Introduction <br />One of the most popular design issue.<br />Terror attacks and accidents.<br />Subject is popularly applied in modern and important buildings.<br />Emerging branch in the field of structural engineering<br />
Principles of blast resistant design<br />Maintain safe separation of attackers and targets i.e. STAND-OFF zones.<br />Design to sustain and contain certain amount of bomb damage. Avoid progressive collapse of the building.<br />Allow for limited localized damage of members<br />Minimize the quantity and hazard of broken glass and blast induced debris.<br />Facilitate rescue and recovery operation with adequate time of evacuation of occupants.<br />
Stand-off zones<br />Blockades, planters, fountains, fences as obstacles to ramming vehicles/truck bomb.<br />Allow only emergency vehicle access. <br />Raise the building 2m above ground level (provide Ramps for barrier free access)<br />
Blast load definition<br />An explosion is a rapid release of potential energy characterized by eruption enormous energy to the atmosphere.<br /> 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<br />
Thermal energy radiation<br />Much of the thermal energy is absorbed .<br />Combustible materials gets ignited.<br />These act fuel for building materials to be burnt<br />Life inside also get burnt.<br />
Effects of shock-waves<br />This travels away from explosion faster than the speed of sound poses threat in close location.<br />Shock front is similar to “moving wall” of highly compressed air accompanied by blast wind.<br />It causes sudden rise in ambient pressure. This is called ‘over-pressure’. Pressure caused by blast wind is called ‘dynamic pressure’.<br />Both these pressure decay rapidly with time.<br />
Pressure and overpressure sinks below ambient pressure before equalizing back to atmospheric pressure.<br />
Air blast<br />This can penetrate basement areas that are open causing high internal pressures and causing high velocity jet of air through openings.<br />Window glass and debris shattered by shockwave acts as missiles in the air jet.<br />High pressure causes lungs damage and ear drum rupture.<br />Air-jets can pick people and hurl them to fixed objects. <br />
Secondary fires<br />These are triggered by blast damages.<br />Damaged gas pipes<br />Flammable building materials.<br />Electric short circuits.<br />Overturned appliances.<br />
Guidelines for analysis<br />Redundancy and alternate load paths.<br />Ductile structure elements.<br />Designing for load reversals.<br />Shear capacities which prevent shear failure prior to flexural failure.<br />
Design procedures<br /><ul><li>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.</li></li></ul><li>1. Pressure- impulse<br />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.<br />
2. Analysis using finite element methods<br /><ul><li>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.</li></li></ul><li>3. Redistribution of loads from removed column<br /><ul><li>Here column from structural frame is removed.
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 </li></li></ul><li>
Ductile detailing of reinforcements<br /><ul><li>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. </li></li></ul><li><ul><li>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</li></li></ul><li>
Acceptable Damage Levels<br /><ul><li>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.</li></li></ul><li>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.<br />
Difference between blast and seismic loads<br />Blast load<br /><ul><li>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.</li></ul>Seismic load<br />The vibrational load of earthquakes which is acts over seconds.<br />Earthquakes load acts at the base of the building.<br />Seismic loads cause global response. <br />Mass worsens earthquake response.<br />
CONCLUSION<br />It is not practical to design buildings to withstand any conceivable terrorist attack.<br />It is possible to improve the performance of structures should one occur in the form of an external explosion.<br />Design process to ensure that appropriate threat conditions and levels of protection are being incorporated.<br />
References<br />Blast safety of the building envelope by Eve Hinman ,PE hinmanengg report.<br />Structural design for external terrorist bomb attack by Jon A. Schmidt structure® magazine march issue (2003)<br />Structure to resist the effects of accidental explosions U.S army nov 1990<br />Blast resistant design technology by Henry Wong WGA Wong Gregerson architects Inc.<br />Constructing and deigning blast resistant buildings by G. Gehring and P. Summers MMI engg Texas<br />Design of structure for blast related progressive collapse resistance by Ronald Hamburger and Andrew Whittaker<br />Blast and progressive collapse Kirk A Marchand and FaridAfawakhiri AISC Inc.<br /> Blast resistant design of R C structures by Dennis M. McCann and Stevan J. Smith an internet webpage in email@example.com<br />