BLAST from the collapse of the structure, impact

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Last updated: August 31, 2019

BLAST LOAD EFFECT OF SIX STORIED FRAMED STRUCTUREWITH SHEAR WALLHanish Chundi, Graduate Student, Carleton University,Ottawa ON Canada.

 Abstract: This paper presents the lateral stability of a 6 storiedshear wall building subjected to blast load. The fundamentals of blast threats and also that of blast waveinteraction with the buildings area studiedhere. This deals with analyzing the structure’s lateral stability inSAP2000. The Six story building is exposed to 500Kg, 400Kg, 300kg, 200kg, 100kgTNT with stand-off distance of Six meters. Section 5 of TM5-1300(UFC 3-340-02)is used for calculating the blast loads for the modelled structure. Dynamicloads of the blast, Nonlinear modelled Time History analysis is used foranalyzing the structure.

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The total drift and displacement of the structures arethe main parameters that are compared with different types of explosives. 1  Introduction As several research papers andauthor defines explosion as an outburst of huge scale of energy andmedium for explosion many be considered as solid or liquid that regulates the speed of thereaction. At present special concern interest is been given for quick-tempered loads on breakthroughstructures, like publicbuildings, high rise buildings, delegations, economic institutions and important buildings of nationalinheritance. Explosions occurs as a result of accidental or unexpected incidents, while unprepared explosive devices, weapon systems anddemolition threats deal with intentional explosions. Examples of someaccidental explosions are burning of flammable solutions and gas due toimproper handling of hassled containers.

Explosives still are the strongest weapon forsome fanatic organizations, for theirday do day work, move from one place to another. The explosion ofany type of explosive in an around a building will cause terrible impacts on the structural integrity of the structure, such as depletion of strength ofinterior and external structural elements like beams, columns slabs and collapse of walls. Loss ofsocial life may occur due to from the collapse of thestructure, impact ofdirect blast, impact dur to flying fragments, fire, smoke and several other reasons. Several cities became target of bomb explosionattacks within the lastdecades either by accident or by intentionally. Some terrorist groups havetargeted buildings round the world.The cases of those attacks have tobe considered for effect of the exposure of buildings to explosion. Inthis paper we have described the available literature on blast load andinterpreted the results with an Structural analyzing software with nonlinearcapabilities which is SAP200 V-15. 2 Explosion The major use of explosionis in: Military applications, Construction or development works like demolitionof old buildings, etc.

It is, also, a very common weapon used by terrorist because it is accessible. Table 1 shows theestimated quantity of explosives that can be fitted in various vehicles.  Table 1 Estimated Quantity ofexplosions of various vehicles Vehicle Type Charge mass/Kg Compact car trunk 115 Trunk of a large car 230 Closed van 680 Closed truck 2270 Truck with a trailer 13610 Truck with two trailers 27200  In order to activate the explosives they haveto be stable and inert, which means the explosion is triggered effect ratherthan a spontaneous one. The explosion is a phenomenon where there will be a rapidand abrupt release of energy. Most of the explosions are detonated byexcitation of inert material which converts in to very hot, dense gas underhigh pressure which results in a release of strong explosion wave. One thirtythree percent of the total chemical energy is released by detonation while theremaining sixty six percent are slowly released during the blast as thesurrounding air and burnt debris mixes with the explosive product. Theexplosion effects are represented in a wave form of high intensity that growsoutwards from the detonation point to the surrounding air.

As the wave passesit depletes its strength and speed as shows in Figure 1.           Figure: 1 Variation of Blast Pressure withDistance 1 After a short period, the pressure willdrop below the ambient pressure in the front (figure 1). During which anegative pressure phase, a partial vacuum is created, this is also accompaniedby high suction winds that carry the debris back to the demolition sourceleading to even more destruction to the surrounding elements. The time historyof blast wave is depicted in Figure 2         Figure: 2 Time History of Blast wave 23.Structural Response to Blast Loading Analyzing the dynamic response ofblast loading on structures is a complex process which involves the effect ofhigh strain rates, non-linear inelastic behavior of materials the timedependent deformation of structures and the uncertainty of blast loadcalculations.

  In order to simplify theanalysis various assumptions are taken related to the structural response andthe loads has been proposed and widely accepted. In order to simplify theanalysis procedure, the structure is considered as a single degree of freedom(SDOF) system and the relation between the positive duration of the blast loadwith the natural period of vibration of the structure is determined. Thisresults in idealization and simplifies the of blast loads calculation.   3.1Elastic Single Degree of Freedom The easiest way to solve a transientproblem is by means of Single degree of freedom approach. The structure can bereplaced with an system of one concentrated mass at each level and one springrepresenting the resistance of the structure against deformation. A pictorialrepresentation of the above has been shown in figure 3.

M represents structuralmass, the whole system is under the effect of external force F with respect totime t, i.e, F(t), with structural resistance R, vertical displacement isexpressed in terms of y, and spring constant as K.         Figure:3 Representation of Single Degree offreedom system 2   The time history of the blast loadingcan be idealized as a triangular pulse having a force of Fm as peak force withpositive phase duration of Td as shows in figure 5. The forcingfunctions are represented as            The blastimpulse is approximated as the area under the force-time curve and is given as (2)       The equationfor motion of un-damped elastic single degree of freedom for time ranging from0 to positive phase duration td,is given by Biggs (1964) as (3)                                                                                       Figure:4 Time History Graph 4 Modelling of Structure Sixstoried reinforced concrete structure with 4 rows and 3 bays or each spanning 6meters and 5 m respectively in dimensions.

The height of first story is 5 m andthe remaining to be 4 m. Shear wall is placed at the mid span through out theheight of the structure. Elevation of the structure is shown in figure 5.

