb Shelter Shelter Areas 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 retire in the event event of a bomb threat threat wa warnin rning. g. These These areas areas must must afford reasonable 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 ternal walls walls and the top floors if the roof is weak. weak. Areas Areas surrounded surrounded by full-heig full-height ht 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 should be b e avoide avoided d while while locating the shelter shelter areas. Basemen Basements ts can sometimes 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 provide sufficient ventilation ventilation and sanitation; 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 should be b e planned to resist any explosion explosion affects. Installation Installation connections connections are critical points to be considered and should be avoided to use in high-risk deformati formation on areas. Areas Areas with high damage receivin receivingg potential potential e.g. external external walls, 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 should be protecte protected d from direct direct attack attacks. s. A reserv reservee install installatio ation n system system should be provided for a potential explosion and should be located remote from the main installation 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. Dept. of Civil Civil Engg. Engg.

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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 vulnera vulnerabil bilit ity y of people people within within the buildin buildingg and damage to the building building itself. itself. The amount amount of glazin glazingg in the facade facade should be minimi minimized. zed. This This will will limit limit the amount amount of inte interna rnall damag damagee from from the glazin glazingg and the amoun amountt of blas blastt that that can enter. 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 Slabs

Treatments for conventional flat slab design are as follows: 1. More attention attention must must be paid paid to the design design and detailin detailingg of exterior exterior bays and lower floors, which are the most susceptible to blast loads. 2. In exterior exterior bays/lo bays/lowe werr floors, floors, drop panels panels and column column capitols capitols are required required to shorten the effective slab length and improve the punching shear resistance. 3. If vertic ertical al clear clearanc ancee is a probl problem em,, shear shear heads heads embed embedded ded in the slab slab will will improve the shear resistance and improve the ability of the slab to transfer moments to the columns. 4. The slab-column interface interface should contain closed-hoop stirrup reinforcement reinforcement properly anchored around flexural bars within a prescribed distance from the column face. 5. Bottom Bottom reinfor reinforceme cement nt must be provid provided ed contin continuous uous through through the column. column. This 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. Th Thee poten potenti tial al for direct direct later lateral al loadi loading ng on the face of the column columns, s, resul resulti ting ng Dept. Dept. of Civil Civil Engg. Engg.

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from the blast pressure and impact of explosive debris, requires that the lower-floor columns be designed with adequate ductility and strength. 2. Th Thee perime perimeter ter column columnss suppo supporti rting ng the lower lower floors floors must ust also also be desig designed ned to resist this extreme blast effect. 3. Encasin Encasingg these these lowe lower-floo r-floorr columns columns in a steel steel jacke jackett will will provid providee confinem confinemen ent, t, increas increasee shear shear capacit capacity y, and impro improve ve the columns columns’’ ductili ductility ty and strength strength.. An alternative, which provides similar benefits, is to embed a steel column within the perimeter concrete columns or wall section. 4. The possibility possibility of uplift uplift must be considered, considered, and, if deemed likely likely,, the columns must be reinforced to withstand a transient tensile force. 5. For smalle smallerr charge charge weigh weights, ts, spiral spiral reinforce reinforcemen mentt provid provides es a measure 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 number of structural structural elements. This loadtransfer 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 transfer girder girder and the column column connectio connections ns must must be properly properly designed 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.


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3.11

Facade And Atrium Atrium

The facade facade is comprised comprised of the glazing glazing and the exterior exterior wall. wall. Better Better glazing glazing has already been discussed above and wall obviously should be hardened to resist the loadi loading. ng. Prese Presence nce of an atriu atrium m along along the face face of the struct structure ure will requir requiree two two protecti protective ve measures measures.. On the outside outside of the structure, structure, the glass glass and glass glass framing 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 Overall Lateral Building Resistance, Shear Walls 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, energy, prior to failure.. failure.. In addition to providing ductile behavior behavior for the structure, the following provisions would improve the blast protection capability of the building: 1. Use a wellwell-dis distri tribute buted d lateral lateral-lo -load ad resisti resisting ng mechani mechanism sm in the horizon horizontal tal 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 shear wa walls lls is not architect architectural urally ly feasible, feasible, a combin combined ed lateral lateral-load load resisti resisting ng mechanis mechanism m can also be used. used. A central central shear shear wa wall ll and a perimeter perimeter moment-resi moment-resisting sting frame will provide for a balanced solution. solution. The perimeter momentresisting frame will require strengthening the spandrel beams and the connections to the outsid outsidee column columns. s. Th This is will will also also resul resultt in better better protecti protection on of the outsid outsidee 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 Lower Floor Exterior

