Analysis on Effect of Blast Load on Sub-Structures

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 04 Issue: 11 | Nov -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 1391
ANALYSIS ON EFFECT OF BLAST LOAD ON SUB-STRUCTURES
Manupriya R1, Ramesh P S2, Ramesh B M3, Dhanalakshmi P1
1P G Student, Computer Aided Design of Structures, Department of Civil Engineering, SJBIT, Bengaluru
2Associate Professor, Department of Civil Engineering, SJBIT, Bengaluru, Karnataka, India
3Assistant Professor, Department of Civil Engineering, SJBIT, Bengaluru, Karnataka, India
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Abstract - Explosions have been the preferred weapon of
terrorist organizations, because they are truly destructive,
easy for fabrication and transport friendly. Analysis of
structures for blast loading is very important specially for
critical civilian buildings since they are not designed for blast
loads. Considering the fact that many studies have been made
only for the super-structures and surface blasts, in this study
we focus mainly on the effects on sub-structure (pile
foundation in soft soil) when the blast loads are applied below
the ground level at certain depth. Different intensities of blast
loads are applied on pile foundation and their effects on piles
as well as the stresses in the soil are analyzed. The analysis is
done using computer aided softwares, SAP2000version19.1.0
and ATBlast.
Key Words: Standoff distance, blast below ground level,
pile foundation, displacement, stresses,hinged support.
1. INTRODUCTION
Recent past blast incidents in the country trigger the minds
of developers, architects and engineers to find solutions to
protect the occupants and structures from blast disasters.
There has been lot of research carriedoutforthedestruction
caused on the super-structure due to the blast loading on
surface of soil. Relatively, there are very less number of
studies that are made on the destruction of sub-structures
due to sub-surface blasts. There are different types of sub-
structures, but, the sub-structure considered here is a pile
foundation. The effects on the pile foundation and the
stresses in the soil were considered in the study. Design
consideration against explosions is very important in high-
rise facilities such as public and commercial tall buildings
and sub-structures, because there are many buildings that
may be under threat of blast loads although not originally
designed for the same.
Trinitrotoluene is usually known as TNT, which is highly
explosive and does not occur naturally. It is a man-made
solid compound which is yellow in color and odorless. It
either absorbs water or dissolves which makes it reliable to
use in wet environmental conditions. Amatol and
composition B, can be the other constituents of this
explosive. It is the most commonly used explosives in
military and industrial applications.Theuseofthisexplosive
for military purposes increased widely in World War 1 and
became one of the dominant military explosives. It’s melting
temperature is around 810ºC, (the temperature below its
detonation temperature) which makes it easy to be melted
and easily be poured in to shells. The chemical formula for
TNT is C6H2 (NO2)3CH3 and its chemical structure is as
shown in Figure 1.
Fig-1. Chemical structure of TNT
2. OBJECTIVES
The main objectives of this study can be summarized as
follows:
i. Understanding the fundamentals of blast hazards
and the interaction of blast waves with sub-
structure (pile foundation).
ii. Modeling and analysis of piles for explosions below
ground level.
iii. To assess the results when the blast loads are
applied on piles.
iv. To determine the lateral displacements of the piles
and the stresses induced in the soil due to the blast
loads.
3. METHODOLOGY
The study is concerned with the behavior of piles when
subjected to blast loading and mainly focuses on the
modeling of a pile structureand a super-structureconsisting
of nine storeys, using SAP2000 program and analyzing its
behavior when subjected to blast loads. The blast loads are
calculated using AT Blast Software. For the analysis of the
sub-structure (i.e. pile foundation) three different charge
weightsare used to examinetheeffectoflateraldisplacement
of the pile at a three different standoff distances and the
variation of the stresses in the soil are analysed.
3.1 Modeling and analysis
Computer modelling of the building wasperformedusingthe
finite element software SAP2000 version 19.1.0. Figure 2
shows the analysis flowchart. Soil blocks are modelled as
solid elements. The parameters of the soft soil were
considered for the analysis. Piles are modelled which are of

three different lengths 10m, 16m and 20m within the soil.
Figures 3 to 5 show the section of the soil and piles of
different lengths. The loads from the super-structure are
transferred on the group of piles as vertical loads through
columns and then the load is divided and applied on
individual piles. The analysis is carried out for different
charge weights of 500kg, 1000kg and 1500kg TNT at three
different standoff distances 3m, 5m and 7m and the results
are tabulated. The reinforced concrete building is framed
structure composed of columns and beams. The loads on the
super-structure are considered as per IS 875 (part 2). Figure
6 shows the 3D view of the super-structure,soilandsupports
provided. The study reflects effects of blasts below the
ground level, ground motion (i.e. stresses in the soil) and the
lateral displacements of the piles. It is assumed that the soft
soil behaves as a elastic medium, so the soil bottom and the
piles are assigned hinged support.
Fig-2. Analysis flowchart
Fig-3. Section of soil and pile of 10m length
Fig-6. 3D view super-structure, soil and pile.
3.2 Analysis Results
The analysis results for different weights of TNT’s, different
standoff distances and different pile lengths are as follows,
A. For sub-structure (pile foundation)
i. Stress versus depth of the soil.
ii. Displacement versus length of the pile.
B. For super-structure (9 storey building)
i. Lateral displacement versus storey height.
ii. Inter-storey drift versus storey height.
Figures 7 to 9 show the stress versus depth of the soil graphs
for different weights of TNT’s when the blast load is applied
at different standoff distances. Similarly, the graphs were
plotted for16m and 20m pile lengths.
Fig-6. Stresses at top of pile for 10m pile length

