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BLAST LOAD AND EFFECTS OF BLAST ON STRUCTURES

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BLAST LOAD AND EFFECTS OF BLAST ON STRUCTURES Powered By Docstoc
					   1.INTRODUCTION

   2. EXPLOSION AND BLAST PHENOMENON

   3. EXPLOSIVE AIR BLAST LOADING

   4. GAS EXPLOSION LOADING AND EFFECT OF
       INTERNAL EXPLOSIONS

   5. FAILURE MODE OF BLAST LOADED STRUCTURE
   5.1 GLOBAL STRUCTURAL BEHAVIOUR
   5.2 LOCALISED STRUCTURAL BEHAVIOUR

   6. BLAST WAVE-STRUCTURE INTERACTION

   7.BLAST EFFECT ON BUILDING FACADE

   8.CONCLUSION

   9.REFERENCE

                                                2
   In past few decades emphasis given to
    problems of blast and earthquake.
   The earthquake problem is rather old, but the
    blast problem is rather new;
   Information about the development in this field is
    made available through publication of the Army
    Corps of Engineers, Department of Defence, U.S.
    Air Force and other governmental office and
    public institutes.
   Due to different accidental or intentional events,
    the behaviour of structural components
    subjected to blast loading has been the subject
    of research effort in recent years.
   Conventional structures - normally not designed
    to resist blast load.
                                                         1
   The magnitudes of design loads lower for
    explosions.

   Conventional structures are susceptible to
    damage from explosions.

   Disasters such as the terrorist bombings of the
    Murrah Federal Building in Oklahoma City in
    1995,demonstrated the need for examination of
    the behaviour of columns subjected to blast
    loads.

   Thus developers, architects and engineers
    seeking solutions for potential blast situations, to
    protect building occupants and the structures.
                                                           2
   An explosion is defined as a large-scale, rapid
    and sudden release of energy.
   Explosions can be categorized on the basis of
    their nature:- as physical, nuclear and chemical
    events.
   In physical explosion: - Energy may be released
    from the catastrophic failure of a cylinder of a
    compressed gas, volcanic eruptions or even
    mixing of two liquid at different temperature.
   In nuclear explosion: - Energy is released from
    the formation of different atomic nuclei by the
    redistribution of the protons and neutrons within
    the inner acting nuclei.
   In chemical explosion: - The rapid oxidation of
    the fuel elements (carbon and hydrogen atoms) is
    the main source of energy.
                                                        3
   Explosive materials classified according to
    their physical state as solids, liquids or gases.
   Solid explosives are high explosives for which
    blast effects are best known.
   They can be classified on the basis of their
    sensitivity to ignition as secondary or primary
    explosive.
   Primary explosive is one that can be easily
    detonated by simple ignition from a spark,
    flame or impact.
   Secondary explosives when detonated create
    blast (shock) waves which can result in
    widespread damage to the surroundings.

                                                        4
   The explosion of a condensed high explosive
    generates hot gases under pressure up to 300
    kilo bar .
   Temperature of about 3000-4000C° created.
   The hot gas expands forcing out the volume it
    occupies.
   A layer of compressed air (blast wave) forms in
    front of this gas volume containing - the energy
    released by the explosion.
   Blast wave instantaneously increases to a value of
    pressure above the ambient atmospheric
    pressure.
   This is referred to the side-on overpressure -
    decays as the shock wave expands outward from
    the explosion source.


                                                         5
   After a short time, the pressure behind the
    front may drop below the ambient pressure.
   During such a negative phase , a partial
    vacuum is created and air is sucked in.
   Thus high suction winds created, carry the
    debris from explosion source faraway.




                                                  6
   The threat for a conventional bomb is defined
    by two elements, the bomb size, or charge
    weight W, and the standoff distance (R)
    between the blast source and the target .

   For eg.:-The Oklahoma bomb in 1995 had a
    charge weight of1814 kg at a stand off of 4.5m
    .

   Terrorist attacks may range from the small
    letter bomb to the gigantic truck bomb as
    experienced in Oklahoma City.

                                                     7
   The exterior building walls capable of
    resisting the blast load- the shock front
    penetrates through window and door
    openings.
   Subjecting the floors, ceilings, walls,
    contents, and people to sudden pressures
    and fragments from shattered windows,
    doors, etc.
   Building components not capable of resisting
    the blast wave will fracture, be fragmented
    and moved by dynamic pressure followed by
    the shock front.
   Building contents and people - displaced and
    tumbled in the direction of blast wave
    propagation.
                                                   8
   In this manner the blast will propagate
    through the building.




                                                    9
                                                    9
                 Figure 2:blast loads on building
   On build-up of fuel in a low turbulence
    environment, typical of domestic gas explosions

   flame propagation on ignition is slow and the
    pressure pulse is extended.

   The specific energy of combustion of hydrocarbon
    fuel is very high (46000 kJ/kg for propane, and
    4520 kJ/kg for TNT) .

   Internal explosions produce complex pressure
    loading profiles of the resulting two loading
    phases.

                                                       10
 The first results from the blast overpressure
  reflection .
 The confinement provided by the structure,
  re-reflection occurs.
 Depending on the degree of confinement of
  the structure:-
 the confined effects of pressures cause
  different degrees of damage to the structure.
 The target structures can be described as
  either vented or un-vented.
 The latter must be strong to resist a specific
  explosion yield than a vented structure .
 Venting following the failure of windows (at
  typically 7 kN/m2) greatly reduces the peak
  values of internal pressures.