The architectural design of the building of interest currently calls for window glass around around the first first floor. Unless Unless this area is construc constructed ted in reinforc reinforced ed concrete, 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 protec protectt again against st a smal smalll charg chargee weig weight ht,, a nomi nominal nal 300 mm (12 in.) in.) thick thick wall with 0.3 percent steel doubly reinforced in both directions might be required.


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

3.14

Stand Stand Off Distance Distance

The keep out distance, within which explosives-laden vehicles may not penetrate, must must be b e maximi maximized zed and guaran guaranteed teed.. As we all know, know, the greater greater the standoff standoff distance, the more the blast forces will dissipate resulting in reduced pressures on the building. building. Several Several recommendations recommendations can be b e made to maintain maintain and improve improve the standoff distance for the building under consideration: 1. Use Use antianti-ram ram bolla bollard rdss or larg largee plan planter ters, s, plac placed ed around around the enti entire re perimet perimeter. er. These barriers must be designed to resist the maximum vehicular impact load that could could be imposed. imposed. For maxim maximum um effectiv effectivenes eness, s, the barrier barriers-bol s-bollard lardss or plant plantersersmust be placed at the curb. 2. The public parking lot at the corner of the building must be secured to guarantee the prescri prescribed bed keepout keepout distance distance from the face of the structure. structure. Preferab Preferably ly,, the parking lot should be eliminated. 3. Street Street parkin parkingg shoul should d not be permit permitted ted on the near near side side of the street street,, adjaadjacent to the building. 4. An additi additiona onall measu measure re to reduce reduce the chances chances of an attack attack woul would d be to prevent vent parking parking on the opposite opposite side of the street. street. While While this does not impro improve ve the keep out distance, it could eliminate the ”parked” bomb, thereby limiting bombings to Park and run.

3.15

Internal Internal Explosion Threat

The blast environment could be introduced into the interior of the structure in four vulnerable vulnerable locations: The entrance lobby, the basement mechanical rooms, the loading dock, and the primary mail mail rooms. Specific modifications modifications to the features of these vulnerable vulnerable spaces can prevent an internal explosion from causing extensive damage and injury inside the building. 1. Walls alls and slabs adjacen adjacentt to the lobby lobby,, loading loading dock, dock, and mail rooms must 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.


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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 external blast wave wave.. Once Once the initial initial blast blast wa wave ve has passed passed the reflected reflected surface surface of the build buildin ing, g, the the peak peak overp overpres ressu sure re decay decayss to zero. As the sides sides and the 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 desig design. n. In colla collaps psee limi limitt desig design n the target target is to prov provid idee enough enough ductili ductility ty to the 15


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. crucial. In the case of an explosion, significant significant translational translational movemen movementt 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 This kind kind of blast loading loading is defined defined as impuls impulsiv ivee loading loading.. If the positiv positivee phase phase is longer than the natural vibration period of the structure, the load can be assumed constant constant when the structure has maximum deformation. deformation. This maximum maximum deformation is a function of the blast loading and the structural rigidity. This kind of blast loading is defined as quasi-static quasi-static loading. Finally Finally, if the positive phase duration is similar to the natural vibration period of the structure, the behavior of the structure becomes becomes quite complic complicated ated.. This This case can be defined defined as dynamic dynamic loading loading.. Frame

Figure Figure 4.2: Enha Enhanced nced beam-tobeam-to-colu column mn connecti connection on details details for steelw steelwork ork and reinreinforced concrete buildings designed to resist gravity, wind loads and earthquake loads in the normal way wa y have have frequen frequently tly been found found to be deficien deficientt in two respects. respects. When When subjected 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. Dept. of Civil Civil Engg. Engg.

