Blast Loading

EFFECT OF BLAST LOADING ON RC STRUCTURES
A SEMINAR REPORT SUBMITTED IN PARTIAL
FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF
Master of Engineering
in
Civil Engineering
By
ALOK B. RATHOD
Bhartiya Vidya Bhavan’s
SARDAR PATEL COLLEGE OF ENGINEERING, MUMBAI
DEPARTMENT OF CIVIL ENGINEERING
2014

Bhartiya Vidya Bhavan’s
SARDAR PATEL COLLEGE OF ENGINEERING, MUMBAI
DEPARTMENT OF CIVIL ENGINEERING
CERTIFICATE
This is to certify that Mr. ALOK BHAKTIRAM RATHOD Roll no. MCSI016 has
successfully completed the seminar work entitled “EFFECT OF BLAST LOADING ON
RC STRUCTURES” in the partial fulfillment of M.E. (Structural Engineering)
Date :
Place : Mumbai

ACKNOWLEDGEMENT
I wish to express my thanks to Prof. Dr. M.M. Murudi, Head of the Civil
Engineering Department, and Prof. Dr. A. A. Bage for being there throughout the
completion of this Report.
I also express my deep sense of gratitude to all my Professors, Department of
Structural Engineering, Sardar Patel College of Engineering, Mumbai for valuable
guidance, constant encouragement and creative suggestions offered during the course
of this seminar and also in preparing this report.
Date: Alok B. Rathod
Place: Mumbai Roll No. MCSI-016
SPCE, Mumbai

I
CONTENTS
CHAPTER NO. TITLE PAGE NO.
ABSTRACT
lll
LIST OF FIGURES
lV
LIST OF TABLES
V
CHAPTER 1 INTRODUCTION 1
1.1 Critical Review 1
1.2 Introduction 1
CHAPTER 2 REVIEW OF LITERATURE 2
2.1 General 2
2.2 Objective and scope of the present work 5
2.2.1 Objective 5
2.2.2 Scope of the study 5
CHAPTER 3 BACKGROUND 6
3.1 Explosion and blast phenomenon 6
3.1.1 Shock waves or blast waves 6
3.1.2 Dynamic loadings 8
3.2 Effects on structures 8
3.3 Structural response or analysis to blast loadings 10

II
CHAPTER 4 CASE STUDIES 12
4.1 Column subjected to blast loadings 12
4.2 Reinforced concrete panels subjected to blast loadings 13
4.2.1 Test specimens 13
4.2.2 Test setting 15
4.2.3 Measurement outline 15
4.2.4 Test results 16
CHAPTER 5 CONCLUSION AND FUTURE SCOPE 20
5.1 Conclusions 20
5.2 Future scope of study 20
REFERENCES 21

III
ABSTRACT
A bomb explosion within or immediately nearby a building can cause catastrophic damage on the
building's external and internal structural frames, collapsing of walls, blowing out of large
expanses of windows, and shutting down of critical life-safety systems. Loss of life and injuries to
occupants can result from many causes, including direct blast-effects, structural collapse, debris
impact, fire, and smoke. The indirect effects can combine to inhibit or prevent timely evacuation,
thereby contributing to additional casualties.
In addition, major catastrophes resulting from gas-chemical explosions result in large dynamic
loads, greater than the original design loads, of many structures. Due to the threat from such
extreme loading conditions, efforts have been made during the past three decades to develop
methods of structural analysis and design to resist blast loads. Studies were conducted on the
behavior of structural concrete subjected to blast loads. These studies gradually enhanced the
understanding of the role that structural details play in affecting the behavior.
The response of simple RC columns subjected to constant axial loads and lateral blast loads was
examined. Also slab panel subjected to various blast loadings was examined, which were made
out of different mix and polymers. Next, a short duration, lateral blast load was applied and the
response time history was calculated.
The analysis and design of structures subjected to blast loads require a detailed understanding of
blast phenomena and the dynamic response of various structural elements. This gives a
comprehensive overview of the effects of explosion on structures.