                                                   11
 Blast loading effects on structural members
  produce local and global responses with different
  failure modes.
 The type of structural response depends mainly
  on the:-
 loading rate,
 orientation of the target


   The failure modes of blast loading can be
    flexure, direct shear or punching shear.
   Local responses characterized by localized
    bleaching and spalling, & result from the close-in
    effects of explosions.
   Global responses are typically manifested as
    flexural failure.

                                                         12
 The global response of structural elements is
 transverse (out-of-plane) loads with long
  exposure time (quasi static loading).
 global membrane (bending) and shear
  responses.
 The second global failure mode is shear
  failure.
 Under the effect of static and dynamic
  loading 4 types of shear failure identified:
  diagonal tension, diagonal compression,
  punching shear, and direct (dynamic) shear .
 The first two types are common in reinforced
  concrete elements under static loading .
                                                  13
   punching shear is local shear failure, the
    example of this is column punching through
    a flat slab.
   Minor structural effect in blast loading and
    thus neglected.
   The fourth type of shear failure is direct
    (dynamic) shear.
   This failure mode is transient short duration
    dynamic loads that result from blast effects.
    It depends on the intensity of the pressure
    waves.




                                                    14
 The close-in effect of explosion cause localized
  shear or flexural failure in the closest structural
  elements.
 This depends on
1. the distance between the source of the
    explosion and the target,
2. the relative strength/ductility of the structural
    elements.
 The localized shear failure takes place in the
  form of localized punching and spalling_ produce
  low and high speed fragments.
 The punching effect is referred to bleaching,
  known in high velocity impact applications .
 Bleaching failures include spalling and scabbing
  of concrete covers.

                                                        15
FIGURE: BREACHING FAILURE DUE TO CLOSE-IN EXPOSION DUE
           TO 6000Kg OF TNT EQUIVALENT

                                                         16
   The structural behaviour of an object exposed to
    blast wave analyzed by dealing with two main
    issues.
   1) blast-loading effects, i.e. forces that are
    resulted directly from the action of the blast
    pressure.
   2) the structural response, or the expected damage
    criteria associated with loading effects.

   In analyzing the dynamic response to blast
    loading, 2 types of target structures were
    considered: diffraction-type and drag-type
    structures.
   The former affected mainly by diffraction
    (engulfing) loading and the latter by drag loading.
   Actual buildings will respond to both types of
    loading .

                                                          17
   The structural response depend on the size, shape
    and weight of the target, firmed attachment to the
    ground, and on the existence of openings in each
    face of the structure.
   Ground or shallow-buried structures subjects to
    ground shock resulting - discharge of explosive
    on/or close to ground surface.
   The energy imparted to the ground by the
    explosion is the main source of ground shock.
   A part of this energy is directly transmitted through
    the ground as directly-induced ground shock, while
    part is transmitted through the air as air-induced
    ground shock.



                                                            18
   When a terrorist bomb explodes in urban area,
    air blast pressure fractures lightweight façades
    like windows, causing catastrophic results.
   Due to façade failure, the blast enters buildings,
    causing superficial structural damage.
   Great disruption to the working environment ,
    the mechanical and electrical services.
   Falling window glass shards cause injuries
    resulting from blast pressure.
   Even with blast-resistant glazing, air blast
    pressure will fracture windows, necessitating
    replacement.

                                                         19
   Advanced computer codes -CFD used to
    simulate the blast effects in the urban
    environment .
   The peak overpressure is 4.1MPa at the
    ground level and reduces rapidly up the
    height of the building.
   The average duration of loading-
    15millisecond.
   Façade damage at different levels was
    assessed based on the blast pressure
    distribution.




                                              20
FIGURE: CFD modeling of blast pressure on building structures

                                                                21
FIGURE: DISTRIBUTION OF BLAST PRESSURE ON BUILDING
         FAÇADE                                   22
   For high-risks facilities such as public and
    commercial tall buildings, design considerations
    against extreme events (bomb blast, high velocity
    impact) is very important.
   It is recommended that guidelines on abnormal
    load cases and provisions on progressive
    collapse prevention should be included in the
    current Building Regulations and Design
    Standards.
   Requirements on ductility levels also help
    improve the building performance under severe
    load conditions.


                                                        23
   1. A. Khadid et al. (2007), “ Blast loaded stiffened plates” Journal
    of Engineering and Applied Sciences,Vol. 2(2) pp. 456-461.

   2. A.K. Pandey et al. (2006) “Non-linear response of reinforced
    concrete containment structure under blast loading” Nuclear
    Engineering and design 236. pp.993-1002.

   3. Alexander M. Remennikov, (2003) “A review of methods for
    predicting bomb blast effects on buildings”, Journal of battlefield
    technology, vol 6, no 3. pp 155-161.

   4. American Society for Civil Engineers 7-02 (1997),
    “Combination of Loads”,pp 239-244.

   5. ANSYS Theory manual, version 5.6, 2000.

   6. Biggs, J.M. (1964), “Introduction to Structural Dynamics”,
    McGraw-Hill, New York.

   7. Dannis M. McCann, Steven J. Smith (2007), “Resistance Design
    of Reinforced Concrete Structures”, STRUCTURE magazine, pp
    22-27, April issue.
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