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very very high forces as the result result of an explosi explosion. on. These These forces will have have a horizon horizontal tal component arising from the walls of the building and a vertical component from the differential differential loading on the upper and lower lower surfaces of floors. Providing Providing additional 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 frames could could still still operate after after an instan instantane taneous ous explosion explosion.. Figure Figure 2.8 shows shows the side-plate side-plate connection detail 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 connection Figure Figure 2.8. These These enhancem enhancement entss are intended intended to reduce 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. ing. Such Such forces can lead to loss loss of load-carr load-carryin yingg capacity capacity of a section. section. In the case of an explosion, columns of a reinforced concrete structure are the most important members that should should be protected. protected. Two types types of wrapping can be b e applied to provide provide this. this. Wrapping rapping with steel steel belts or wrappin wrappingg with with carbon fiberreinfo fiberreinforced rced polymers 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 internal explosion explosion is greatest. Lightwei Lightweight ght 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. wave. The air-blast shock wave wave is the primary damage mechani mechanism sm in an explosion. explosion. The pressures pressures it exerts on buildi building ng surfaces surfaces may be several several orders of magnitude magnitude greater than the loads for which the building building is designed. 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 enve envelope lope of the building. building. The explosion explosion pushes on the exterior exterior wa walls lls at the lower 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. Dept. of Civil Civil Engg. Engg.

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slabs

Figure 4.3: Shock Front from Air Burst

Figure 4.4: Shock Front from Surface Burst

4.3

Compariso Comparison n of Blast Blast And And Seismic Seismic Loading Loading

Blast wave and seismic loading are two different type of extreme force that may cause structural structural failure. However, However, they share some common common similarities. similarities. SimilariSimilarities between seismic and blast loading includes the following: 1. Dynamic loads and dynamic structural response. 2. Involve inelastic structural response. Dept. Dept. of Civil Civil Engg. Engg.

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3. Design Design considera consideratio tions ns will will focus on life life safety safety as opposed opposed to preven preventing ting strucstructural damage. 4. Other considerations: Nonstructural damage and hazards. 5. Performance based design: life safety issues and progressive collapse. 6. Structural integrit integrity: y: includes includes ductility ductility, continuity continuity,, and redundancy; redundancy; balanced balanced design. The differences between these two types of loading include: 1. Blast loading loading is due to a propagating propagating pressure wave wave as opposed to ground shaking. shaking. 2. Blast Blast result resultss in direc directt press pressure ure loadin loadingg to struct structure ure;; press pressure ure is in all all direc direc-tions, whereas a Seismic event is dominated by lateral load effects. 3. Blas Blastt load loadin ingg is of high higher er ampl amplit itude ude and and very short short durati duration on compa compared red with a seismic event. 4. Magni Magnitu tude de of blas blastt loadi loading ng is diffic difficul ultt to predi predict ct and and not based based on geogra geograph ph-ical location. 5. Blast Blast effects are confined confined to structur structures es in the immediate immediate vicinit vicinity y of event 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 Damage Evaluati Evaluation on Procedure For For Building Building Subjected Subjected To Blast 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 summar summary y, wh whil ilee the the effect effect of blas blastt load loadin ingg is local localiz ized ed compar compared ed with with an earthquake, the ability to sustain local damage without total collapse (structural integrity integrity)) is a key similarity similarity betw b etween een seismic-resis seismic-resistant tant and blast-resista blast-resistant nt design. In this study, the evaluation data that had been listed in inspection form is adapted and modified from inspection form for building building after an earthquake. 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. Dept. of Civil Civil Engg. Engg.

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Chapter 5 Case Study

5.1

World Trade 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 equipment failed failed to operate. In order to separate the fact from the fiction, I have have attempted to quantify quantify various various details of the collapse. The major events include the following: The airp The airpla lane ne impac impactt with with damag damagee to the the colu column mns. s. Th Thee ensui ensuing ng fire with loss of steel strength and distortion (figure 5.3) The collapse, collapse, which which generally generally occurred inward inward without significant significant tipping.(figure 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 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 Th is produce producess a heig height ht-t -too-wi width dth ratio ratio of 6.8. 6.8. Th Thee total total weig weight ht of the structur structuree 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 section on 100 cm centers(fi centers(figure gure 3). This This permitted permitted windows windows more more than than oneone-ha half lf meter meter wide. wide. Insi Inside de this this oute outerr tube tube ther theree was a 27 m 40 m core, core, which which was designed designed to support the weigh weightt of the towe tower. r. 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. i.e. 95 percent percent air. air. The egg-crat egg-cratee constructi construction on made a redundan redundantt structure structure (i.e., (i.e., if one or two columns were lost, the loads would shift into adjacent columns and 20


