Blast Resistant Buildings

BLAST RESISTANT

STRUCTURES

Guided By Presented By

A Technical Seminar

On

Blast resistant structures2

OVERVIEW…i. Introductionii. Explosion and Blast Phenomenon

Types of Explosions Blast Load Explosion Process for High Explosives Blast Threat Definition Building Damages Due to Blast Accepted Damage Levels

iii. Principles of Blast resistant design Structural Aspect Architectural aspect

iv. Case Studiesv. Conclusionvi. References

Blast resistant structures

INTRODUCTION

Emerging branch in the field of structural engineering

Terror attacks and accidents - cannot be prevented

Damage to the assets, loss of life and social panic are factors

that have to be minimized if the threat of terrorist action

cannot be stopped

Popularly applied in modern and important buildings,

especially ‘Icon Buildings’

Current engineering knowledge can enhance the new and

existing buildings to mitigate the effects of an explosion3


EXPLOSION AND BLAST

PHENOMENON

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An explosion is defined as a large-scale, rapid and sudden

release of energy

Classified based on nature:

• Physical

• Chemical

• Nuclear

5


Explosive materials classification

i. Based on their physical state:

• Solid explosives

• Liquid explosives

• Gaseous explosives

ii. Based on the sensitivity to ignition:

• Primary explosives e.g. Lead azide

• Secondary explosives e.g. TNT , ANFO

CONT..

6


TYPES OF EXPLOSIONS

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Unconfined Explosions

Confined Explosion

Explosions caused by explosives attached to the

structure


Air burst with ground reflections

Surface burst

Unconfined explosionsOccur as :

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Air-burst• Detonation of explosives

occur above the ground level

Surface burst• Detonation of explosives

occur close to or on the ground surface


• Confined Explosions

Explosion occurs within a building

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Extent of Venting Fully Vented

Partially Vented

Fully Confined


Fully vented

Partially vented Fully confined Fully vented, partially vented and fully confined explosions

CONT..

10


• Explosions caused by explosives attached to the Structure

Detonating explosive is in contact with a structural componente.g. a column

The arrival of detonation wave will generate intense stress waves in the material and will result in crushing of the material

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BLAST LOAD

A blast is a destructive wave of highly compressed

air spreading outward from an explosion

Accompanied by rapid release of potential energy

A part of this energy is converted to thermal

energy radiation and a part is coupled as air blast

and shock waves which expand radially12


CONT..

Blast load

Thermal energy

radiation

Audible blast

Air blast

Dynamic pressure

Ground shock wave

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EXPLOSION PROCESS FOR HIGH EXPLOSIVES

An explosion occurs when a gas, liquid or solid material goes through a rapid chemical reaction

The gas products of the reaction are formed at a very high temperature and pressure at the source

These high pressure gasses expand rapidly into the surrounding area and a blast wave is formed

Blast waves propagate at supersonic speeds and gets reflected as they meet objects

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As the blast wave continues to expand away from the source of the explosion its intensity diminishes and its effect on the objects is also reduced

It is convenient to separate out the different types of loading experienced by the surrounding objects away from the source

Three effects have been identified in three categories:i. Air shock waveii. Dynamic pressureiii. Ground shock wave

CONT..

15


•

CONT..

16

• The effect rapidly compressing the surrounding air

Air Shock Wave

• The air pressure and air movement effect due to the accumulation of gases from the explosion chemical reactions

Dynamic Pressure

• The effect rapidly compressing the ground

Ground Shock Wave


Overpressure:The air shock wave produces an instantaneous increase in pressure above the ambient atmospheric pressure at a point some distance from the source

A pressure differential is generated between the combustion gases and the atmosphere causing a reversal in the direction of flow, back towards the center of the explosion, known as a negative pressure phase

Equilibrium is reached when the air is returned to its original state

CONT..

17


CONT..

Blast wave pressures plotted against time

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Schematic diagram showing blast load action

CONT..

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BLAST THREAT DEFINITION

The blast threat for a building

structure is defined by two

parameters:

The stand-off distance

Distance between the

blast source and target

The charge weight

Bomb size in TNT

equivalent

20

Blast threat parameters


Explosive

Weapon

Approximate TNT

Equivalence

Pipe Bomb 5 pounds TNT

Briefcase Bomb 50 pounds TNT

Car Bomb 500 pounds TNT

Van Bomb 1,500 pounds TNT

Truck Bomb 10,000 pounds TNT

The following table displays a rough TNT equivalence guide for various explosive weapons:

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BUILDING DAMAGES DUE TO BLAST

Damage due to explosions may be divided into:

i. Localized / Direct air-blast effects ii. Progressive collapse

Direct air-blast effects refer to damage caused by the high-intensity pressures of the air-blast close to the explosive source

Progressive collapse refers to the spread of an initial local failure from element to element, eventually resulting in a disproportionate extent of collapse relative to the zone of initial damage

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ACCEPTED DAMAGE LEVELS

Levels of damage computed by means of analysis may be described by the terms:

i. Minorii. Moderateiii. Major

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CONT..

