Architectural And Structural Design Of Blast Resistant Buildings

BLAST RESISTANT BUILDINGS

A seminar report submitted in partial fulfillment of the requirementsfor 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

Department of Civil EngineeringSreepathy Institute of Management & Technology

Vavanoor, Palakkad-679533

CERTIFICATE

This is to certify that the seminar entitled ”BLAST RESISTANT BUILD-INGS” is a bonafide record of the seminar presented by PAUL JOMY (Reg No.SYAKECE033) under our supervision and guidance. The seminar report has beensubmitted to the Department of Civil Engineering of SIMAT Vavanoor, Palakkad-679533 in partial fulfillment of the award of the Degree of Bachelor of Technologyin Civil Engineering, during the year 2013-2014.

Mr.ARUN ASHOKGuideAsst. ProfessorCivil EnggSIMAT, Vavanoor

Mr.SUDHEER.K.VHead of the DeptCivil EnggSIMAT, VavanoorPalakkad

ABSTRACT

The increase in the number of terrorist attacks especially in the last few years hasshown that the effect of blast loads on buildings is a serious matter that should betaken into consideration in the design process. Although these kinds of attacks areexceptional cases, man-made disasters; blast loads are in fact dynamic loads thatneed to be carefully calculated just like earthquake and wind loads.

The objective of this study is to shed light on blast resistant building design the-ories, the enhancement of building security against the effects of explosives in botharchitectural and structural design process and the design techniques that should becarried out. Firstly, explosives and explosion types have been explained briefly. Inaddition, the general aspects of explosion process have been presented to clarify theeffects of explosives on buildings. To have a better understanding of explosives andcharacteristics of explosions will enable us to make blast resistant building designmuch more efficiently. Essential techniques for increasing the capacity of a buildingto provide protection against explosive effects is discussed both with an architecturaland structural approach.

ACKNOWLEDGEMENT

I am extremely thankful to our Principal Dr.S.P. SUBRAMANIAN for giving hisconsent for this seminar. And also i’m thankful to Mr.SUDHEER.K.V, Head ofthe Department of Civil engineering, for his valuable suggestions and support. Thevaluable help and encouragement rended in this endeavour by my guide Mr.ARUNASHOK, Asst.Professors, Dept.of Civil Engineering for his constant help and sup-port throughout the presentation of the seminar by providing timely advices andguidance. 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 theyhave given me during the course of my work.

PAUL JOMY (SYAKECE033)

Eight Semester 2010 AdmissionDept. of Civil Engg.

SIMAT, Vavannoor, Palakkad

Contents

List of Figures iii

1 Introduction 11.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Objective Of The Blast Design . . . . . . . . . . . . . . . . . . . . . . 2

2 Literature survey 32.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Explosion - Major of All Terrorist Activities . . . . . . . . . . . . . . 3

2.2.1 Expected Terrorist Blast On Structures . . . . . . . . . . . . . 32.2.2 Major Cause of Life Loss After The Blast . . . . . . . . . . . 3

2.3 Goals of Blast Resistant Design . . . . . . . . . . . . . . . . . . . . . 42.4 Basic Requirements To Resist Blast Loads . . . . . . . . . . . . . . . 4

2.4.1 Mechanics of a Conventional Explosion . . . . . . . . . . . . . 42.5 Types of Explosions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.5.1 Unconfined Explosion . . . . . . . . . . . . . . . . . . . . . . . 52.5.2 Confined Explosions . . . . . . . . . . . . . . . . . . . . . . . 6

2.6 Explosion Process For High Explosive . . . . . . . . . . . . . . . . . . 6

3 Architectural Aspect of Blast Resistant Building Design 83.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2 Planning And Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Structural Form and Internal Layout . . . . . . . . . . . . . . . . . . 83.4 Bomb Shelter Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.5 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.6 Glazing And Cladding . . . . . . . . . . . . . . . . . . . . . . . . . . 103.7 Floor Slabs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.8 Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.9 Transfer Girders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.10 External Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.11 Facade And Atrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.12 Overall Lateral Building Resistance, Shear Walls . . . . . . . . . . . . 133.13 Lower Floor Exterior . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.14 Stand Off Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.15 Internal Explosion Threat . . . . . . . . . . . . . . . . . . . . . . . . 14

i

4 Structural Aspect of Blast Resistant Building 154.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2 Structural Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.3 Comparison of Blast And Seismic Loading . . . . . . . . . . . . . . . 184.4 Damage Evaluation Procedure For Building Subjected To Blast Impact 19

5 Case Study 205.1 World Trade Center Collapse . . . . . . . . . . . . . . . . . . . . . . 20

5.1.1 The Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.1.2 The Details of The Impact . . . . . . . . . . . . . . . . . . . . 21

5.1.2.1 The Airplane Impact . . . . . . . . . . . . . . . . . . 215.1.2.2 The Collapse . . . . . . . . . . . . . . . . . . . . . . 24

5.1.3 Can Building Resist Direct Airplane Hits . . . . . . . . . . . . 255.1.4 How Can We Minimize The Chance of Progressive Collapse . 26

5.2 Israel as a Case Study And Paradigm . . . . . . . . . . . . . . . . . . 27

6 Design Principles for Protection of Structures 316.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316.2 Preventative Measures . . . . . . . . . . . . . . . . . . . . . . . . . . 316.3 Hardening of the structure . . . . . . . . . . . . . . . . . . . . . . . . 326.4 Hardening of the structure . . . . . . . . . . . . . . . . . . . . . . . . 33

7 Conclusion 357.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

References 37

ii

List of Figures

2.1 Air burst with ground reflections . . . . . . . . . . . . . . . . . . . . 52.2 Surface burst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Fully vented, partially vented and fully confined explosions . . . . . . 62.4 Blast wave pressures plotted against time . . . . . . . . . . . . . . . . 7

3.1 Schematic layout of site for protection against bombs . . . . . . . . . 93.2 Internal planning of a building . . . . . . . . . . . . . . . . . . . . . . 9

