Blast Load – Effects on Structures

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6 September 2019 - 14:33, by , in blast mitigation, Comments off

BLAST LOAD: EXPLOSIONS AND EFFECTS ON STRUCTURES

 

EXPLOSIONS

  • The sudden release of potential energy and the rapid expansion of gases.
  • The released potential energy is accompanied by
    – heat,
    – sound and,
    – high pressure
  • Expanded hot gases pushes out surrounding
  • Layer of compressed air is formed in front of the hot gases, called shock front
  • The shock front of the blast wave produces instantaneous increase in pressure
  • The blast wave lasts for a few tens of milliseconds.
  • Pressure generated during explosion reaction is about 30 GPa
  • Temperature ranges between 3000 to 4000 °C
  • Blast waves are omnidirectional and reflects off surfaces with higher densities than the travelling medium
  • Reflected waves have higher pressure than incident waves.
  • Arrival time: time taken for the incident wave to travel from centre of detonation to a point of interest
  • The blast wave has two phases
    – The positive phase: where blast pressure is higher than atmospheric pressure
    – Negative phase: where blast pressure is lower than atmospheric pressure

 

EXPLOSIONS: Pressure-time History of a blast wave

 

EXPLOSIONS: DISTANCE TO GROUND

  • Spherical (Free air burst)
    – Very common scenario
    – Centre of detonation elevated above ground
    – Pressure is incident pressure
  • Hemispherical ( Surface burst)
    – Explosion on ground surface or close to the ground
    – Blast wave instantaneously reflected
    – Energy lost due to cratering

 

EXPLOSIONS: Illustration of an explosion

 

EXPLOSIONS: blast wave on a concrete face

 

EXPLOSIONS: Types of Explosives

  • TNT (Trinitrotoluene)
  • ANFO (Ammonium Nitrate Fuel Oil)
  • Explosives can be in
    – solid
    – gels and
    – liquid states

 

CLASSIFICATIONS OF EXPLOSIONS

  • Nature of Explosion
    – Physical: rupture of compressed gas containers, volcanic eruptions
    – Nuclear: Sudden release of energy through fusion or fission of atomic nuclear.
    – Chemical: the oxidation of fuel elements such as carbon and hydrogen atoms
  • Accidental explosions occur in industries or homes
  • Planned explosions are sometimes controlled explosions used in the mining industry and demolition of structure.
    – Planned explosions such Terrorist attacks lead to loss of lives and destruction of property

 

CLASSIFICATIONS OF EXPLOSIONS: Accidental explosions

  • GoilFilling Station Explosion
    – June 04, 2015
    – Circle, Accra, Ghana
    – Explosion of underground fuel storage facility when mixed with flood water
    – Fire eruption and spreading aided by the rains and flood
    – Over 150 lives lost
    – Destruction of property
  • Explosion at Nsawam
  • December 24, 2015
  • Peabo, Nsawam, Eastern Region
  • Detonating of explosives abandoned in metallic containers
  • 1 dead and several injured
  • Over 1000 residents displaced
  • Destruction to buildings

 

CLASSIFICATIONS OF EXPLOSIONS: Planned Explosions

  • Oklahoma City Bombing
    – April 19, 1995
    – Oklahoma, USA
    – Attack on the Alfred P. MurrahFederal Building
    – Detonation of truck full of explosives
    – 2300 kg of ammonium nitrate
    – 168 dead, 680 injured
    – 324 buildings destroyed or damaged

 

EFFECTS OF EXPLOSIONS ON STRUCTURES

  • Blast Pressure
  • Fragmentation
  • Ground shock
  • Cratering
  • Fireball

 

FAILURE MODES –STRUCTURAL ELEMENTS

  • Shear failure
    – Under high but short duration blast loads
    – Failure near supports of the structural element
  • Flexure failure
    – under low but long duration blast loads
    – Failure near midspanof structural element
  • Cratering
    – Removal of concrete on the compression face of the elements
  • Spalling
    – the ejection of concrete from the back face of a structural member
    – dominant failure mode in close-in (near-field) explosions
    – Result of tensile failure
    – Magnitude of reflected tensile waves exceeding concrete tensile strength
    – Ejection of fragments called spall

 

MITIGATION MEASURES

  • Provide anti-climb measure for walls
  • Provide vehicular an pedestrian gates
  • Landscaping barriers
    – Bollards
    – Trees
    – Gullies
  • Blast Curtains for windows
  • Blast Doors

 

RETROFFITING STRUCTURAL ELEMENTS

  • Using Steel Jackets
  • Wall retrofitting systems
    – Steel plates
    – Concrete skin
    – FRP sheets

 

Source: Article written by LAWRENCE ABLADEY

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