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Minor

• Non-structural failure of building elements as windows, doors, and cladding

• Injuries may be expected, and fatalities are possible but unlikely

Moderate

• Structural damage is confined to a localized area and is usually repairable

• Structural failure is limited to secondary structural members, such as beams, slabs and non-load bearing walls

• Injuries and possible fatalities are expected


CONT..

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Major

• Loss of primary structural components such as columns or transfer girders promotes loss of additional adjacent members that are adjacent or above the lost member

• Extensive fatalities are expected


PRINCIPLES OF BLAST RESISTANT DESIGN

Maintain safe separation of attackers and targets i.e. Stand-off zones

Design to sustain and contain certain amount of bomb damage. Avoid progressive collapse of the building

Allow for limited localized damage of members

Minimize the quantity and hazard of broken glass and blast induced debris

Facilitate rescue and recovery operation with adequate time of evacuation of occupants

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STRUCTURAL ASPECT OF

BLAST RESISTANT BUILDING

DESIGN

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1. FLOOR SLABS

A reinforced-concrete flat-plate structural slab is an economical system

Softening of the moment-resisting capacity of the slabs will reduce the lateral load-resisting capacity of the system

The loss of contact between the slab and the columns may result the whole building to become laterally unstable

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Design & detailing of exterior bays & lower floors

Drop panels and column capitals may be used

Anchorage of the reinforcement

Column Heads And Drop panels

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2. TRANSFER GIRDER REINFORCEMENT

Damage to a plate girder leaves many columns unsupported

Designed to be continuous over several supports

Substantial structural framing should be provided to create redundancy

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Schematic Depiction of failure of a transfer girder under blast loading

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3. SHEAR WALLS

Provide shear walls around the building

This will act as a well distributed lateral load resisting mechanism

Construction of shear walls

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4. CURTAIN WALL PROTECTION

Non-bearing exterior enclosure that is supported by a building’s frame

Flexible than conventional window systems

Mullion support would absorb a portion of the blast energy and improve the performance of glazing

Blast Curtain wall

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5. ENERGY-ABSORBING CATCH SYSTEM

Absorb and dissipate large amounts of blast energy

Prevent the debris from entering the occupied space

Catch System

34


6. COLUMN REINFORCEMENTS

Providing adequate longitudinal reinforcement

Closely spaced ties

Steel columns may be encased in concrete to prevent premature buckling

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Direct Pressure

Conventional RC columns not designed to resist bending, so may be prone to damage

Should therefore be designed with adequate ductility and strength

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7. STEEL JACKET

Spiral reinforcement provides a measure of core confinement

Improves capacity and behavior of reinforced concrete columns under extreme loads

Steel jackets

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ARCHITECTURAL ASPECT OF BLAST RESISTANT DESIGN

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1. PERIMETER PROTECTION

Anti Ram bollards around the perimeter

It should be able to resist the maximum vehicular load

Public parking lot should be secured

Street parking should not be permitted adjacent to the building

Anti ram bollards

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2. BOMB SHELTER AREAS

Specially designated within the building

Personnel can retire in case of an external bomb threat

warning

Should accommodate all the occupants of the building

Located away from doors, windows and external walls

Provide sufficient ventilation and sanitation

Provide alternate means of escape

Limit the blast pressure to less than ear drum rupture

pressure 40


3. PLANNING AND LAYOUT

Adequate placing of shelter areas within a building

The priority should be to create as much stand-off distance between an external bomb and the building as possible

Necessity of blast protection for structural and non-structural members should be considered

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Layout for site protection against bombs

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4. STRUCTURAL FORM AND INTERNAL LAYOUT

Structural form is a parameter that greatly affects the blast loads on the building

Arches and domes are the types of structural forms that reduce the blast effects on the building

Complex shapes that cause multiple reflections of the blast wave should be discouraged

It should be noted that single story buildings are more blast resistant compared with multi-story buildings if applicable

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Partially or fully embed buildings are quite blast resistant

The internal members of the building should be designed to resist the fire

Highest exterior threat is separated by the greatest distance from the highest value asset

Double - doorings should be used and should be arranged eccentrically

CONT..