4.1 Sequence of air-blast effects . . . . . . . . . . . . . . . . . . . . . . . 154.2 Enhanced beam-to-column connection details for steelwork and rein-

forced concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3 Shock Front from Air Burst . . . . . . . . . . . . . . . . . . . . . . . 184.4 Shock Front from Surface Burst . . . . . . . . . . . . . . . . . . . . . 18

5.1 A cutaway view of WTC structure . . . . . . . . . . . . . . . . . . . 215.2 A graphic illustration of WTC . . . . . . . . . . . . . . . . . . . . . . 225.3 Airplane’s impact on WTC . . . . . . . . . . . . . . . . . . . . . . . . 235.4 Collapse of WTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.5 Entrance to an underground shelter in Israel . . . . . . . . . . . . . . 275.6 Shelter used as a playroom . . . . . . . . . . . . . . . . . . . . . . . . 285.7 Shelter used as a playroom . . . . . . . . . . . . . . . . . . . . . . . . 285.8 The change from underground shelters to protected spaces . . . . . . 295.9 Example of Israeli structural blast desing . . . . . . . . . . . . . . . . 295.10 Example of Israeli structural blast desing . . . . . . . . . . . . . . . . 305.11 Example of traditional American structual blast desing . . . . . . . . 30

iii

Chapter 1

Introduction

1.1 General

The increase in the number of terrorist attacks especially in the last few years hasshown that the effect of blast loads on buildings is a serious matter that should betaken into consideration in the design process. Although these kinds of attacks areexceptional cases, man-made disasters; blast loads are in fact dynamic loads thatneed to be carefully calculated just like earthquake and wind loads.

The objective of this study is to shed light on blast resistant building design the-ories, the enhancement of building security against the effects of explosives in botharchitectural and structural design process and the design techniques that should becarried out. Firstly, explosives and explosion types have been explained briefly. Inaddition, the general aspects of explosion process have been presented to clarify theeffects of explosives on buildings. To have a better understanding of explosives andcharacteristics of explosions will enable us to make blast resistant building designmuch more efficiently. Essential techniques for increasing the capacity of a buildingto provide protection against explosive effects is discussed both with an architecturaland structural approach.

Damage to the assets, loss of life and social panic are factors that have to beminimized if the threat of terrorist action cannot be stopped. Designing the struc-tures to be fully blast resistant is not an realistic and economical option, howevercurrent engineering and architectural knowledge can enhance the new and existingbuildings to mitigate the effects of an explosion.

1


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 buildingthat is no less than that for persons outside the buildings in the event of an explo-sion. Evidence from past incidents has shown that many of the fatalities and seriousinjuries were due to collapse of buildings onto the persons inside the building. Thisobjective is to reduce the probability that the building itself becomes a hazard inan explosion.

Preventing cascading events due to loss of control of process units not involvedin the event is another objective of blast resistant design. An incident in one unitshould not affect the continued safe operation or orderly shutdown of other units.

Preventing or minimizing financial losses is another objective of blast resistantdesign. Buildings containing business information, critical or essential equipment,expensive and long lead time equipment, or equipment which if destroyed, wouldconstitute significant interruption or financial loss to the owner should be protected.

Dept. of Civil Engg. 2 SIMAT, Vavanoor

Chapter 2

Literature survey

2.1 General

The need and requirements for blast resistance in buildings have evolved over re-cent years. Buildings have become more complex and have increased in size thusincreasing the risk of accidental explosions. Such explosions have demolished thebuildings, in some cases resulting in substantial personnel causalities and businesslosses. Such events have heightened the concerns of the industry, plant management,and regulatory agencies about the issues of blast protection in buildings have thepotential for explosions. Generally, these issues relate to plant building safety andrisk management to prevent or minimize the occurrence of such incidents and tositing, design, and operations.

2.2 Explosion - Major of All Terrorist Activities

The probability that any single building will sustain damage from accidental ordeliberate explosion is very low, but thecost for those who are unprepared is veryhigh.

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


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 causedby terrorist bombings. The threat for a conventional bomb is defined by two equallyimportant elements, the bomb size, or charge weight, and the standoff distance - theminimum guaranteed distance between the blast source and the target.

- Another requirement is to keep the bomb as far away as possible, by maximiz-ing the keepout distance. No matter what size the bomb, the damage will be lesssevere the further the target is from the source.

- Structural hardening should actually be the last resort in protecting a struc-ture; detection and prevention must remain the first line of defense . As terroristattacks range from the small letter bomb to the gigantic truck bomb as experiencedin Oklahoma City, the mechanics of a conventional explosion and their effects on atarget 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 blastpressures resulting from this hemispherical explosion decay as a function of the dis-tance from the source as the ever-expanding shock front dissipates with range. Theincident peak pressures are amplified by a reflection factor as the shock wave encoun-ters an object or structure in its path. Except for specific focusing of high intensityshock waves at near 45 incidence, these reflection factors are typically greatest fornormal incidence (a surface adjacent and perpendicular to the source) and diminishwith the angle of obliquity or angular position relative to the source. Reflection fac-tors depend on the intensity of the shock wave, and for large explosives at normalincidence these reflection factors may enhance the incident pressures by as much asan 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 situatedfurther away produce a lower-intensity, longer-duration uniform pressure distribu-tion over the entire structure. In short by purely geometrical relations, the larger



the standoff, the more uniform the pressure distribution over the target. Eventually,the entire structure is engulfed in the shock wave, with reflection and diffraction ef-fects 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, theweakened structure may be subjected to impact by debris that may cause additionaldamage

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 andintermediate amplification of the wave caused by ground reflections occurs prior tothe arrival of the initial blast wave at a building Figure 2.1.

As the shock wave continues to propagate outwards along the ground surface, afront commonly called a Mach stem is formed by the interaction of the initial waveand the reflected wave.

However a surface burst explosion occurs when the detonation occurs close toor on the ground surface. The initial shock wave is reflected and amplified by theground surface to produce a reflected wave. Figure 2.2. Unlike the air burst, thereflected wave merges with the incident wave at the point of detonation and formsa single wave. In the majority of cases, terrorist activity occurres in built-up areasof cities, where devices are placed on or very near the ground surface.



