CALCULATION OF BLAST LOAD AS PER IS : 4991 - 1968

CALCULATION OF BLAST LOAD AS PER IS : 4991 - 1968BYSUNIL KUMAR

UNDER THE GUIDANCE OF Prof. DEVARAJ.VCONTENTSIntroductionDefinitionsGeneral characteristics of blast and effect on structuresBlast forceScaling lawBlast load on above ground structure

IntroductionOver the last decades considerable attention has been raised on the behavior of engineering structures under blast or impact loading. The use of explosives by terrorist groups around the world that target civilian buildings and other structures is becoming a growing problem in modern societies. Explosive devices have become smaller in size and more powerful than some years ago, leading to increased mobility of the explosive material and larger range effects. Usually the casualties from such a detonation are not only related to instant fatalities as a consequence of the direct release of energy, but mainly to structural failures that might occur and could result in extensive life loss therefore it is essential to consider appropriate blast loading for the design of structure.DefinitionBlast wave : An explosion can be defined as a very fast chemical reaction involving a solid, dust or gas, during which a rapid release of hot gases and energy takes place. The phenomenon lasts only some milliseconds and it results in the production of very high temperatures and pressures. During detonation the hot gases that are produced expand in order to occupy the available space, leading to wave type propagation through space that is transmitted spherically through an unbounded surrounding medium. The blast wave contains a large part of the energy that was released during detonation and moves faster than the speed of sound.General characteristics of blast and effect on structuresShock wave : As a result of explosion, a shock wave is generated in the air which moves outward in all directions from the point of burst with high speed causing time-dependent pressure and suction effects at all points in its way. The shock wave consists of an initial positive pressure phase followed by a negative (suction) phase at any point as shown in Fig. I. The shock wave is accompanied by blast wind causing dynamic pressures due to drag effects on any obstruction coming in its way. The peak values depend upon the size of explosion, the distance of the surface from the source, and other factors like ambient pressure and temperature in air.The peak positive intensity quickly drops down to zero the total duration of the positive phase being a few milliseconds. The maximum negative overpressure is much smaller than the peak positive overpressure, its limiting value being one atmosphere. But the negative phase duration is 2 to 5 times as long as that of the positive phase.

Blast ForceMaximum values of reference explosion : The maximum values of the positive side-on overpressure pso, reflected over pressure pro and dynamic pressure qo, as caused by the explosion of one tone explosive at various distances from the point of explosion, the duration of the positive phase of the blast t, and the equivalent time duration of positive phase td are given in Table 1 of the IS 4991-1968.Decay of pressure with time : The pressure varies with time according to the following relation.

Table - 1 From IS : 4991 - 1968

Notes :The value of pa the ambient air pressure may be taken as 1 kg/cm2 at mean sea level.One tone of explosive referred to in this table is equivalent to 1.5 X 109 calories.Velocity of sound in m/s may be taken ( 331.5 + 0.607 T ) where T is the ambient temperature in centigrade.Scaling law For any explosion other than the reference explosion, the peak pressures and time durations may be found from the peak values given in Table 1 by the cube root scaling laws as given below:

Where,W = yield of explosion in equivalent weight of the reference explosive measured in tones,x = scaled distance for entering the Table 1 for reading peak values, andto = scaled time read from Table 1 against scaled distance.

NOTE - Actual distance is measured from the ground zero to the point under consideration. Actual time is the time for actual explosion.Blast load on above ground structureTypes of structure : There are mainly two types of structure:Diffraction type of structures : these are closed structures without openings, with total area opposing the blast. These are subjected to both shock wave overpressure and the dynamic pressure caused by blast wind.Drag type of structure : These are the open structures composed of elements like beams, columns, trusses, etc. which have small projected area opposing the shock wave. These are mainly subjected to dynamic pressure q

Closed rectangular structure : Front Face As the shock wave strikes the vertical face of a structure normal reflection occurs and the pressure on the front face instantaneously increases to the reflected overpressure pro given by the following equation:

Where,Pa = the ambient atmospheric pressure.

Above ground rectangular structure

The net pressure acting on the front face at any time t is the reflected overpressure pr. or ( ps + Cd q), whichever is greater:whereCd = drag coefficient given in Table 2, andpr = the reflected overpressure which drops from the peak value pro to overpressure (ps + Cd q) in clearance time tc, given by:tc = 3S/U or td whichever is lesswhere,S = H or B/2 whichever is less (as shown in Fig)U = shock front velocity = M x awhere,a = velocity of sound in air which may be taken as 344 m/s at mean sea level at 20oC, andM = Mach number of the incident pulse given Sqrt(1+6Pso/7Pa). The values of M for various conditions are also tabulated in Table 1.The net average loading on the front face (B x H) as a function of time is shown in below fig. depending on whether tc, is smaller than or equal to td. The pressures Pro, Pso and q0 and time td are for actual explosion. Determined according to the scaling laws

Pressure versus time for front face

Rear Face - Using the pressures for the actual explosion, the average loading on the rear face is taken as shown in below fig. where the time has been considered from the instant the shock first strike the front face. The time interval of interest are the following.L/U = The travel time of shock from front to rear face, and4S/U = Pressure rise time on back face.

Pressure versus time for rear face

Roof and Side Walls : As considered for rear face the average pressure versus time curve for roof and side walls is given in below fig. when td is greater than the transit time tt = L/U. When tt is greater than td the load on roof and side walls may be considered as a moving triangular pulse having the peak value of overpressure ( Pso+ Cd q0 ) and time td as shown in below fig.

Average pressure diagram when td > t

Moving pressure pulse for tt > td

Table -2 From IS 4991 - 1968

ExampleBlast parameters due to the detonation of a 0.1 tone explosive are evaluated on an above ground rectangular structure, 3 m high, 10 m wide and 8 m long, situated at 30 m from ground zero.a) Characteristics of the blastScaled distance x = 30 / (0.1)1/3 = 64.65m

Referring to Table 1 assuming Pa = 1.00 kg/cm2 and linearly interpolating between 63 m and 66 m for the scaled distance 64.65 m, the pressure are directly obtained.

Pso = 0.35 kg/cm2Pro = 0.81 kg/cm2qo = 0.042 kg/cm2

The scaled times to and td obtained from Table 1 for scaled distance 64.65 m are multiplied by (0.1 )1/3 to get the values of the respective quantities for the actual explosion of 0.1 tone charge.to = 37.71 (0.1)1/3 = 17.5 millisecondtd=28.32 (0.1)1/3 = 13.15 millisecondM=sqrt(1+(6/7)(Pso/Pa)) = 1.14a = 344 m/s U = 392 m/s = 0.392 m/millisecond

b) Pressure on the buildingHere H= 3m, B=10m, and L=8m then S = 3mthen S=3mtc=3S/U=(3x3)/0.392=23.0millisecond > tdtt=L/U=8/0.392=20.4millisecond > tdtr=4S/U=(4x3)/0.392=30.6millisecond>tdAs tr>td no pressure on the back face is considered.For roof and side Cd=-0.4Pso+Cd q0=0.35-0.4x0.042=0.33 kg/cm2The pressure diagrams are shown below

Recommended values of blast pressure for designFor general guidance the buildings may be designed for a bare charge of 100 kg at distances given in Table 7.Table - 7

Load combinationWind or earthquake forces shall not be assumed to occur simultaneously with blast effects. Effects of temperature and shrinkage shall be neglected.Live load on floors shall be considered as per IS : 875-I 964* depending upon the class of building. No live load shall be considered on the roof at the time of blast.THANK YOU