Figure 5.1: A cutaway view of WTC structure

the building would remain standing). standing). The WTC was primarily primarily a lightw lightweight eight 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 Detai Details ls of The Impac Impactt

5.1.2. 5.1.2.1 1

The Airpl Airplane ane Impac Impactt

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. surprising. Furthermore, urthermore, since there was no significant significant wind on September September 11, the outer perimeter columns were only stressed before the impact to around 1/3 of their 200 MPa design allowable. The only only indi indivi vidu dual al metal metal compo componen nentt of the the aircr aircraf aftt that that is compar comparabl ablee in strength to the box perimeter columns of the WTC is the keel beam at the bottom of the aircraf aircraftt fuselag fuselage. e. While While the aircraft aircraft impact undoubtedl undoubtedly y destroy destroyed ed several 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 aircraft aircraftss weigh weight, t, ignited. ignited. The ensuing ensuing fire wa wass clearly clearly the principal principal cause of the collapse (see figure 5.2)


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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 media report (and many scientist scientistss believe believe)) that the steel steel melted. melted. It is argued that the jet fuel fuel burns burns very very hot, especially especially with so much much fuel present. present. This This is not true. Part of the problem is that people often confuse temperature and heat. While they are related, related, they are not the same. same. Thermody Thermodynami namicall cally y, the heat contai contained ned 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 definitely not capable of melting steel. In combustion combustion science, there are three basic types of flames, namely, namely, a jet burner, a pre-mix pre-mixed ed flame, and a diffuse diffuse flame. A jet burner generally generally involv involves es mixing mixing the


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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 constan constant-v t-volu olume me cham chamber. ber. Since Since the combusti combustion on products products cannot expand 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 pre-mixed flame, the same nearly stoichiometric stoichiometric mixture 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 values within the flammable flammable range. A fireplace flame is a diffuse flame burning in air, as was the WTC fire. Diffuse Diffuse flames generate generate the lowe lowest st heat intens intensiti ities es of the three flame types. If the fuel and the oxidant start at ambient temperature, a maximum flame temperatu temperature re can be defined. defined. For carbon burning burning in pure oxygen, oxygen, the maxim maximum um is


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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. water. If pure oxygen oxygen is used, this heat only only needs needs to heat two two molecul molecules es (carbon monoxide and water), while with air, these two molecules must be heated plus four molecule moleculess of nitroge nitrogen. n. Th Thus, us, burning burning hydrocar hydrocarbons bons in air produces only onethird the temperature increase as burning in pure oxygen because three times as many many molecul molecules es must be heated heated when air is used. The maximu maximum m flame flame temperatemperature increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000Chardly sufficient to melt steel at 1,500C. 5.1. 5.1.2. 2.2 2

The The Coll Collap apse se

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 primary structur structural al member, member, such such as a column column.. Howe Howeve ver, r, when multipl multiplee members members Dept. Dept. of Civil Civil Engg. Engg.

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fail, the shifting loads eventually overstress the adjacent members and the collapse occurs like a row of dominoes falling down. The perimeter perimeter tube design of the WTC wa wass highly highly redundan redundant. t. It survived 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 able to support support approx approxima imatel tely y 1,300 1,300 t beyond beyond its own weight. weight. The total weight 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 estimated speed of 200 km per hour. If it had been free fall, fall, with no restrain restraint, t, the collapse would have only taken eight seconds and would have impacted at 300 km/h.

5.1.3

Can Buildin Building g Resist Direc Directt Airplane Airplane Hits Hits

If the design design terrorist terrorist attack attack is simila similarr to that of Sept. 11, can buildings buildings be given 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, 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 physically and economically economically impossible. impossible. So why did the World Trade Center Towers not collapse immediately due to the impact impact load on the system? system? The planes planes did not stop in a few feet, 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. center. This This means means that there there wa wass only only 26 inches inches clear clear between between each column. 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. Dept. of Civil Civil Engg. Engg.