Figure 2.2: Surface burst

2.5.2 Confined Explosions

When an explosion occurs within a building, the pressures associated with the initialshock front will be high and therefore will be amplified by their reflections withinthe building.

Figure 2.3: Fully vented, partially vented and fully confined explosions

This type of explosion is called a confined explosion. In addition and dependingon the degree of confinement, the effects of the high temperatures and accumulationof gaseous products produced by the chemical reaction involved in the explosion willcause additional pressures and increase the load duration within the structure.

Depending on the extent of venting, various types of confined explosions arepossible. Figure2.3

2.6 Explosion Process For High Explosive

An explosion occurs when a gas, liquid or solid material goes through a rapid chem-ical reaction. When the explosion occurs, gas products of the reaction are formedat a very high temperature and pressure at the source. These high pressure gassesexpand rapidly into the surrounding area and a blast wave is formed. Because thegases are moving, they cause the surrounding air move as well. The damage causedby explosions is produced by the passage of compressed air in the blast wave. Blastwaves propagate at supersonic speeds and reflected as they meet objects. As theblast wave continues to expand away from the source of the explosion its intensity



diminishes and its effect on the objects is also reduced. However, within tunnels orenclosed passages, the blast wave will travel with very little diminution.

Close to the source of explosion the blast wave is formed and violently hot andexpanding 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 conve-nient to separate out the different types of loading experienced by the surroundingobjects. Three effects have been identified in three categories. The effect rapidlycompressing the surrounding air is called air shock wave. The air pressure and airmovement effect due to the accumulation of gases from the explosion chemical re-actions is called dynamic pressure and the effect rapidly compressing the ground iscalled ground shock wave.

Figure 2.4: Blast wave pressures plotted against time

The air shock wave produces an instantaneous increase in pressure above theambient atmospheric pressure at a point some distance from the source. This iscommonly referred to as overpressure. As a consequence, a pressure differential isgenerated between the combustion gases and the atmosphere, causing a reversal inthe direction of flow, back towards the center of the explosion, known as a negativepressure phase. This is a negative pressure relative to atmospheric , rather than ab-solute negative pressure Figure 2.4. Equilibrium is reached when the air is returnedto its original state.

As a rough approximation, 1kg of explosive produces about 1m3 of gas. As thisgas expands, its act on the air surrounding the source of the explosion causes it tomove and increase in pressure. The movement of the displaced air may affect nearbyobjects and cause damage. Except for a confinement case, the effects of the dynamicpressure diminish rapidly with distance from source.

The ground shock leaving the site of an explosion consists of three principalcomponents . A compression wave which travels radially from the source; a shearwave which travels radially and comprises particle movements in a plane normal tothe radial direction where the ground shock wave intersects with the surface and asurface or Raleigh wave. These waves propagate at different velocities and alternateat different frequencies.


Chapter 3

Architectural Aspect of Blast Resistant Building

Design

3.1 General

The target of blast resistant building design philosophy is minimizing the conse-quences to the structure and its inhabitants in the event of an explosion. A primaryrequirement is the prevention of catastrophic failure of the entire structure or largeportions of it. It is also necessary to minimize the effects of blast waves transmittedinto the building through openings and to minimize the effects of projectiles on theinhabitants of a building. However, in some cases blast resistant building designmethods, conflicts with aesthetical concerns, accessibility variations, fire fightingregulations and the construction budget restrictions.

3.2 Planning And Layout

Much can be done at the planning stage of a new building to reduce potentialthreats and the associated risks of injury and damage. The risk of a terrorist attack,necessity of blast protection for structural and non-structural members, adequateplacing of shelter areas within a building should be considered for instance. Inrelation to an external threat, the priority should be to create as much stand-offdistance between an external bomb and the building as possible. On congestedcity centers there may be little or no scope for repositioning the building, but whatsmall stand-off there is should be secured where possible. This can be achieved bystrategic location of obstructions such as bollards, trees and street furniture. Figure4.1 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 effectson the building compared with a cubicle form. The plan-shape of a building alsohas a significant influence on the magnitude of the blast load it is likely to experi-ence. Complex shapes that cause multiple reflections of the blast wave should bediscouraged. 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 moreblast 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 struc-tures take the advantage of the shock absorbing property of the soil covered by. Thesoil provides protection in case of a nuclear explosion as well.

The internal layout of the building is another parameter that should be under-taken with the aim of isolating the value from the threat and should be arrangedso that the highest exterior threat is separated by the greatest distance from the



highest value asset. Foyer areas should be protected with reinforced concrete walls;double-dooring should be used and the doors should be arranged eccentrically withina corridor to prevent the blast pressure entering the internals of the building. En-trance to the building should be controlled and be separated from other parts ofthe building by robust construction for greater physical protection. An underpassbeneath or car parking below or within the building should be avoided unless accessto it can be effectively controlled.

A possible fire that occurs within a structure after an explosion may increase thedamage catastrophically. Therefore the internal members of the building should bedesigned to resist the fire.

3.4 Bomb Shelter Areas

The bomb shelter areas are specially designated within the building where vulner-ability from the effects of the explosion is at a minimum and where personnel canretire in the event of a bomb threat warning. These areas must afford reasonableprotection against explosions; ideally be large enough to accommodate the person-nel involved and be located so as to facilitate continual access. For modern-framedbuildings, shelter areas should be located away from windows, external doors, ex-ternal walls and the top floors if the roof is weak. Areas surrounded by full-heightconcrete walls should be selected and underground car parks, gas storage tanks,areas light weight partition walls, e.g. internal corridors, toilet areas, or conferenceshould be avoided while locating the shelter areas. Basements can sometimes beuseful shelter areas, but it is important to ensure that the building does not collapseon top of them. The functional aspects of a bomb shelter area should accommodateall the occupants of the building; provide adequate communication with outside;provide sufficient ventilation and sanitation; limit the blast pressure to less than theear drum rupture pressure and provide alternative means of escape.

