BLAST like demolition of old buildings, etc. It

BLAST LOAD EFFECT OF SIX STORIED FRAMED
STRUCTURE WITH SHEAR WALL

Hanish Chundi, Graduate Student, Carleton
University, Ottawa ON Canada.

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Abstract: The main aim of  this
report is to show the lateral stability of a G+5 storied shear wall building
under blast loading condition. The basic interaction and stability of the
structure to the blast load is determined here. This deals with determining the
structural stability in SAP 2000 using time history function. The framed shear
wall building is subjected to a blast charge   of 0.5 Ton, 0.4Ton, 0.3Ton,
0.2Ton, 0.1Ton of  TNT with stand off
blast distance of 7m.  Total lateral
displacement and inter story displacement are the main parameters to be
discussed here. The building is analysed with dynamic loading as non linearly
analyzed structure with time history function.

 

1  Introduction

 

Explosion is usually
described as sudden burst or release of energy with excessive heat and freely
flying fragments either small or large depending on the intensity of the blast.
Now a days separate codal provisions and standards are been given for
structures like public buildings, high rise buildings, economic institutes and
other national heritages to withstand such unpredictable loads. Explosion is
usually accidental or uncontrolled phenomenon’s while dealing with weapons
systems or highly explosive materials improperly. Examples of such accidents
are burning or gas pipelines or flammable solutions due to improper handling of
such materials. Some organizations consider explosives as their strongest
weapons for their dad to day work like building demolishers, etc. Detonation of
any kind of explosive in or around a structure can cause terrible impact on
structural integrity of the structure and degrades the strength of it structural
elements, sometime even loss of social life due to flying fragments of debris
and progressive collapse of structure and many other reasons. As important
cities and places has become a victim of blast either intentionally or
accidentally, it has been a major factor to study the structure when subjected
to blast pressure before construction or to study the stability of a structure in
case or any explosion entire accidentally or intentionally. In this paper we
have described the available literature on blast load and interpreted the
results with an Structural analyzing software with nonlinear capabilities which
is SAP200 V-15.

 

 

2 Explosion

 

Explosives are mostly used in:
Defence applications, Construction or development works like demolition of old
buildings, etc. It is one of the most
common weapon used by terrorist because it is
accessible easily. Table 1 shows the estimated quantity of explosives
that can be fitted in various vehicles.

 

Table 1 Estimated Quantity of
explosions 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 have
to be stable and inert, which means the explosion is triggered effect rather
than a spontaneous one. The explosion is a phenomenon where there will be a
rapid and abrupt release of energy. Most of the explosions are detonated by
excitation of inert material which converts in to very hot, dense gas under
high pressure which results in a release of strong explosion wave. One thirty
three percent of the total chemical energy is released by detonation while the
remaining sixty six percent are slowly released during the blast as the
surrounding air and burnt debris mixes with the explosive product. The
explosion effects are represented in a wave form of high intensity that grows
outwards from the detonation point to the surrounding air. As the wave passes
it depletes its strength and speed as shows in Figure 1.

 

 

 

 

 

 

 

 

 

 

Figure: 1 Variation of Blast Pressure with
Distance 1

 

After a short period, the pressure
will drop below the ambient pressure in the front (figure 1). During which a
negative pressure phase, a partial vacuum is created, this is also accompanied
by high suction winds that carry the debris back to the demolition source
leading to even more destruction to the surrounding elements. The time history
of blast wave is depicted in Figure 2

 

 

 

 

 

 

 

 

 

Figure: 2 Time History of Blast wave 2

 

 

3.
Structural Response to Blast Loading

 

Analyzing the dynamic response of
blast loading on structures is a complex process which involves the effect of
high strain rates, non-linear inelastic behavior of materials the time
dependent deformation of structures and the uncertainty of blast load
calculations.  In order to simplify the
analysis various assumptions are taken related to the structural response and
the loads has been proposed and widely accepted. In order to simplify the
analysis procedure, the structure is considered as a single degree of freedom
(SDOF) system and the relation between the positive duration of the blast load
with the natural period of vibration of the structure is determined. This
results in idealization and simplifies the of blast loads calculation. 