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form a Vierendeel bridge over the hole created in one side of each of the towers. Both of these these facts that the plane plane was not stopped stopped at the exterio exteriorr and that the columns and spandrels spandrels were extremely dense were necessary necessary to prevent 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 We Minimize Minimize The Chance Chance of Progressiv Progressive e Collapse Collapse

This This is still still one more more questio question n that some people people are asking. asking. Because Because the towers 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 progress progressiv ive? e? Somethi Something ng breaks, breaks, and then something something else breaks, breaks, and so on. NorNormally, 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 orld Trade Center Center have have good resistan resistance ce to progress progressiv ivee collapse 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 wa ys to prevent progressive progressive collapse 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 chief is talking talking about losing losing a who whole le part of a buildi building. ng. As the event 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-


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pletely and immediately immediately disabled or destroyed destroyed upon impact: fireproofing, automatic sprinklers, sprinklers, compartmental compartmentalization ization and pressurization, pressurization, lighting, structure structure and exit stairs.

5.2

Israel Israel as a Case Case Study Study And And Parad Paradigm igm

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 When the state state wa wass found founded ed in 1948, 1948, Israe Israell had alrea already dy const construc ructed ted und undererground 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 long periods periods of time. time. In additi addition on , wh when en the shel shelter terss were were not in use they they coul could d be used for recreational purposes (Einstein, 2003 ). Many shelters were turned into librari libraries es and meeting meeting places places for youth groups groups (see Figures Figures 2.10 2.10 and 2.11). These These underground shelters became a part of Israeli culture.

Figure 5.5: Entrance to an underground shelter in Israel

In the 1970s 1970s civi civili lians ans in Israe Israell were being being threa threaten tened ed along along its its border border with with Lebanon. Lebanon. Katusha Katusha rockets rockets were were being launched launched over over the Lebanese Lebanese border into the


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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 tection from the attacks. attacks. Througho Throughout ut northern northern Israel rooms designed designed to protect 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 with the Gulf wa war. r. Sadd Saddam am Hussei Hussein n threaten threatened ed Israel Israel with with Scud missile missiless 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 withi within n ever every y home, home, office, office, and publi publicc space. space. Th Thee wind window owss had had to be able 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. Dept. of Civil Civil Engg. Engg.

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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 ments (Einste (Einstein, in, 2003). 2003). These These regulat regulation ionss were were fully fully integrate integrated d into into the Israeli 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. Dept. of Civil Civil Engg. Engg.

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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 protecti protecting ng its citizen citizenss from explosio explosions. ns. Israel Israel has success successful fully ly integr integrated ated blast protection into its society and buildings as a result of years of terror and threats. By making blast protection a permanent permanent part of the building building code professionals professionals have 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. Dept. of Civil Civil Engg. Engg.

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Chapter 6 Design Principles for Protection of Structures

6.1

General

Designing a building to withstand blasts includes more than just hardening the struct structure ure.. A lot lot of thought thought must must go into into the design design to take take into into accoun accountt more 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 standing in the case of localized damage. While discussing discussing the principles principles of blast design I will focus on the protection of structures in the event of a close range range bomb, bomb, most most simila similarr to presen presentt terroris terroristt activit activities ies.. This This include includess explos explosions ions due to suicide bombers near or inside a building, truck and car bombs near or driven inside a building, and package bombs.