3.5 Installation

Gas, water, steam installations, electrical connections, elevators and water storagesystems should be planned to resist any explosion affects. Installation connectionsare critical points to be considered and should be avoided to use in high-risk de-formation areas. Areas with high damage receiving potential e.g. external walls,ceilings, roof slabs, car parking spaces and lobbies also should be avoided to locatethe electrical and other installations. The main control units and installation feed-ing points should be protected from direct attacks. A reserve installation systemshould be provided for a potential explosion and should be located remote from themain installation system.

3.6 Glazing And Cladding

Glass from broken and shattered windows could be responsible for a large numberof injuries caused by an explosion in a city centre. The choice of a safer glazing ma-



terial is critical and it has been found out that laminated glass is the most effectivein this context. On the other hand, applying transparent polyester anti-shatter filmto the inner surface of the glazing is as well an effective method.

For the cladding, several aspects of design should be considered to minimize thevulnerability of people within the building and damage to the building itself. Theamount of glazing in the facade should be minimized. This will limit the amountof internal damage from the glazing and the amount of blast that can enter. Itshould also be ensured that the cladding is fixed to the structure securely witheasily accessible fixings. This will allow rapid inspection after an explosion so thatany 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 andlower floors, which are the most susceptible to blast loads.

2. In exterior bays/lower floors, drop panels and column capitols are required toshorten the effective slab length and improve the punching shear resistance.

3. If vertical clearance is a problem, shear heads embedded in the slab will im-prove the shear resistance and improve the ability of the slab to transfer momentsto the columns.

4. The slab-column interface should contain closed-hoop stirrup reinforcement prop-erly anchored around flexural bars within a prescribed distance from the column face.

5. Bottom reinforcement must be provided continuous through the column. Thisreinforcement serves to prevent brittle failure at the connection and provides analternate mechanism for developing shear transfer once the concrete has punchedthrough.

6. The development of membrane action in the slab, once the concrete has failed atthe column interface, provides a safety net for the postdamaged structure. Contin-uously tied reinforcement, spanning both directions, must be detailed properly toensure that the tensile forces can be developed at the lapped splices. Anchorage ofthe reinforcement at the edge of the slab is required to guarantee the developmentof the tensile forces.

3.8 Columns

Treatment for conventionally designed columns to improve blast resisting mecha-nism:

1. The potential for direct lateral loading on the face of the columns, resulting



from the blast pressure and impact of explosive debris, requires that the lower-floorcolumns be designed with adequate ductility and strength.

2. The perimeter columns supporting the lower floors must also be designed toresist 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 al-ternative, which provides similar benefits, is to embed a steel column within theperimeter concrete columns or wall section.

4. The possibility of uplift must be considered, and, if deemed likely, the columnsmust be reinforced to withstand a transient tensile force.

5. For smaller charge weights, spiral reinforcement provides a measure of coreconfinement that greatly improves the capacity and the behavior of the reinforcedconcrete columns under extreme load.

3.9 Transfer Girders

The building relies on transfer girders at the top of the atrium to distribute theloads 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 ar-chitectural space for the entrance to the building.

Transfer girders typically concentrate the load-bearing system into a smallernumber of structural elements. This loadtransfer system runs contrary to the con-cept of redundancy desired in a blast environment. The column connections, whichsupport the transfer girders, are to provide sustained strength despite inelastic de-formations. The following recommendations must be met for transfer girders:

1. The transfer girder and the column connections must be properly designed anddetailed, using an adequate blast loading description.

2. A progressive-collapse analysis must be performed, particularly if the blast load-ing exceeds the capacity of the girder.

3.10 External Treatments

The two parameters that most directly influence the blast environment that thestructure 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 standoffdistance.



3.11 Facade And Atrium

The facade is comprised of the glazing and the exterior wall. Better glazing hasalready been discussed above and wall obviously should be hardened to resist theloading. Presence of an atrium along the face of the structure will require twoprotective measures. On the outside of the structure, the glass and glass framingmust be strengthened to withstand the loads. On the inside, the balcony parapets,spandrel beams, and exposed slabs must be strengthened to withstand the loadsthat 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 theductility of the load-resisting system.This means that the structure has to be ableto deform in elastically under extreme overload, thereby dissipating large amountsof energy, prior to failure.. In addition to providing ductile behavior for the struc-ture, the following provisions would improve the blast protection capability of thebuilding:

1. Use a well-distributed lateral-load resisting mechanism in the horizontal floorplan. This can be accomplished by using several shear walls around the plan of thebuilding this will improve the overall seismic as well as the blast behavior of thebuilding.

2. If adding more shear walls is not architecturally feasible, a combined lateral-load resisting mechanism can also be used. A central shear wall and a perimetermoment-resisting frame will provide for a balanced solution. The perimeter momen-tresisting frame will require strengthening the spandrel beams and the connectionsto the outside columns. This will also result in better protection of the outsidecolumns.

Several recommendations were presented for each of the identified features. Theimplementation of these recommendations will greatly improve the blast-resistingcapability of the building under consideration.

3.13 Lower Floor Exterior

The architectural design of the building of interest currently calls for window glassaround the first floor. Unless this area is constructed in reinforced concrete, thedamage to the lower floor structural elements and their connections will be quitesevere. 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.) thickwall with 0.3 percent steel doubly reinforced in both directions might be required.



2. For intermediate charge weight protection, a 460 mm (18 in.) thick wall with 0.5percent 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 dis-tance, the more the blast forces will dissipate resulting in reduced pressures on thebuilding. Several recommendations can be made to maintain and improve the stand-off 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 thatcould be imposed. For maximum effectiveness, the barriers-bollards or planters-must be placed at the curb.

2. The public parking lot at the corner of the building must be secured to guaran-tee the prescribed keepout distance from the face of the structure. Preferably, theparking lot should be eliminated.