IV
LIST OF FIGURES
FIGURE NO. TITLE PAGE NO.
Fig no. 3.1 Blast wave propagation 6
Fig no. 3.2 Generalized Blast Pressure History 7
Fig no. 3.3 Blast loads on buildings 8
Fig no. 3.4 Blast Pressure Effects on a Structure 9
Fig no. 3.5 3.5 (a) SDOF System 10
3.5 (b) Blast Loading 10
Fig no. 4.1 Simplified Blast Loading 12
Fig no. 4.2 3D model of the column using explicit code LS-Dina 13
Fig no. 4.3 Test specimen geometry 14
Fig no. 4.4 Overview of supporting steel box 15
Fig no. 4.5 Test Setup 15
Fig no. 4.6 Sensor location of the concrete specimen 16
4.6 (a) Steel Strain 16
4.6 (b) Concrete Strain 16
Fig no. 4.7 Blast pressure on concrete specimen 16
4.7 (a) Free field pressure on CFRP 16
4.7 (b) Reflect pressure on CFRP 16
Fig no. 4.8 Behavior of conc. specimen (NSC, CFRP) 18
Fig no. 4.9 Error bar of disp. under blast loading 18
Fig no. 4.10 Error bar of disp. under blast loading 18
4.10(a) Normal strength concrete specimen 18
4.10(b) Failure of each FRP specimen 18

V
LIST OF TABLES
TABLE NO. TITLE PAGE NO.
Table No. 4.1 Mixed proportion of concrete slab specimen 14
Table No. 4.2 Material properties of retrofitted materials 14
Table No.4. 3 Blast test results 17

1
CHAPTER 1
INTRODUCTION
1.1 CRITICAL REVIEW
The blast explosion nearby or within structure is due to pressure or vehicle bomb or quarry
blasting. These causes catastrophic damage to the building both externally and internally
(structural frames). Resulting in collapsing of walls, blowing out of windows, and shutting down
of critical life-safety systems. Buildings, bridges, pipelines, industrial plants dams etc. are the
lifeline structures and they play an important role in the economy of the country and hence they
have to be protected from dynamic and wind loading.
These structures should be protected from the blast effects, which are likely to be the targets of
terrorist attacks. The dynamic response of the structure to blast loading is complex to analyze,
because of the nonlinear behavior of the material. Explosions result in large dynamic loads,
greater than the original design loads, for which the structures are analyzed and designed.
Analyses and design of blast loading requires detailed knowledge of blast and its phenomena. A
critical review is presented in this paper to estimate the blast loading and its dynamic effects on
various components of structure treating the effects as SDOF system.
1.2 INTRODUCTION
The study of blast effects on structures has been an area of formal technical investigation for
over 60 years. A bomb explosion within or immediately nearby a building can cause catastrophic
damage on the building's external and internal structural frames, collapsing of walls, blowing out
of large expanses of windows, and shutting down of critical life-safety systems. Loss of life and
injuries to occupants can result from any causes,including direct blast-effects,structural collapse,
debris impact, fire, and smoke. The indirect effects can combine to inhibit or prevent timely
evacuation, thereby contributing to additional casualties. In addition, major catastrophes
resulting from gas-chemical explosions result in large dynamic loads, greater than the original
design loads, of many structures.
Strategies for blast protection have become an important consideration for structural designers
as global terrorist attacks continue at an alarming rate. Conventional structures normally are not
designed to resist blast loads and because the magnitudes of design loads are significantly lower
than those produced by most explosions, conventional structures are susceptible to damage
from explosions. No civilian buildings can be designed to withstand the kind of extreme attack
that happened to the World Trade Centre in USA. Building owners and design professionals alike,
however, can take steps to better understand the potential threats and protect the occupants
and assets in an uncertain environment. With this in mind, developers, architects and engineers
increasingly are seeking solutions for potential blast situations, to protect building occupants and
the structures.

