Blast Load – Effects on Structures

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

To extend the longevity of the structure for many decades to come, structural strengthening is a safe, secure choice. We provides fully comprehensive, complete systems for all kinds of structural strengthening. Contact us now!

Source: Article written by LAWRENCE ABLADEY