3. Street parking should not be permitted on the near side of the street, adja-cent to the building.

4. An additional measure to reduce the chances of an attack would be to pre-vent parking on the opposite side of the street. While this does not improve thekeep out distance, it could eliminate the ”parked” bomb, thereby limiting bombingsto Park and run.

3.15 Internal Explosion Threat

The blast environment could be introduced into the interior of the structure in fourvulnerable locations:

The entrance lobby, the basement mechanical rooms, the loading dock, and theprimary mail rooms. Specific modifications to the features of these vulnerable spacescan prevent an internal explosion from causing extensive damage and injury insidethe building.

1. Walls and slabs adjacent to the lobby, loading dock, and mail rooms must behardened to protect against the hand delivered package bomb, nominally a 10-20kg explosive. This hardening can be achieved by redesigning the slabs and erectingcast-in-place reinforced-concrete walls, with the thickness and reinforcement deter-mined 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.


Chapter 4

Structural Aspect of Blast Resistant Building

4.1 General

The front face of a building experiences peak overpressures due to reflection of anexternal blast wave. Once the initial blast wave has passed the reflected surfaceof the building, the peak overpressure decays to zero. As the sides and the topfaces of the building are exposed to overpressures (which has no reflections and arelower than the reflected overpressures on the front face), a relieving effect of blastoverpressure is experienced on the front face. The rear of the structure experiencesno pressure until the blast wave has traveled the length of the structure and a com-pression wave has begun to move towards the centre of the rear face. Therefore thepressure built up is not instantaneous. On the other hand, there will be a time lagin 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 directionof 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. Sim-ilar to the static loaded case design, blast resistant dynamic design also uses thelimit state design techniques which are collapse limit design and functionality limitdesign. In collapse limit design the target is to provide enough ductility to the

15


building so that the explosion energy is distributed to the structure without overallcollapse. For collapse limit design the behavior of structural member connections iscrucial. In the case of an explosion, significant translational movement and momentoccur and the loads involved should be transferred from the beams to columns. Thestructure doesnt collapse after the explosion however it cannot function anymore.

Functionality limit design however, requires the building to continue functional-ity after a possible explosion occurred. Only non-structural members like windowsor cladding may need maintenance after an explosion so that they should be de-signed ductile enough.

When the positive phase of the shock wave is shorter than the natural vibrationperiod 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 islonger than the natural vibration period of the structure, the load can be assumedconstant when the structure has maximum deformation. This maximum deforma-tion is a function of the blast loading and the structural rigidity. This kind of blastloading is defined as quasi-static loading. Finally, if the positive phase duration issimilar to the natural vibration period of the structure, the behavior of the structurebecomes quite complicated. This case can be defined as dynamic loading. Frame

Figure 4.2: Enhanced beam-to-column connection details for steelwork and rein-forced concrete

buildings designed to resist gravity, wind loads and earthquake loads in the normalway have frequently been found to be deficient in two respects. When subjectedto blast loading; the failure of beam-to-column connections and the inability of thestructure to tolerate load reversal.Beam-to-column connections can be subjected to



very high forces as the result of an explosion. These forces will have a horizontalcomponent arising from the walls of the building and a vertical component from thedifferential loading on the upper and lower surfaces of floors. Providing additionalrobustness to these connections can be a significant enhancement.

In the connections, normal details for static loading have been found to be inad-equate for blast loading. Especially for the steelwork beam-to-column connections,it is essential for the connection to bear inelastic deformations so that the momentframes could still operate after an instantaneous explosion. Figure 2.8 shows theside-plate connection detail in question . The main features to note in the reinforcedconcrete connection are the use of extra links and the location of the starter bars inthe connection Figure 2.8. These enhancements are intended to reduce the risk ofcollapse or the connection be damaged, possibly as a result of a load reversal on thebeam.

It is vital that in critical areas, full moment-resisting connections are made inorder to ensure the load carrying capacity of structural members after an explosion.Beams acting primarily in bending may also carry significant axial load caused bythe blast loading.

On the contrary, columns are predominantly loaded with axial forces under nor-mal loading conditions, however under blast loading they may be subjected to bend-ing. Such forces can lead to loss of load-carrying capacity of a section. In the caseof an explosion, columns of a reinforced concrete structure are the most importantmembers that should be protected. Two types of wrapping can be applied to providethis. Wrapping with steel belts or wrapping with carbon fiberreinforced polymers(CFRP).

Cast-insitu reinforced concrete floor slabs are the preferred option for blast resis-tant buildings, but it may be necessary to consider the use of precast floors in somecircumstances. Precast floor units are not recommended for use at first floor wherethe risk from an internal explosion is greatest. Lightweight roofs and more partic-ularly, glass roofs should be avoided and a reinforced concrete or precast concreteslab is to be preferred.

4.2 Structural Failure

An explosion will create blast wave. The air-blast shock wave is the primary damagemechanism in an explosion. The pressures it exerts on building surfaces may beseveral 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 thatthe building may not have been designed for, such as upward force on the floorsystem. In terms of sequence of response, the air-blast first impinges on the weakestpoint in the vicinity of the device closest to the explosion, typically the exteriorenvelope of the building. The explosion pushes on the exterior walls at the lowerstories and may cause wall failure and window breakage. As the shock wave continuesto expand, it enters the structure, pushing both upward and downward on the floor



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 maycause structural failure. However, they share some common similarities. Similari-ties between seismic and blast loading includes the following:

1. Dynamic loads and dynamic structural response.

2. Involve inelastic structural response.



3. Design considerations will focus on life safety as opposed to preventing struc-tural 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; balanceddesign.

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 direc-tions, whereas a Seismic event is dominated by lateral load effects.

3. Blast loading is of higher amplitude and very short duration compared witha seismic event.

4. Magnitude of blast loading is difficult to predict and not based on geograph-ical location.

5. Blast effects are confined to structures in the immediate vicinity of event be-cause 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 pres-sure 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 anearthquake, the ability to sustain local damage without total collapse (structuralintegrity) is a key similarity between seismic-resistant and blast-resistant design. Inthis study, the evaluation data that had been listed in inspection form is adaptedand modified from inspection form for building after an earthquake. Even though,seismic loading will cause global response to building compared to blast loadingwhich will cause localized response, but similar damage assessment procedure couldbe used.