2
CHAPTER 2
REVIEW OF LITERATURE
2.1 GENERAL
In the past, few decades’ considerable emphasis has been given to problems of blast and
earthquake. The earthquake problem is rather old, but most of the knowledge on this subject has
been accumulated during the past fifty years. The blast problem is rather new; information about
the development in this field is made available mostly through publication of the Army Corps of
Engineers, Department of Defense, U.S. Air Force and other governmental office and public
institutes.
Much of the work is done by the Massachusetts Institute of Technology, The University of Illinois,
and leading educational institutions and engineering firms. Due to different accidental or
intentional events, the behavior of structural components subjected to blast loading has
been the subject of considerable research effort in recent years. Conventional structures are
not designed to resist blast loads; and because the magnitudes of design loads are
significantly lower than those produced by most explosions. Further, often conventional
structures are susceptible to damage from explosions. With this in mind, developers, architects
and engineers increasingly are seeking solutions for potential blast situations, to protect building
occupants and the structures.
This study is very much useful for design the buildings constructed for industries where chemical
process is the main activity. An increasing number of research programs on the sources of these
impact loads a dynamic analysis and preventive measures are being undertaken. Just in design
some areas takes into account the effects of earthquakes, hurricanes, tornadoes and
extremes snow loads, likewise even explosive or blast loads has to be taken into design
consideration. This does not mean design and consideration of specialshelter facilities but simply
the application of appropriate design techniques to ordinary buildings, so that one can achieve
some degree of safety from sudden attacks.
Philip Esper in 2003, after the Four major bombing incidents took place in Mainland UK within
the last ten years; the 1992 St Mary's Axe, the 1993 Bishopsgate, the 1996 Docklands and
Manchester bombs the author was involved in the investigation of damage and reinstatement of
numerous commercial buildings, and in providing advice to building owners and occupiers on
blast protection measures for both existing and proposed buildings. These detonation devices
were estimated as 450 kg, 850 kg, 500 kg and 750 kg of TNT equivalent, respectively. As a result,
the author was involved in the investigation of damage and reinstatement of numerous
commercial buildings, and in providing advice to building owners and occupiers on blast
protection measures for both existing and proposed buildings. Numerical modeling as well as
laboratory and on-site testing were used in the investigation of damage and assessing the
dynamic response of these buildings and their floor slabs to blast loading. The finite element (FE)

