BLAST LOAD

By:Jivani Dipak

Need of considering blast load

• loading To Grow awareness of the risks associated with important structures has led to advances in the application of analysis of blast and impact.

Blast loading.

During the explosion,there is sudden expansion of small amount of air surrounding the explosive,which causes increased pressure in that small area around the explosive.

1.Overpressure

2.Propagation of Overpressure.

Overpressure time history with critical blast parameters.

Blast Impulse.

Blast effect on building.

• Crater effect

• Ground shaking.

• Overpressure loading.

• Other effects.

Magnitude of blast loading acting on structure vary greatly with

• Properties of explosive.

- Type of material.

- Quantity of explosive.

- Energy output.

• Location of detonation.

• Stand-off distance.

• Reflection of shock front.

Action of loading.• Exterior Blasts:

• Interior Blasts:

Load combination

• Wind and EQ shall not be considered simultaneously with blast.

• Effect of temperature and shrinkage shall be neglected.• Live load - IS:875-1964• No live load consider on roof at the time of blast.• For general guideline the building may be designed for a

charge of 100kg at a distance given in table.

BLAST FORCE

• As per the code IS:4991-1968 the blast force will be calculated as,[ Clause:5]

Ps=Pso[1-(t/to)e]-b*t/to

Where, T is the time to zero pressure.(mille seconds)

b is wave form parameter.

• This is a time decay function.

• Scaling Laws

Closed structure

• Front facepeak value

Reflected overpressure which drops from peak value

Open structure

Partly Open structure

• Area of opening >50% of area of walls- drag type structure.

• Area of opening< 50% of area of walls- closed type structure.

• For intermediate condition, direct interpolation may be made between the two conditions of both maximum pressure and time duration.

Air craft loading.

• Design loads resulting from aircraft impacts are governed by the absorption of kinetic energy from the aircraft by the building at its maximum deflection.

• Total impulsive load at the interface of the collapsing aircraft and the building is given by.

- is coefficient for change in momentum=1

- is constant which can be determined from design acceleration for failure of the aircraft.

- is the velocity of aircraft

Case study- WTC

• World trade center.