Chapter 5

Case Study

5.1 World Trade Center Collapse

The collapse of the World Trade Center (WTC) towers on September 11, 2001, wasas sudden as it was dramatic; the complete destruction of such massive buildingsshocked nearly everyone. Immediately afterward and even today, there is widespreadspeculation that the buildings were structurally deficient, that the steel columnsmelted, or that the fire suppression equipment failed to operate. In order to separatethe 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 ofsteel strength and distortion (figure 5.3)The collapse, which generally occurred inward without significant tipping.(figure5.4)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 eachtower 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 structurewas roughly 500,000 t. The building is a huge sail that must resist a 225 km/h hur-ricane. 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 architectsselected a lightweight perimeter tube design consisting of 244 exterior columns of 36cm square steel box section on 100 cm centers(figure 3). This permitted windowsmore than one-half meter wide. Inside this outer tube there was a 27 m 40 mcore, which was designed to support the weight of the tower. It also housed theelevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tallconnected the core to the perimeter at each story. Concrete slabs were poured overthese 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


Figure 5.1: A cutaway view of WTC structure

the building would remain standing). The WTC was primarily a lightweight steelstructure; however, its 244 perimeter columns made it one of the most redundantand 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 ofthe aircraft; however, when one recognizes that the buildings had more than 1,000times the mass of the aircraft and had been designed to resist steady wind loadsof 30 times the weight of the aircraft, this ability to withstand the initial impact ishardly surprising. Furthermore, since there was no significant wind on September11, the outer perimeter columns were only stressed before the impact to around 1/3of their 200 MPa design allowable.

The only individual metal component of the aircraft that is comparable instrength to the box perimeter columns of the WTC is the keel beam at the bottomof the aircraft fuselage. While the aircraft impact undoubtedly destroyed severalcolumns in the WTC perimeter wall, the number of columns lost on the initial im-pact was not large and the loads were shifted to remaining columns in this highlyredundant structure. Of equal or even greater significance during this initial impactwas the explosion when 90,000 Lgallons of jet fuel, comprising nearly 1/3 of theaircrafts weight, ignited. The ensuing fire was clearly the principal cause of thecollapse (see figure 5.2)



Figure 5.2: A graphic illustration, from the USA Today newspaper web site, of theWorld Trade Center points of impact. Click on the image above to access the actualUSA Today feature

The fire is the most misunderstood part of the WTC collapse.Even today, themedia report (and many scientists believe) that the steel melted. It is argued thatthe 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 theyare related, they are not the same. Thermodynamically, the heat contained in amaterial 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 withthe quantity of material, while the heat is an extensive property, which does varywith the amount of material. One way to distinguish the two is to note that if a sec-ond log is added to the fireplace, the temperature does not double; it stays roughlythe same, but the length of time the fire burns, doubles and the heat so producedis doubled. Thus, the fact that there were 90,000 L of jet fuel on a few floors of theWTC does not mean that this was an unusually hot fire. The temperature of the fireat 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



Figure 5.3: Flames and debris exploded from the World Trade Center south towerimmediately after the airplanes impact. The black smoke indicates a fuel-rich fire

fuel and the oxidant in nearly stoichiometric proportions and igniting the mixturein a constant-volume chamber. Since the combustion products cannot expand inthe constant-volume chamber, they exit the chamber as a very high velocity, fullycombusted, jet. This is what occurs in a jet engine, and this is the flame type thatgenerates the most intense heat.

In a pre-mixed flame, the same nearly stoichiometric mixture is ignited as it exitsa nozzle, under constant pressure conditions. It does not attain the flame velocitiesof 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, butflow together in an uncontrolled manner and combust when the fuel/oxidant ratiosreach values within the flammable range. A fireplace flame is a diffuse flame burningin air, as was the WTC fire. Diffuse flames generate the lowest heat intensities ofthe three flame types.

If the fuel and the oxidant start at ambient temperature, a maximum flametemperature can be defined. For carbon burning in pure oxygen, the maximum is



3,200C; for hydrogen it is 2,750C. Thus, for virtually any hydrocarbons, the maxi-mum flame temperature, starting at ambient temperature and using pure oxygen, isapproximately 3,000C. This maximum flame temperature is reduced by two-thirdsif air is used rather than pure oxygen. The reason is that every molecule of oxygenreleases the heat of formation of a molecule of carbon monoxide and a moleculeof water. If pure oxygen is used, this heat only needs to heat two molecules (car-bon monoxide and water), while with air, these two molecules must be heated plusfour molecules of nitrogen. Thus, burning hydrocarbons in air produces only one-third the temperature increase as burning in pure oxygen because three times asmany molecules must be heated when air is used. The maximum flame tempera-ture increase for burning hydrocarbons (jet fuel) in air is, thus, about 1,000Chardlysufficient 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 burnedfloors gave way, causing the perimeter wall columns to bow outward and the floorsabove them to fall. The buildings collapsed within ten seconds, hitting bottom withan estimated speed of 200 km/hr

Nearly every large building has a redundant design that allows for loss of oneprimary structural member, such as a column. However, when multiple members



fail, the shifting loads eventually overstress the adjacent members and the collapseoccurs like a row of dominoes falling down.

The perimeter tube design of the WTC was highly redundant. It survived theloss of several exterior columns due to aircraft impact, but the ensuing fire led toother steel failures. Many structural engineers believe that the weak pointswere theangle clips that held the floor joists between the columns on the perimeter wall andthe core structure .With a 700 Pa floor design allowable, each floor should have beenable to support approximately 1,300 t beyond its own weight. The total weight ofeach tower was about 500,000 t.