Thedimensions and design of the structural elements are designed according toIS-456 and SP-16 of India codes under normal loading conditions as momentresisting frame. The detailing of the elements are shown in the figure 6.                   FigurFigure 5 Modelled structure in SAP2000                                                                                    a) Typical Beam Detailing                                                 b) Typical Column Detailing       c) Detaining of Shear wallFigure:6 Detailing of elements Grade of concrete usedin M25 5 Blast Load Determination  Fromthe procedure described in section 5 of TM5 – 1300 (1990): Structures to resistthe effect of accidental explosions, which was developed by US Department ofDefense are used to calculate the maximum blast load that can be applied to theframed structure and its structural elements of the building. The blast loadsare calculated for various charge weights of TNT like 500Kg, 400Kg, 300Kg, 200Kg,100Kg which are exploded at 6m standoff distance form the structure.

Thecalculated blast load is distributed all over a single side of the building.Dynamic parameters of the blast loads which are duration, Time of arrival,shock front velocity, impulse are calculated accordingly. The blast loading isconsidered as pressure loading on the structure. The calculated pressure isconverted into point loads in each nodes in the various levels of the structurein the front face using time history function. For instance, the total pressuredue to blast load on the top floor is multiplied with surface area of therequired structure, which has 5 columns on the front face. The calculated loadis evenly divided to the 5 nodes. Table 2 gives the variation of blast pressurein bay 3, along the height if the structure for a 100Kg TNT charge weightexploded at 6 m standoff distance.

 Table 2 Variation of Blast Pressure on Bay 3 alongthe height of the structure Bay 10 Height (m) A (deg) Arrival time (ms) Load duration(ms) Pr (psi) Load (kN) Story 1 5 13.54 6.38 2.64 228 10997 2 9 34.95 7.95 3.07 148 5354 3 13 48.

67 10.79 4.63 70 2532 4 17 58.4 14.85 3.36 37 1338 5 21 65.38 19.

93 7.67 24 868 6 25 70.02 25.77 9.12 16 579 Thegraph shown on figure 7 shows the time history function defined in SAP2000 foranalysing the structure for Blast pressure with varying load with respect totime                      Figure 7 Time history function with for 100kG ChargeTNT 6 Results and Discussions Assessingthe structural behaviour under blast loading is very difficult and depends onvarious factors which affects the performance and structural integrity of thestructure.

In order to investigate the lateral stability of the structuredynamic analysis is carried out. Figure 8 represents the maximum lateraldeflection of the structure with different charges which are 122.7mm, 94.68mm,74.98mm, 58.54mm and 35.

4mm for 500kg, 400kg, 300kg, 200kg, 100kg of chargeweight of TNT respectively at stand off distance of 6m.  Accordingto Indian Standard code of 1893 the maximum allowable lateral Displacement isH/500 which is 50mm. So only 100Kg TNT charge satisfies the maximal lateraldisplacement of 35.4 mm and the rest charges are not satisfying with the IScode provision.

 Themaximum inter story drift for 500kg, 400kg, 300kg, 200kg, 100kg of chargeweight of TNT are 25.94mm, 22.42mm, 17.

78mm, 13.79mm, 8.39mm respectively whenplaced at a standoff distance of 6m.

Figure 9 represents the inter story driftof the structure with various charge of TNT. As the codal provision of IS1893the maximum allowable inter story drift is 0.004 x h, where h is the storyheight which is 20mm. Therefore, the maximum inter story drift are satisfied by300kg, 200kg, 100kg charge of TNT which others exceed the permissible limit. Thevariation of pressure with different charge weights at standoff distance of 6mis shows in figure 11                      Figure 8 Maximum Displacement of the structure   Figure 9 Inter story drift of the structure   The StressDistribution of the shear wall bay of the structure is shown in figure 10                       Figure 10 Stress Distribution of the shear wall under100kg TNT Charge DifferentVariation of blast pressure is for different charge weight at 6m standoffdistance along the height od the structure are shown in Table 3  Table 3 variation of Blast pressure along the heightof the structure with different charge weights at 6m standoff distance Story Different Charge weight of TNT 100kg 200kg 300kg 400kg 500kg 1 303 593 853 1095 1323 2 159 326 489 640 783 3 64 120 183 248 315 4 37 59 81 107 133 5 24 36 46 56 67 6 16 24 31 37 43                   Figure 11variation of pressure with different charge weights at standoff distance of 6m From theabove graph it is found that reflective positive pressure increases withincrease in charge weight. 7 Reference 1.

    CSI AnalysisReference Manualfor SAP2000, (2007).Computers and Structures Inc., USA.2.    Department of Defense DOD. 2003.”Unified Facilities Criteria UFC DoD minimum antiterrorism standards forbuildings.” UFC-4-010-01, Washington, D.

C.3.    DOE/TIC-11268. A Manual for the Prediction ofBlast and Fragment Loading on Structures, U.

S. Department of Energy, WashingtonDC, 1992.4.    IS 456: 2000. “IndianStandard Code of Practicefor Plain and Reinforced Concrete”, Bureau of Indian Standards, New Delhi.5.

    Lucia FIGULI, Vlasdimir KAVICKY, Kamil BOC, Dagmar VIDRIKVA,Stefan JANGl “ANALYSIS OF BLAST LOADED STRUCTURES” Science andMilitary.6.    SP 16: 1980. “Design Aids for Reinforced Concrete to IS 456:2000″Bureau of Indian Standards, New Delhi.7.

    Van Der Meer, L. J.  Dynamic response ofhigh-rise building structure to blast load, Research report: A-2008.3, O-2008; 2008            

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