Design of Blast resistant structures

Introduction • One of the most popular design issue.• Terror attacks and accidents.• Subject is popularly applied in modern and

important buildings.• Emerging branch in the field of structural

engineering

Moving vehicle attack

Stationary vehicle bomb

Exterior attack

Arsons

Others

Ballistic attacks

Types of blasts

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.

Stand-off zones

• Blockades, planters, fountains, fences as obstacles to ramming vehicles/truck bomb.

• Allow only emergency vehicle access. • Raise the building 2m above ground level

(provide Ramps for barrier free access)

Blast load definitionAn explosion is a rapid release of potential energy characterized by eruption enormous energy to the atmosphere.A part of energy is converted to thermal energy radiation(flash) and a part is coupled as air blast and shock waves which expand radially

Blast load

Thermal energy

radiation

Audible blast

Air blast

Dynamic pressure

Over pressure

Thermal energy radiation

• Much of the thermal energy is absorbed .• Combustible materials gets ignited.• These act fuel for building materials to be

burnt• Life inside also get burnt.

Effects of shock-waves

• This travels away from explosion faster than the speed of sound poses threat in close location.

• Shock front is similar to “moving wall” of highly compressed air accompanied by blast wind.

• It causes sudden rise in ambient pressure. This is called ‘over-pressure’. Pressure caused by blast wind is called ‘dynamic pressure’.

• Both these pressure decay rapidly with time.

• Pressure and overpressure sinks below ambient pressure before equalizing back to atmospheric pressure.

Air blast• This can penetrate basement areas that are open

causing high internal pressures and causing high velocity jet of air through openings.

• Window glass and debris shattered by shockwave acts as missiles in the air jet.

• High pressure causes lungs damage and ear drum rupture.

• Air-jets can pick people and hurl them to fixed objects.

Secondary fires

• These are triggered by blast damages.• Damaged gas pipes• Flammable building materials.• Electric short circuits.• Overturned appliances.

Guidelines for analysis

• Redundancy and alternate load paths.• Ductile structure elements.• Designing for load reversals.• Shear capacities which prevent shear failure

prior to flexural failure.

Design procedures

• There is no equivalent static design procedure available

• But both material linear and material non-linear capacities are considered and designed

• Work energy method , dynamic response and such solutions are found satisfactory.

1. Pressure- impulse• Pressure impulse diagrams are the graphically present

asymptotic limits for cases where change in peak pressures changes insignificantly over the time to maximum response and for cases where the blast load duration is on short with respect to time to maximum response.

2. Analysis using finite element methods• It includes calculation of non-linear dynamic single

degree freedom of member i.e. blast pressure/load and then comparing with calculated single degree of freedom response (i.e. calculation of blast load within response limit of the trial member)

• Here balanced design of beams is done considering beam is weaker than column and failure of beam is desired first.

• Sufficient shear transfer is provided to even slabs and transfer girders avoided near blast threat regions.

3. Redistribution of loads from removed column• Here column from structural frame is removed.• Missing column loads are distributed.• The frame is proportioned with sufficient strength

to resist twice the D.L and L.L .• Procedure may be conceptually incorrect but

provides desired solution.• Beams and columns assumed to distribute twice

the vertical loads.• Based on theory related to instantaneous

application of loads.• To prevent progressive collapse

Fig1. Redistribution of loads from removed column in building with a continuous moment resting frame along column lines

Fig2. Load resisting mechanisms upon removal of supporting columns

Ductile detailing of reinforcements• The need for fire resistance, strength and

ductility favors reinforced concrete as a construction material for floors. Blast-resistant design philosophy allows structural elements to undergo large inelastic (plastic) deformations in response to blast loading.

• A ductile structure that undergoes large deformations without failure can absorb much more energy than a brittle structure of the same static strength.

• Tensile reinforcement between 0.5 and 2 percent of the cross-sectional area of the concrete element will usually insure ductile behavior while providing the required strength.

• Compression steel in flexural members serves two purposes. After a structural member is deflected by blast loads, it attempts to spring back or rebound. Dynamic rebound causes load reversal and, under certain circumstances, can result in catastrophic failure

Acceptable Damage Levels• 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. However, if the building has been designed for loss of primary members, localized loss of columns may be accommodated without initiating progressive collapse. Injuries and possible fatalities are expected.

• Major: Loss of primary structural components such as columns or transfer girders precipitates loss of additional adjacent members that are adjacent or above the lost member. In this case, extensive fatalities are expected. Building is usually not repairable.

Difference between blast and seismic loads

Blast load• Explosion loads are single

high pressure impulses acting over milliseconds .

• Explosion loads act directly on the exterior envelope.

• Explosion loads generally cause localized damage.

• Mass helps resist explosion loads.

Seismic load• The vibrational load of

earthquakes which is acts over seconds.

• Earthquakes load acts at the base of the building.

• Seismic loads cause global response.

• Mass worsens earthquake response.

CONCLUSION• It is not practical to design buildings to

withstand any conceivable terrorist attack.• It is possible to improve the performance of

structures should one occur in the form of an external explosion.

• Design process to ensure that appropriate threat conditions and levels of protection are being incorporated.

References• Blast safety of the building envelope by Eve Hinman ,PE hinman engg report.• Structural design for external terrorist bomb attack by Jon A. Schmidt structure®

magazine march issue (2003)• Structure to resist the effects of accidental explosions U.S army nov 1990• Blast resistant design technology by Henry Wong WGA Wong Gregerson architects

Inc.• Constructing and deigning blast resistant buildings by G. Gehring and P. Summers

MMI engg Texas• Design of structure for blast related progressive collapse resistance by Ronald

Hamburger and Andrew Whittaker• Blast and progressive collapse Kirk A Marchand and Farid Afawakhiri AISC Inc.• Blast resistant design of R C structures by Dennis M. McCann and Stevan J. Smith

an internet webpage in dmccann@exponent.com