As the joists on one or two of the most heavily burned floors gave way and theouter box columns began to bow outward, the floors above them also fell. The floorbelow (with its 1,300t design capacity) could not support the roughly 45,000 t of tenfloors (or more) above crashing down on these angle clips. This started the dominoeffect that caused the buildings to collapse within ten seconds, hitting bottom withan 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 300km/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 giventhe capacity to meet this demand? To answer this question, it is important to un-derstand 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 peopleinside the building, the plane cannot be allowed to penetrate the exterior wall. Tostop a Boeing 767 traveling in excess of 500 miles per hour in a distance of a fewfeet 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 amore 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, todaysbuildings would need to be 100 times stronger than dictated by code, which is bothphysically and economically impossible.

So why did the World Trade Center Towers not collapse immediately due tothe impact load on the system? The planes did not stop in a few feet, but had aneffective stopping distance of over 100 feet. This would drop the deceleration forcedown to something close to the capacity of the building. Another part of the answerto this question lies in the way that the exterior of the building was structured. Theexterior columns were 14-inch square welded steel box columns spaced at 40 incheson 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 asolid wall of steel with perforations for windows. This wall construction was able to



form a Vierendeel bridge over the hole created in one side of each of the towers.

Both of these facts that the plane was not stopped at the exterior and that thecolumns and spandrels were extremely dense were necessary to prevent the buildingfrom 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 towersultimately collapsed with one floor crashing down upon the next, it has been calleda progressive collapse.

Again, it is important to think carefully about the question. Arent all collapsesprogressive? Something breaks, and then something else breaks, and so on. Nor-mally, when the term progressive collapse is used, it specifically refers to the loss ofone 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 oneface 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? Bynormal use of the term progressive collapse it did. The collapse that did ultimatelyoccur was progressive, like all collapses, but was not progressive collapse that someinternational 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 build-ings that are most susceptible to progressive collapse are buildings that are well-tiedtogether (i.e., able to transfer building loads from one element to another, such asa column). Yet, virtually every structural engineer will advise that one of the bestways to prevent progressive collapse is to tie the building together. How can therebe this kind of a contradiction?

The difference is that the engineer is thinking about losing a column or twoand the fire chief is talking about losing a whole part of a building. As the eventthat initiates the progressive collapse becomes larger than losing a column, the riskbecomes that the strong horizontal ties of a building will cause the collapse to prop-agate horizontally.

Any discussion of code provisions with respect to progressive collapse must rec-ognize that both the engineer and the fire chief are right depending on the kind ofhazard that is defined.

At least six safety systems present in the World Trade Center towers were com-



pletely and immediately disabled or destroyed upon impact: fireproofing, automaticsprinklers, compartmentalization and pressurization, lighting, structure and exitstairs.

5.2 Israel as a Case Study And Paradigm

Over the course of its history, Israel has adapted military blast design to blast de-sign to be used as a part of civilian structures. Israels methods for integrating blastprotection into its society can be used as an example for the rest of the world as itis increasingly subjected to more security threats.

When the state was founded in 1948, Israel had already constructed under-ground shelters across the country (see Figure 5.5). Underground shelters were thefirst forms of civilian blast protection because one of the most effective methods ofproviding protection for a structure is to bury it (Smith and Hetherington, 1994).Underground bomb shelters do have some benefits; they are generally larger thanwhat could be provided for inside of a building so they are more comfortable forlong periods of time. In addition , when the shelters were not in use they couldbe used for recreational purposes (Einstein, 2003 ). Many shelters were turned intolibraries and meeting places for youth groups (see Figures 2.10 and 2.11). Theseunderground 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 withLebanon. Katusha rockets were being launched over the Lebanese border into the



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 pro-tection from the attacks. Throughout northern Israel rooms designed to protecta buildings inhabitants from an explosion were included in most homes as well asschools and public buildings (Sandler, 2003). This was the beginning of the transi-tion from underground shelters, separate from the buildings. To shelters integratedinto daily structures.

The biggest change in Israels policy toward protecting its citizens came in 1991with the Gulf war. Saddam Hussein threatened Israel with Scud missiles and thisnot only increased the treat due to explosions, but also introduced the strong pos-sibility of bio-chemical threats. People were now required to have protected spaceswithin every home, office, and public space. The windows had to be able to besealed 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



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 struc-tures, and how to design these spaces were developed and known as Haga require-ments (Einstein, 2003). These regulations were fully integrated into the Israelibuilding 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 needto provide protection against chemical warfare , the importance of regulating the in-tegration of protected spaces into buildings remains and extend into blast protection.

Protecting a building from explosions is now an integral part of a buildings designout security risks while preserving the essence of the design (Einstein, 2003). Israelisociety cannot have all of its buildings feel like concrete fortified structures even ifthey rely (Figures 2.13,2.14,2.15) are examples of Israeli blast designed structures,



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 dueto terrorism, it has become apparent that the U.S. must also change its approachto protecting its citizens from explosions. Israel has successfully integrated blastprotection 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 havebeen forced to come up with new ways of designing building s that protect their in-habitants but still maintain peoples quality of life (Einstein, 2003). Because of theincreased and continuing threat to the United States it is clear that structural engi-neers here too will have to make blast design an integral part of all structures. Themore this mentality is put into practice the sooner blast design will be able t coex-ist with current structural design consideration such as architecture, sustainability,usability, and economics.


Chapter 6

Design Principles for Protection of Structures

6.1 General

Designing a building to withstand blasts includes more than just hardening thestructure. A lot of thought must go into the design to take into account moreconceptual design aspects such as preventing an attack to begin with, maintaininga large stand-off distance in case of an attack, and designing the building so that itwill remain standing in the case of localized damage. While discussing the principlesof blast design I will focus on the protection of structures in the event of a closerange bomb, most similar to present terrorist activities. This includes explosionsdue to suicide bombers near or inside a building, truck and car bombs near or driveninside a building, and package bombs.

