blast resistant structures
- 1. Design of Blast resistant structures
- 2. 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
- 4. Principles of blast resistant designMaintain 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 membersMinimize the quantity and hazard of broken glass and blast induced debris.Facilitate rescue and recovery operation with adequate time of evacuation of occupants.
- 5. Stand-off zonesBlockades, 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)
- 6. 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
- 8. Thermal energy radiationMuch of the thermal energy is absorbed .Combustible materials gets ignited.These act fuel for building materials to be burntLife inside also get burnt.
- 9. Effects of shock-wavesThis 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.
- 10. Pressure and overpressure sinks below ambient pressure before equalizing back to atmospheric pressure.
- 11. Air blastThis 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.
- 12. Secondary firesThese are triggered by blast damages.Damaged gas pipesFlammable building materials.Electric short circuits.Overturned appliances.
- 13. Guidelines for analysisRedundancy and alternate load paths.Ductile structure elements.Designing for load reversals.Shear capacities which prevent shear failure prior to flexural failure.
- 15. Design proceduresThere is no equivalent static design procedure available
- 16. But both material linear and material non-linear capacities are considered and designed
- 17. Work energy method , dynamic response and such solutions are found satisfactory.1. Pressure- impulsePressure 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.
- 18. 2. Analysis using finite element methodsIt 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)
- 19. Here balanced design of beams is done considering beam is weaker than column and failure of beam is desired first.
- 20. Sufficient shear transfer is provided to even slabs and transfer girders avoided near blast threat regions.3. Redistribution of loads from removed columnHere column from structural frame is removed.
- 21. Missing column loads are distributed.
- 22. The frame is proportioned with sufficient strength to resist twice the D.L and L.L .
- 23. Procedure may be conceptually incorrect but provides desired solution.
- 24. Beams and columns assumed to distribute twice the vertical loads.
- 25. Based on theory related to instantaneous application of loads.
- 26. To prevent progressive collapse
- 27. Ductile detailing of reinforcementsThe 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.
- 28. 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.
- 29. 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
- 30. Acceptable Damage LevelsMinor: Non-structural failure of building elements as windows, doors, and cladding. Injuries may be expected, and fatalities are possible but unlikely.
- 31. 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.
- 32. Difference between blast and seismic loadsBlast loadExplosion loads are single high pressure impulses acting over milliseconds .
- 33. Explosion loads act directly on the exterior envelope.
- 34. Explosion loads generally cause localized damage.
- 35. Mass helps resist explosion loads.Seismic loadThe 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.
- 37. CONCLUSIONIt 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.
- 38. ReferencesBlast safety of the building envelope by Eve Hinman ,PE hinmanengg 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 1990Blast 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 TexasDesign of structure for blast related progressive collapse resistance by Ronald Hamburger and Andrew WhittakerBlast and progressive collapse Kirk A Marchand and FaridAfawakhiri AISC Inc. Blast resistant design of R C structures by Dennis M. McCann and Stevan J. Smith an internet webpage in dmccann@exponent.com