 

3.1
Elastic Single Degree of Freedom

 

The easiest way to solve a transient
problem is by means of Single degree of freedom approach. The structure can be
replaced with an system of one concentrated mass at each level and one spring
representing the resistance of the structure against deformation. A pictorial
representation of the above has been shown in figure 3. M represents structural
mass, the whole system is under the effect of external force F with respect to
time t, i.e, F(t), with structural resistance R, vertical displacement is
expressed in terms of y, and spring constant as K.

 

 

 

 

 

 

 

 

 

Figure:3 Representation of Single Degree of
freedom system 2

 

 

 

The time history of the blast loading
can be idealized as a triangular pulse having a force of Fm as peak force with
positive phase duration of Td as shows in figure 5. The forcing
functions are represented as

 

         

 

The blast
impulse is approximated as the area under the force-time curve and is given as

 

(2)    

 

 

The equation
for motion of un-damped elastic single degree of freedom for time ranging from
0 to positive phase duration td,
is given by Biggs (1964) as

 

(3)   

 

 

 

 

                                                                          

 

 

 

 

 

Figure:4 Time History Graph

 

4 Structural Layout and Details

 

The
G+5 storied framed shear wall concrete structure is modelled using SAP200 with 5
rows of column in y direction and 3 bays in x direction each spanning 6 meters
and 5 meters respectively. 5 meters is the ground floor roof lever from the ground
level followed by 5 stories of 4 meter each. Shear wall is placed at the mid span through out the
height of the structure. Elevation of the structure is shown in figure 5. The
dimensions and design of the structural elements are designed according to
IS-456 and SP-16 of India codes under normal loading conditions as moment
resisting frame. The detailing of the elements are shown in the figure 6.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5 Modelled structure in SAP2000

 

 

 

                                                                       

 

 

 

 

 

 

 

 

 

a) Typical Beam Detailing                                                 
b) Typical Column Detailing

 

 

 

 

 

 

 

c) Detaining of Shear wall

Figure:6 Detailing of structural elements

 

Grade of concrete
used is M25 with HYSD 415 Steel rebars with a cover of 0.03 m for all elements.

 

5 Blast Load Determination

 

 

The method used to
calculate the blast pressure is developed by US Department of Defence under
Section 5 of TMT – 1300 (1990): Structure to resist the effect of accidental explosion,
for framed structure and its elements of a high-rise building. The loading parameters
are calculated for 0.5 Ton, 0.4Ton, 0.3Ton, 0.2Ton, 0.1Ton of  TNT  charge weight with stand off blast distance of
7m from the base of the building along longer direction. The blast pressure calculated
is multiplied with area and converted into blast loads and evenly distributed
to all the nodes at various levels at the face of the structure. For instance, the total pressure due to
blast load on the top floor is multiplied with surface area of the required
structure, which has 5 columns on the front face. Dynamic parameters of the
blast loads which are duration, Time of arrival, shock front velocity, impulse
are calculated accordingly Table 2 depicts the intensity of blast pressure in
bay 3, along the height if the structure for a 0.1 Ton of TNT charge weight
exploded at 7 m standoff distance.

 

 

 

 

 

 

Table 2 Intensity of Blast Pressure on Bay 3 at different
levels

Bay 3

Height  in m

Angle  in deg

Arrival time in ms

Duration of load in ms

Pr in psi

Load in kN

Floor Level

6

25

69.8

24.99

9.09

17

578

5

21

64.4

20.1

6.85

26

865

4

17

57.9

15.05

6.85

37

1341

3

13

47.87

9.98

4.58

73

2534

1

9

36.87

7.87

3.17

151

5359

G

5

12.84

5.97

2.85

227

10997

 

The
graph shown on figure 7 shows the time history function defined in SAP2000 for
analysing the structure for Blast pressure with varying load with respect to
time 

x

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