3
analysis technique used in this investigation is described, and the correlation between the results
of the FE analysis and laboratory and on-site testing is highlighted. It was concluded that the
ductility and natural period of vibration of a structure governs its response to an explosion.
Ductile elements, such as steel and reinforced concrete, can absorb significant amount of strain
energy, whereas brittle elements, such as timber, masonry, and monolithic glass, fail abruptly.
LUCCIONI et al in 2005, studied the effects of mesh size on pressure and impulse distribution of
blast loads with the aid of hydro-codes. A computational dynamic analysis using AUTODYN-3D
was carried out over the congested urban environment that corresponds to the opposite rows of
buildings of a block, in the results obtained for different positions of the explosive charge are
presented and compared. The effect of mesh size for different boundary conditions is also
addressed. It is concluded that the accuracy of numerical results is strongly dependent on the
mesh size used for the analysis. On the other side the mesh size is also limited by the dimensions
of the model and the computer capacity. One of the major features in the numerical simulation
of blast wave propagation in large urban environments is the use of an adequate mesh size.
The accuracy of numerical results is strongly dependent on the mesh size used for the analysis. A
10 cm mesh is accurate enough for the analysis of wave propagation in urban ambient.
Nevertheless, it may be too expensive to model a complete block with this mesh size.
Alternatively, a coarser mesh can be used in order to obtain qualitative results for the comparison
of the loads produced by different hypothetical blast events. Even coarse meshes, up to 50 cm of
side, give a good estimation of the effects of moving the location of the explosive charges.
Ghani Razaqpur et al in 2006, investigated the behavior of reinforced concrete panels, or slabs,
retrofitted with glass fiber reinforced polymer (GFRP) composite, and subjected to blast load
Eight 1000 x 1000 x 70 mm panels were made of 40 MPa concrete and reinforced with top and
bottom steel meshes. Five of the panels were used as control while the remaining four were
retrofitted with adhesively bonded 500 mm wide GFRP laminate strips on both faces, one in each
direction parallel to the panel edges. The panels were subjected to blast loads generated by the
detonation of either 22.4 kg or 33.4 kg ANFO explosive charge located at a 3-m standoff. Blast
wave characteristics, including incident and reflected pressures and impulses, as well as panel
central deflection and strain in steel and on concrete/FRP surfaces were measured. The post-
blast damage and mode of failure of each panel was observed, and those panels that were not
completely damaged by the blast were subsequently statically tested to find their residual
strength. It was determined that the reflected blast pressure and impulse measured at the same
location during different shots using the same charge size and standoff distance were generally
reasonably close, but in some cases significant deviation occurred. The results of this study
indicate that the GFRP retrofit may not be suitable in every situation and that quantifying its
strengthening effects will need more actual blast testing rather than merely theoretical modeling
or pseudo-dynamic testing.

4
Ray Singh Meena in 2009, focused on the design techniques for the loading on roof structures
and the resistance of open web steel joists, a common roof component. Blast loads are dynamic,
impulsive and non-simultaneous over the length of a roof.
To design against explosions, a procedure has been developed to devise a uniform dynamic load
on a roof that matches the response from blast loads. The objective of this research was to test
and compare its results to the deflections from blast loads using FEM of analysis and to compare
them to equivalent loading response. It is recommended that additional research is to be done
on the prediction of blast pressures on roofs and on the development of an equivalent uniform
dynamic load. It is also recommended that an analytical resistance function for open web steel
joists be clearly defined, which includes all failure limit states.
Ngo ET AL in 2007, carried an analytical study on RC column subjected to blast loading and
progressive collapse analysis of a multi-storied building were carried out. The 3D model of the
column was analyzed using the nonlinear explicit code LS-Dyna 3D (2002) which takes into
account both material nonlinearity and geometric nonlinearity. It was observed that the increase
in flexural strength was greater than that of shear strength. Thus, the increase in the material
strengths under dynamic conditions may lead to a shift from a ductile flexural failure to a brittle
shear failure mode. In the progressive collapse analysis study which is based on the local
damage assessment due to bomb blast at ground level, progressive collapse analyses was
performed on the example building. The structural stability and integrity of the building were
assessed by considering the effects of the failure of some perimeter columns, spandrel beams
and floor slabs due to blast overpressure or aircraft impact. In addition to material and geometric
nonlinearities, the analyses considered membrane action, inertia effects, and other influencing
factors. The results show that the ultimate capacity of the floor slab is approximately 16.5kPa
which is 2.75 times the total floor load (dead load plus 0.4 live load).
Alok Goyal in 2008, discussed through an overview to quantify blast loads as high pressure, short
duration shock loading for the building as a whole and on each individual structural component.
The study concluded that the most difficult part of the blast-resistance design is to define the
blast wave parameters with acceptable probability of exceedance, and to quantify desired
performance parameters in terms of crack widths, rotations, ductility factor capacities of
elements or story drifts. Considerable efforts and skill is required to numerically predict the blast
induced pressure field and highly non-linear response. Even then, the results may be meaningless
due to modeling limitations and uncertainties associated with blast loads. The developed systems
therefore should be tested in field and the data collected should be used to improve the design
and the mathematical model.