6.2 Preventative Measures

The first step in making a building blast resistant is to try to prevent a terroristattack from occurring in the first place. This can be accomplished by making aterrorists attack from occurring in the first place. This can be accomplished bymaking a terrorists job as difficult as possible. There isles of a chance of a terroristtargeting 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. Heavysecurity as well as physical barriers can make entering a building difficult. Also, ifspace allows, spreading out a complex makes an effective terrorist attack more dif-ficult to execute. A bomb in one location will have less overall effect on a buildingif all of the buildings assets are spread out (Mays and smith, 1995). This strategyis only effective for building that is not set in the middle of the city and can affordto expand outwards. In addition, sites that could be possible terrorist targets suchas intelligence or defence building should be kept anonymous if possible (Mays andSmith. 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 theconsequences of an attack can be much less severe (Mays and Smith, 1995). It iimportant to prevent the placement of explosives near 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 mem-bers are the most critical (Eytan, 2003). By using tinted glass you can hid the exactstructural system from outside viewers as well.

One of the most important principles with blast design is to keep a large standoffdistance between the building and the potential blast. The strength of a blastdecreases in relation to the cube of the standoff distance from the explosion, thisindicates that as l you get farther away from the blast the intensity of the peakpressure dies off substantially. Smith and Hetherington illustrate this by sayingthat keeping vehicle bombs away from your structure is probably the sing, mostcost 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 casethat a blast does take place. The main way to harden a structure is to design thestructure with a lot of ductility integrated thought-out the system. Explosions gen-erate an enormous amount of energy and the role of the structures ductility i toabsorb this energy. As a result steel and reinforced concrete are the bets materialsto use in a blast resistant structure. Other structural concerns include how the floorsare attached to the rest of the structural frame. Floors need to be securely tied tothe frame and be able to withstand stresses in the direction opposite the normalgravity loads. Explosion cause a strong uplift reassure that can dislodge floors fromtheir supports if they are not tied securely. Floors many times work as a diaphragmthat carries lateral load in a structure, as a result, if the floor is removed from therest of the structural system progressive collapse can ensue.

Glazing is a major concern when hardening a building. Because normal glass isa 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 thoughthe air can be very severe and are usually very frequent. There are several tech-niques for increasing the blast resistance of glazing these techniques in combinationwith dynamic design of the structural frame can greatly increase the performanceof 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 fromentering 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 designthe building so that it is at least three bays wide. This provides space so that in thecase of an explosion people can move away from the exterior of a building. Also,



the centre area of the structure should be designed as a concrete core. This concretecore can be designed as a hardened area that can be used as protected space for thebuilding occupants.

In addition to all of these design techniques Eytan has developed a method ofhardening a structure in layers. Hardening structure inlayers is effective because itensures that the failure of one hardening layer will not lead to the catastrophic failureof the structure due to redundancy of the protective systems. The first hardeninglayer is the layer furthers away from the structure. The role of this layer is to pre-vent 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 ofthis layer is to prevent a terrorists forced entry further into the building. It shouldshield the rest of the building from the building from flying debris and shrapnel froma bomb. In addition, it should protect main structural elements from close rangeexplosions. And, of course it should further protect the structure from the pressurewave created by a bomb outside of the building.

The third layer is the layer that protects the internal structural system. Thislayer needs to protect the building from all of the things that the second layer isdesigned to protect. In addition, this layer must be able to protect the structurefrom 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 isthat a building be designed so that progressive structural collapse does not occurin the case of severe structural damage. As we have seen in events such as theWorld Trade Centre bombing, as destructive as the explosion itself was, the great-est damage and loss of life was due to the eventual collapse of the structure thatwas as a result of structural damage. Preventing structural collapse is necessary sothat as many people as possible can get out of a building safely after an attack. Ifprogressive collapse occurs it magnifies the effect of any terrorist event and allowsa terrorist to accomplish more damge than they ever could on their own. There areseveral guidelines that should be kept in mind in order to design a building to beprotected from structural collapse:

a.Create many different load paths and redundancies within a structure so thatit will not collapse in the case of several columns of critical members being damagedor 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



loading to prevent severe damage to the most important members.

e.Design critical members to be surrounded by energy absorbing materials or mem-bers.

Some other technique for protecting columns inside a structure include: usingcomposite material shields around the column with an air gap between the shieldand the column. Columns can be designed to be part of heavy walls so that theywill not experience local failure. The strength of the columns is improved if theyare designed as part of a moment frame where the connections can carry a largeamount of moment and dissipate a lot of energy. Also, columns should be designedto withstand a greater buckling load in case its unsupported length is increased bydamage to adjacent beams, joists, and slabs.


Chapter 7

Conclusion

7.1 General

The aim in blast resistant building design is to prevent the overall collapse of thebuilding and fatal damages. Despite the fact that, the magnitude of the explosionand the loads caused by it cannot be anticipated perfectly, the most possible sce-narios 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 extentof this danger. Most importantly human safety should be provided. Moreover, toachieve functional continuity after an explosion, architectural and structural factorsshould be taken into account in the design process, and an optimum building planshould be put together.

This study is motivated from making buildings in a blast resistant way, pio-neering to put the necessary regulations into practice for preventing human andstructural loss due to the blast and other human-sourced hazards and creating acommon sense about the explosions that they are possible threats in daily life. Inthis context, architectural and structural design of buildings should be speciallyconsidered.

During the architectural design, the behavior under extreme compression load-ing of the structural form, structural elements e.g. walls, flooring and secondarystructural elements like cladding and glazing should be considered carefully. In con-ventional design, all structural elements are designed to resist the structural loads.But it should be remembered that, blast loads are unpredictable, instantaneous andextreme. Therefore, it is obvious that a building will receive less damage with aselected 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 connec-tions, structural elements designed adequately, moment frames that transfer suffi-cient 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


the existing structures, retrofitting of the structural elements might be essential.Although these precautions will increase the cost of construction, to protect specialbuildings with terrorist attack risk like embassies, federal buildings or trade centersis unquestionable.


References

[1] Koccaz Z. (2004) Blast Resistant Building Design, MSc Thesis, Istanbul Tech-nical 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, SCIPublication, 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

Architectural And Structural Design Of Blast Resistant Buildings

Design

Transcript of Architectural And Structural Design Of Blast Resistant Buildings