Blast load and its analysis

Blast Load and Its AnalysisDesign of Blast Resistant Structures

Typical Examples of Blasts in several important

structures

Attack on World Trade Centre on 11/09/01

Khobar Towers bombing in Saudi Arabia 25/06/1996

Oklahoma City Bombing 19/04/1995

Need:

Increase in number of terrorist attacks.

To minimize damage to the assets

To minimize the loss of life.

To protect historical monuments and important buildings

To subside social panic

Moving vehicle attack

Stationary vehicle

bomb

Exterior attack

Arsons

Others

Ballistic attacks

What is blast ?The detonation of a condensed high explosive generates hot gases under pressure upto 300 kilobar and a temperature about 3000-4000°C. The resulted compressed air expands outward from the centre of the blast and causes formation of wave in a wind medium having a velocity greater than sound. So this blast wave causes air pressure to rise and which is known as side on overpressure. As the blast wave or shock wave traverses path the overpressure gradually decreases. Now a vaccum pressure zone gets generated behind the shock wave.

An 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

Response of blast at various distances from the source of explosion

Figure: Blast wave pressure – Time history

Pso = Peak side on overpressurePso - = Negative max overpressuretd + is always less than td – and Pso + is always greater than Pso -

Design ConsiderationAs the impulse of the negative zone is less than the impulse of the positive zone, the negative face is usually not taken into account for the design purpose.

1.Explosive charge weight 2.Stand off distance

Determining factors for blast parameter

This is the distance from the source of explosion at which the blast effect caused by standard charge weight is just equivalent to as caused by W charge at distance R .Scaled distance Z =

Scaled Distance

Empirical relationships :1.Brode

(1955) :

2. Newmark and Hansen (1961) :

3. Mills (1987) :

Dynamic pressure:

The air behind the front of the blast wave moves along the same direction as wind but with smaller velocity. These winds result in loading of duration more than positive phase. So the pressure caused by these winds is called dynamic pressure. q(t) =qo * f(t)

The maximum value of dynamic pressure = Fig: Positive phase duration of over pressure

(parentheses) and dynamic pressure for 1 KN burst

The positive phase duration for overpressure and dynamic pressure can be selected from fig

Reflected Pressure :When wave front is obstructed by a structure a reflected blast wave gets generated having higher pressure than the incident pressure. This pressure is called reflected pressure. The reflection factor depends upon the orientation of the structure.

The maximum reflected overpressure =

Where,Po = atmospheric or ambient pressure

As Pr > Pso , Pr is taken as design parameter.

Fig : Reflected overpressure

IS Code provision:

As per IS 4991 – 1968 , the value of the Pso, qs , Pr

computed from Table 1 for 1 tonne detonation amount.

The pressure time relationship in the positive phase are idealised by using a straight line starting with the maximum pressure value but terminating at a time td or tq .

Fig : Assumed Plot

Fig : Actual Plot

Blast load on above ground structure:

Closed Rectangular StructureOpen or Drag type structures

There are two types of structures :

Closed Rectangular Structure

Front face—The net pressure acting on the front face at any time t is reflected overpressure Pr or (Ps + Cd * q) whichever is greater.

Terminology : Cd = drag coefficient given in Table – 2 of IS 4991 - 1968 Pr = reflected overpressure which drops from the peak value Pro to overpressure (Ps + Cd * q) in clearance time tc

tc = Where S = H or B/2 whichever is less U = Shock front velocity = M.a Where a= velocity of sound in air which may be taken as 344m/s at mean sea level at 20° C and M = Mach no. of the incident pulse given by The value of the M for various conditions are also tabulated in Table -1 of IS 4991 - 1968

The maximum reflected overpressure,The net average loading on the front face (B X H) as function of time is

shown in fig depending on whether tc is smaller than or equal to td. The pressure Pro, Pso and qo and time td are for actual explosion determined according to the Scaling laws.