6.2

Prev Preventat entativ ive e Measur Measures es

The first step in making a building blast resistant is to try to prevent a terrorist attac attack k from from occurri occurring ng in the first place. place. Th This is can can be accom accompl plis ishe hed d by making making a terroris terrorists ts attack attack from from occurring occurring in the first place. This This can be accompl accomplish ished ed 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). Preventin Preventingg access into the building is the first way to deter a terrorist. Heavy Heavy security security as well as physical physical barriers can make entering a building difficult. Also, if space allows, spreading out a complex makes an effective terrorist attack more difficult ficult to execute. execute. A bomb bomb in one location location will have have less less overal overalll effect effect on a buildi building ng if all of the building buildingss assets assets are spread spread out (Mays (Mays and smith, smith, 1995). This This strategy 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 conseque consequences nces of an attack attack can be much much less severe severe (Mays (Mays and Smith, Smith, 1995). It i important important to prevent prevent the placement placement of explosives explosives near sensitive sensitive structural members. Ways of accomplishing this include hiding columns and other important structural 31


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 dista distance nce between between the build buildin ingg and and the the poten potenti tial al blas blast. t. The strengt strength h of a blas blastt 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 pressure dies off substan substantia tially lly.. Smith Smith and Hetheri Hetheringto ngton n illust illustrate rate this this by sayin sayingg that keeping vehicle bombs away from your structure is probably the sing, most cost effective device you can employ.

6.3 6.3

Hard Harden enin ing g o off the the str struc uctu ture re

The next principle in structural blast design is to harden the structure in the case that a blast does take place. place. The main way way to harden harden a structur structuree is to design 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 resistant structure. structure. Other structural structural concerns include how how the floors flo ors 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 very severe severe and are usually usually very frequen frequent. t. There There are several several techtechniques 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 explosion people can move move away away from the exterior exterior of a buildi building. ng. Also, Also, Dept. Dept. of Civil Civil Engg. Engg.

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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 building occupants. 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 structure due to redundan redundancy cy of the protectiv protectivee systems systems.. The first hardeni hardening ng 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 outside of the building. The second layer layer is the envelo envelope pe of the externa externall structur structural al system. 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. bomb. In addition, addition, it should should protect protect main main structur structural al elements elements from close close range 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 layer is the laye layerr that protects protects the internal internal structura structurall system system.. This This layer needs to protect the building from all of the things that the second layer is designed designed to protect. protect. In addition, addition, this layer layer must be able to protect protect the structur structuree from explosions donated inside of the structure.

6.4 6.4

Hard Harden enin ing g o off the the str struc uctu ture re

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 case of seve severe re struct structur ural al dama damage. ge. As we hav have seen seen in event eventss such 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. Dept. of Civil Civil Engg. Engg.

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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 techni technique que for protecti protecting ng columns columns inside inside a structur structuree includ include: e: using using composite material shields around the column with an air gap between the shield and the column. column. Columns Columns can be designed designed to be part of heavy heavy walls walls so that they will will not experience experience local failure. failure. The strength strength of the columns columns is impro improve ved d 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.


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Chapter 7 Conclusion

7.1

General

The aim in blast resistant building design is to prevent the overall collapse of the buildi building ng and fatal damages. damages. Despit Despitee the fact that, that, the magnitude magnitude of the explosion 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. danger. Most importan importantly tly human human safety safety should should be b e provid provided. ed. Moreov Moreover, er, to achieve achieve functional continuity continuity after an explosion, explosion, architectural and structural 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 common sense about the explosio explosions ns that they are possible possible threats threats in daily 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 structural form, structural structural elemen elements ts e.g. wa walls lls,, flooring flooring and secondary secondary structural structural elements like cladding and glazing should be b e considered considered carefully. 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. extreme. Therefor Therefore, e, it is obvio obvious us that a buildi building ng will receive receive less less damage damage with a selected safety safety level and a blast resistant resistant architectural architectural design. 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 Every single single member member should be designed designed to bear the possible possible blast loading loading.. For 35


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. unquestionable.


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References

[1] Koccaz Z. (2004) Blast Resistant Resistant Building Design, MSc Thesis, Istanbul Istanbul TechTechnical University, Istanbul, Turkey. [2] Smith P.D., P.D., Hetherington Hetherington J.G. (1994) Blast and ballistic loading loading of structures. structures. Butterworth Heinemann. [3] Yandzio E., Gough M. (1999). Protection of Buildings Buildings Against Explosions, Explosions, SCI Publication, Publication, Berkshire, Berkshire, U.K. [4] Website : www.iitk.ac.in/nicee/wcee/article/14-05-01-0536.PDF [5] Civil engineering engineering articles articles at google.com

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Architectural And Structural Design Of Blast Resistant Buildings

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