Fig-9. Stresses at bottom of pile for 10m pile length
Fig-12. Displacements at top of pile for 10m pile length

Fig-15. Displacements of super-structure for different pile
lengths
Fig-16. Inter-storey drifts of super-structure for different
pile lengths
4. DISCUSSIONS
The following are the discussions made on thepresentstudy
i.e., the blast is applied at a depth of 2m below the ground
level and the lateral displacement of thepileandthestresses
in the soil are analysed. It is found that the percentage
change in the stress for 10m pile was higher with increasing
standoff distances when compared to 16m and 20m piles
(i.e. for 10m pile, percentage change in stress from3mto7m
standoff distance is 70.1 - 49.35 = 20.75. For 16m pile,
percentage change in stress from3mto7mstandoffdistance
is 60.36 – 48.97 = 11.39. For 20m pile, percentage change in
stress from 3m to 7m standoff distance is 48.1 – 46.74 =
1.36).
The following tables 1 to 3 show the percentage change in
the stresses due to different weights of TNT applied at
different standoff distances.
Standoff
distance,
m
Stress
due to
500kg
TNT,
N/mm2
Stress
due to
1000kg
TNT,
N/mm2
Stressdue
to 1500kg
TNT,
N/mm2
%
change
in
stress
3 149.22 250.01 252.36 70.1
5 143.10 240.53 241.65 68.8
7 123.74 147.48 187.77 49.35
Table-1. Comparison of stress in soil for a 10m depth pile
Standoff
distance,
m
Stress
due to
500kg
TNT,
N/mm2
Stress
due to
1000kg
TNT,
N/mm2
Stressdue
to 1500kg
TNT,
N/mm2
%
change
in
stress
3 161.23 240.37 258.56 60.36
5 160.22 179.83 245.32 53.11
7 150.92 160.38 224.84 48.97
Standoff
distance,
m
Stress
due to
500kg
TNT,
N/mm2
Stress
due to
1000kg
TNT,
N/mm2
Stressdue
to 1500kg
TNT,
N/mm2
%
change
in
stress
3 180.43 231.56 267.23 48.1
5 170.6 183.25 251.6 47.4
7 162.5 178.6 238.46 46.74
The following tables 4 to 6 show the percentage change in
displacement of the pile for different weights of TNT and
standoff distances. It is found that, the percentage change in
the displacement is higher from 10m to 16m pile but
decreases for 20m pile (i.e. for 10m pile, percentage change
in displacement from 3m to 7m standoff distance is 55.02 –
46.56 = 8.46. For 16m pile, the percentage change is 42.03 –
31.14 = 10.89. For 20m pile, the percentagechangeis48.43–
45.52 = 2.91).
Standoff
distance,
m
Disp.
due to
500kg
TNT,
mm
Disp.
due to
1000kg
TNT, mm
Disp.
due to
1500kg
TNT, mm
%
change
in Disp.
3 41.3 44.4 64.1 55.02
5 38.5 43.7 58.6 52.2
7 37.8 40.1 55.4 46.56
Table-4. Comparison of displacement in soil for a 10m
depth pile

Standoff
distance,
m
Disp. due
to 500kg
TNT, mm
Disp. due
to 1000kg
TNT, mm
Disp. due
to 1500kg
TNT, mm
%
change
in
Disp.
3 35.4 38.2 51.7 42.03
5 34.1 36.0 46.0 34.89
7 33.4 34.3 43.8 31.14
depth pile
Standoff
distance,
m
Disp. due
to 500kg
TNT, mm
Disp. due
to 1000kg
TNT, mm
Disp. due
to 1500kg
TNT, mm
%
change
in
Disp.
3 25.6 32.9 38.8 48.43
5 25.10 29.5 37.01 47.4
7 24.6 28.6 35.80 45.52
depth pile
5. CONCLUSIONS
The conclusions are:
1. As the standoff distance increases the stresses
will be decreased at the top of the pile to the
depth of 10m and further it remains constant
with the variation of the pile length.
2. As the length of the pile increases, stresses in
the soil are increased.
3. As the intensity of the blast load (weight of the
TNT) increases, stresses in the soil also
increases.
4. As the standoff distances of the blast load
increases, the displacement in the pile
decreases.
5. As the length of the pileincreases,displacement
values of the pile decreases.
6. As the intensity of the blast load (weight of the
TNT) increases, lateral displacement of the pile
increases.
7. For the super-structure, the displacement
values gradually decreases when the length of
the pile increases.
8. For the super-structure, the inter-storey drift
values gradually decreases when the length of
the pile increases.
ACKNOWLEDGEMENT
I would like to thank my college Principal, Head of the
Department and Guide for their untiring guidance,
continuous support and for extending all the required
facilities for successful completion of this paper work.
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