The average loading on the rear face (B X H) is taken as shown in Fig.-7 where the time has been reckoned from the instant the shock first strikes the front face. The time interval are the following: = The travel time of shock from front to rear face = Pressure rise time on back face

Rear face:

Roof and side walls :

When tr > td ,the load on roof and side walls may be considered as moving triangular pulse having the peak value of overpressure Pro or (Ps0 + Cd * q0) and time td as shown in fig 7b.

When td > Transit time (tr) = ,the pressure diagram is given in fig.

Open or Drag Type StructureThe net translation pressure on the obstructing areas of element may be taken as shown in fig.

Where Cd= drag coefficient depending upon shape of the structure tq = ½ to

Blast Load

Problem on

Problem : Calculate the impulse of a blast load on a building for positive phase using the given parameters.Given Parameters

Detonation Amount 0.1tonne

Distance from ground Zero 30m

Height of Building 3m

Width of Building 10m

Length of Building 8m

Pa (ambient pressure) 1kg/cm2

a) Characteristics of the blast Scaled Distance 64.63304m

From table -1 Assume Pa=1 kg/cm2 P 0.35367kg/cm2 Pr 0.806452kg/cm2 Q 0.04219kg/cm2 t 37.70826mil sec t 28.32257mil sec Actual t 17.50262mil sec Actual t 13.14617mil sec M (Mach No.) 1.141554 Velocity of sound 344m/s Shock front velocity (U) 392.6946m/s 0.392695m/mil sec

b)Pressures on the building S (H or B/2 whichever is less) 3 m

Clearance time(t) 22.91858mil sec> td Transit time (t) 20.37207mil sec

Pressure rise time on back face (t) 30.5581mil sec

0 2 4 6 8 10 12 140

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.81

0

Average front face loading

Time (mili sec)

Peak

refl

ecte

d ov

erpr

essu

re

(Kg/

cm2)

Solution: From the graph-Total impulse of force = = (0.81+0) * 13.15 * 102 *(Area) = (0.81+0) * 13.15 * 102 *10*3 N-Sec = 15977 N-sec

Problem: A 25m high full water tank is subjected to a force as shown in the previous graph. Natural time period Tn = 1.12 sec.The force is caused by above ground explosion. Neglecting effect of damping, calculate base shear & moment at the base of tower supporting the tank. The mass of the water tank = 10000 kg.

Lateral deflection at any time, u = umax = = = 0.285 m

Base Shear = K*umax = mw2 *umax

= 10000 * (2∏/1.12)2 * 0.285

= 89604 N = 89.6 KNMaximum Bending Moment = Base Shear * Height = 89.6 * 25 = 2240 KN-m

Variation of Peak Reflected Overpressurewith Detonation Amount

Charge Weight (tonne)

Pr(kg/cm2)

t (mil sec)

t (mil sec)

0.2 1.266034 13.91818 21.72694

0.3 1.679215 14.40166 20.87335

0.4 2.102653 14.5682 20.13651

0.5 2.468371 14.83824 19.58105

1 4.2 15.39 17.63895

Stand off Distance = 30m

0 2 4 6 8 10 12 14 160

0.2

0.4

0.6

0.8

1

1.2

1.4

0

1.266034

Reflected overpressure Vs td

Equivalent Pulse time (mili sec)Refle

cted

ove

rpre

ssur

e (k

g/cm

2)

For Charge weight = 0.2 tonne

0 2 4 6 8 10 12 14 1600.20.40.60.8

11.21.41.61.8

0

1.679215



Refle

cted

ove

rpre

ssur

e (

kg/c

m2)

Equivalent Pulse time (mili sec)

0 2 4 6 8 10 12 14 160

0.5

1

1.5

2

2.52.102653

0



Refle

cted

ove

rpre

ssur

e (

kg/c

m2)



Refle

cted

ove

rpre

ssur

e (

kg/c

m2)

