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
Blast resistant structures
EXPLOSION AND BLAST
PHENOMENON
4
Blast resistant structures
An explosion is defined as a large-scale, rapid and sudden
release of energy
Classified based on nature:
• Physical
• Chemical
• Nuclear
5
Blast resistant structures
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
Blast resistant structures
TYPES OF EXPLOSIONS
7
Unconfined Explosions
Confined Explosion
Explosions caused by explosives attached to the
structure
Blast resistant structures
Air burst with ground reflections
Surface burst
Unconfined explosionsOccur as :
8
Air-burst• Detonation of explosives
occur above the ground level
Surface burst• Detonation of explosives
occur close to or on the ground surface
Blast resistant structures
• Confined Explosions
Explosion occurs within a building
9
Extent of Venting Fully Vented
Partially Vented
Fully Confined
Blast resistant structures
Fully vented
Partially vented Fully confined Fully vented, partially vented and fully confined explosions
CONT..
10
Blast resistant structures
• 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
11
Blast resistant structures
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
Blast resistant structures
CONT..
Blast load
Thermal energy
radiation
Audible blast
Air blast
Dynamic pressure
Ground shock wave
13
Blast resistant structures
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
14
Blast resistant structures
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
Blast resistant structures
•
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
Blast resistant structures
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
Blast resistant structures
CONT..
Blast wave pressures plotted against time
18
Blast resistant structures
Schematic diagram showing blast load action
CONT..
19
Blast resistant structures
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
Blast resistant structures
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:
21
Blast resistant structures
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
22
Blast resistant structures
ACCEPTED DAMAGE LEVELS
Levels of damage computed by means of analysis may be described by the terms:
i. Minorii. Moderateiii. Major
23
Blast resistant structures
CONT..
24
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
Blast resistant structures
CONT..
25
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
Blast resistant structures
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
26
Blast resistant structures
STRUCTURAL ASPECT OF
BLAST RESISTANT BUILDING
DESIGN
27
Blast resistant structures
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
28
Blast resistant structures
Design & detailing of exterior bays & lower floors
Drop panels and column capitals may be used
Anchorage of the reinforcement
Column Heads And Drop panels
29
Blast resistant structures
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
30
Blast resistant structures
Schematic Depiction of failure of a transfer girder under blast loading
31
Blast resistant structures
3. SHEAR WALLS
Provide shear walls around the building
This will act as a well distributed lateral load resisting mechanism
Construction of shear walls
32
Blast resistant structures
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
33
Blast resistant structures
5. ENERGY-ABSORBING CATCH SYSTEM
Absorb and dissipate large amounts of blast energy
Prevent the debris from entering the occupied space
Catch System
34
Blast resistant structures
6. COLUMN REINFORCEMENTS
Providing adequate longitudinal reinforcement
Closely spaced ties
Steel columns may be encased in concrete to prevent premature buckling
35
Blast resistant structures
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
36
Blast resistant structures
7. STEEL JACKET
Spiral reinforcement provides a measure of core confinement
Improves capacity and behavior of reinforced concrete columns under extreme loads
Steel jackets
37
Blast resistant structures
ARCHITECTURAL ASPECT OF BLAST RESISTANT DESIGN
38
Blast resistant structures
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
39
Blast resistant structures
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
Blast resistant structures
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
41
Blast resistant structures
Layout for site protection against bombs
42
Blast resistant structures
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
43
Blast resistant structures
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..
44
Blast resistant structures
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
45
Blast resistant structures46
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
Blast resistant structures
CASE STUDIES
47
Blast resistant structures
CASE STUDY 1 : OKLAHOMA CITY BOMBING
Extent of damage – Murrah building48
Blast resistant structures
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
49
BUILDING DESCRIPTION AND EXPLOSION
Blast resistant structures
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
50
Blast resistant structures
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
51
Blast resistant structures
Blast and Progressive Collapse Damage 52
Blast resistant structures
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
53
Blast resistant structures
CASE STUDY 2 KHOBAR TOWERS BOMBING
Extent of damage – Khobar Towers54
Blast resistant structures
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
55
BUILDING DESCRIPTION AND EXPLOSION
Blast resistant structures
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
56
Blast resistant structures
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
Blast resistant structures
A close up of the precast concrete wall system
58
Blast resistant structures
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
59
Blast resistant structures
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
60
Blast resistant structures
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
Blast resistant structures62