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5. 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 amount of glazing in the facade should be minimized

It should also be ensured that the cladding is fixed to the structure securely with easily accessible fixings

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The choice of a safer glazing material is critical and it has been found out that laminated glass is the most effective in this context

Applying transparent polyester anti-shatter film to the inner surface of the glazing is as well an effective method

Laminated Glass


CASE STUDIES

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CASE STUDY 1 : OKLAHOMA CITY BOMBING

Extent of damage – Murrah building48


Study was conducted by Federal Emergency Management Agency (FEMA) Building Performance Investigation Team (BPAT)

The Alfred P. Murrah Federal Office Building in Oklahoma City, USA consists of a nine-storey office building and a multilevel parking garage

On April 19, 1995, a rental truck filled with a high explosive substance was detonated about 10 feet away from the building

Casualties 167Injured 782

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BUILDING DESCRIPTION AND EXPLOSION


The crater formed by blast was measured to be only about 10 feet away from Column G20 of the Murrah Building

4,800 lbs of ammonium nitrate and fuel oil explosive, also known as ANFO was detonated

Resulting pressures on the nine-story portion of the Murrah Building due to this air-blast wave were a maximum of 10,000 psi at the area closest to the detonation

BLAST LOADING

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COLLAPSE AND FAILURE INVESTIGATION

Three columns, G16, G20, and G24 where directly effected by the blast from the truck bomb

Destruction of these columns resulted in the transfer girder at Level 3 to be unsupported and a progressive collapse of the floors

Glazing of the curtain wall provided no resistance to the blast wave

The failure of the slabs above Level 5 was merely a result of the disproportionate collapse

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Blast and Progressive Collapse Damage 52


SUGGESTIONS EVOLVED Confinement of the entire length of columns and beams provided through the use of ties

Upward loads on slabs accounted for through the use of continuous top reinforcement

Continuous bottom reinforcement in slabs along column lines

Floor systems designed to develop full strength of reinforcement

Outer bays are redundantly designed to account for the loss of a ground floor column

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CASE STUDY 2 KHOBAR TOWERS BOMBING

Extent of damage – Khobar Towers54


Study was conducted by Federal Emergency Management Agency (FEMA) Building Performance Investigation Team (BPAT)

Building 131, one building of a larger military complex named the Khobar Towers in Dhahran, Saudi Arabia was attacked

On the night of June 25, 1996, a sewage tanker filled with explosives exploded in front of the Khobar Towers

Casualties 19 Injured 372

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BUILDING DESCRIPTION AND EXPLOSION


BLAST LOADING

The truck was filled with at least 5,000 pounds of home-made plastic explosive made from fertilizers

This equates to 20,000 pounds of TNT and ranks as the most powerful detonated terrorist attack

The blast sent debris and pressure waves crashing into Building 131’s side

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COLLAPSE AND FAILURE INVESTIGATION

The majority of the casualties were due to flying glass and debris

Did not result in a progressive collapse of the structure

Precast wall and floor systems formed a redundant system which later proved to save countless lives

Concrete partitions offered several redundant load paths for the forces to be redistributed

The explosion pressures, primary, and secondary debris caused the lower levels of the façade to buckle into the building 57


A close up of the precast concrete wall system

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SUGGESTIONS EVOLVED

The importance of addressing the standoff distance to each building

The significance of blast resistance facade design

Site planning and security measures should be observed

Stronger and more ductile glazing systems

Redundancy of the precast wall system should be utilized

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CONCLUSIONBlast resistant building design is done to prevent the overall collapse of the building and fatal damages

It is not practical to design buildings to withstand any conceivable terrorist attack

Design process to ensure that appropriate threat conditions and levels of protection should be incorporated

Architectural and structural factors should be taken into account in the design process

For the existing structures, retrofitting of the structural elements might be essential

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REFERENCES1. Zeynep Koccaz, Fatih Sutcu, Necdet Torunbalci (2008): ‘Architectural And Structural Design For Blast Resistant Buildings’, The 14th World Conference on Earthquake Engineering, Beijing, China, October 12-17, 2008 2. Tod Rittenhouse (1995): ‘Designing Terrorist Resistant Buildings’, Journal of Fire Engineering, Volume-148, Issue 11 3. Robert Smilowitz (2011), ‘Designing Building To Resist Explosive Threats’, Whole building Design Guide, October 2011 4. Jon A. Schmidt (2003), ’Structural Design For External Bomb Attacks’, Structure Magazine, March 2003 Edition 5. Eve Hinman (2011), ‘Blast Safety Of the Building Envelope’, Whole building Design Guide, October 2011 6. Corley, W. G., Sozen, M. A., Thorton, C. H., and Mlakar, P. F. (1996), ‘The Oklahoma City Bombing: Improving Building Performance through Multi-hazard Mitigation’, FEMA Bulletin 277, Federal Emergency Management Agency, Washington D.C. 61


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