Equivalent Pulse time (mili sec)0 2 4 6 8 10 12 14 160

0.5

1

1.5

2

2.5

3

2.468371

0


0 0.5 1 1.5 2 2.5 3 3.5 4 4.502468

1012141618

0

15.39


For Charge weight = 1 tonne

Refle

cted

ove

rpre

ssur

e (

kg/c

m2)


Tonn

e

Pr

0.1 0.8064520.2 1.2660340.3 1.6792150.4 2.1026530.5 2.468371

1 4.2

0 0.2 0.4 0.6 0.8 1 1.20

0.5

1

1.5

2

2.5

3

3.5

4

4.5

f(x) = 0.64899312172 x³ − 1.82762927891 x² + 5.06024347196 x + 0.31832085472R² = 0.999963434967115

Detonation Wt. vs Peak Re-flected Pressure

Detonation Wt (tonne)

Peak

Refl

eced

Pre

ssur

e (K

g/cm

2)

Variation of Peak Reflected Overpressurewith Stand-off Distance

Stand off Distance

(m)

Pr(kg/cm2)

t (mil sec)

t (mil sec)

10 12.13074 4.48978 13.6224

15 3.62087 7.473143 18.19483

20 1.857502 9.611829 20.55338

25 1.146493 11.4432 22.0064

30 0.806452 13.14617 22.91858

Detonation amount = 0.1 tonne

For stand off Distance = 10 m

Refle

cted

ove

rpre

ssur

e (

kg/c

m2)

Equivalent Pulse time (mili sec)0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

2

4

6

8

10

12

1412.13074

0


Refle

cted

ove

rpre

ssur

e (

kg/c

m2)



0 0.5 1 1.5 2 2.5 3 3.5 40

1

2

3

4

5

6

7

8

0

7.473143


Refle

cted

ove

rpre

ssur

e (

kg/c

m2)



0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

2

4

6

8

10

12

0

9.611829

Refle

cted

ove

rpre

ssur

e (

kg/c

m2)



0 0.2 0.4 0.6 0.8 1 1.2 1.40

2

4

6

8

10

12

14

0

11.4432


Refle

cted

ove

rpre

ssur

e (

kg/c

m2)



0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

2

4

6

8

10

12

14

0

13.14617


Distance from ground Zero (m)

Pr(kg/cm2)

10 12.1307411 8.92781112 6.83063813 5.26922714 4.15947915 3.6208716 3.10677717 2.65202220 1.85750230 0.806452

0 5 10 15 20 25 30 350

2

4

6

8

10

12

14

f(x) = 3013.46426515556 x^-2.45810607648953R² = 0.987446828711927

Distance from ground zero vs Peak reflected overpressure plot (for 0.1tonne detonation

amount)

Distance from ground zero (m)

Peak

refl

ecte

d pr

essu

re

(kg/

cm2)

Scope of the Work :• The main objective of the present study is to design a G+6 storey RCC building considering blast load, earth quake load, wind load.

• It is required to compare the design of the same building without blast load and including the blast load.

• Ductile detailing of the structural members need to be done.

References:

• Ngo, T., Mendis, P., Gupta, A. & Ramsay, J. “Blast Loading and Blast Effects on Structures – An Overview” The University of Melbourne, Australia (2007)

• Activity A5 - Blast Simulation Technology Development “Calculation of Blast Loads for Application to Structural Components” Administrative Arrangement No JRC 32253-2011 with DG-HOME (2013)

• IS 4991 ‘‘CRITERIA FOR BLAST RESISTANT DESIGN OF STRUCTURES FOR EXPLOSIONS ABOVE GROUND” (1968)

• Remennikov, A “A Review of Methods for Predicting Bomb Blast Effects on Buildings” University of Wollongong (2003), [email protected]

• Draganić, H., Sigmund V.“BLAST LOADING ON STRUCTURES” (2003)

• NBM Media article on “Blast Loading and Its Effects on Structures”

Blast load and its analysis

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