5
2.2 OBJECTIVE AND SCOPE OF THE PRESENT WORK
2.2.1 OBJECTIVE:
 To analyze and design the structures against the abnormal loading conditions like blast loads,
strong wind pressure etc. requiring detailed understanding of blast phenomenon.
 To study the dynamic response of various structural elements like column, beam, slab and
connections in steel and RCC structures.
 The main objective of the research presented in this report is to analytically and
numerically study the structural behavior of HSC and NSC column subjected to blast loading.
2.2.2 SCOPE OF THE STUDY:
In order to achieve the above-mentioned objectives the following tasks have been carried out:
 All the computation of dynamic loading on a rectangular structure with and without openings
and open frame structures to evaluate the blast pressure.
 Computation of the blast loading on the column.
 Modeling of a simple RC column in ANSYS.
 Response of a simple RC column under the Blast loading.

6
CHAPTER 3
BACKGROUND
3.1 EXPLOSION AND BLAST PHENOMENON
An explosion is a rapid release of stored energy characterized by a bright flash and an audible
blast. Part of the energy is released as thermal radiation (flash); and part is coupled into the air
as air-blast and into the soil (ground) as ground shock, both as radially expanding shock waves.
To be an explosive, the material will have the following characteristics.
1. Must contain a substance or mixture of substances that remains unchanged under ordinary
conditions, but undergoes a fast chemical change upon stimulation.
2. This reaction must yield gases whose volume—under normal pressure, but at the high
temperature resulting from an explosion—is much greater than that of the original
substance.
3. The change must be exothermic in order to heat the products of the reaction and thus to
increase their pressure. Common types of explosions include construction blasting to break
up rock or to demolish buildings and their foundations, and accidental explosions resulting
from natural gas leaks or other chemical/explosive materials.
3.1.1 SHOCK WAVES OR BLAST WAVES
The rapid expansion of hot gases resulting from the detonation of an explosive charge gives rise
to a compression wave called a shock wave (Fig1), which propagates through the air. The front
of the shock wave can be considered infinitely steep, for all practical purposes. That is, the time
required for compression of the undisturbed air just ahead of the wave to full pressure just
behind the wave is essentially zero. From the figure 1 it can be concluded that if the explosive
source is spherical, the resulting shock wave will be spherical, since its surface is continually
increasing, the energy per unit area continually decreases.
Fig 3.1: Blast wave propagation

7
Consequently, as the shock wave travels outward from the charge, the pressure in the front of
the wave, called the peak pressure, steadily decreases. At great distances from the charge, the
peak pressure is infinitesimal, and the wave can be treated as a sound wave. Behind the shock
wave front, the pressure in the wave decreases from its initial peak value. At some distance from
the charge, the pressure behind the shock front falls to a value below that of the atmosphere and
then rises again to a steady value equal to that of the atmosphere. The part of the shock wave in
which the pressure is greater than that of the atmosphere is called the positive phase and,
immediately following it, the part in which the pressure is less than that of the atmosphere is
called the negative or suction phase.
The rapid oxidation of fuel elements develops chemical explosions. This reaction releases
heat and produces gas, which expands. Low-end explosives create quasi static loads. High
explosives (chemical and nuclear) in a surrounding medium, such as air or water, cause shock
waves in the medium. The blast releases high-pressure gases at high tempera- tires. These gases
naturally expand, and the surrounding medium is consequently compressed. The compressed
medium, or for the specific case of air, forms a shock front. The shock front travels in a radial
direction. As the explosive gases cool and slow their movement, the amount of "overpressure"
the shock front carries decreases. The gases release energy to reach equilibrium towards the
atmospheric pressure.
Fig 3.2: Generalized Blast Pressure History
However, due to the high pressure and mass of the gases, more expansion is necessary to actually
reach equilibrium. This causes the pressure in the shock wave to drop below the atmospheric
pressure. After sufficient "under pressure" is expended, the state returns to the atmospheric
pressure. The air behind the shock front also places a load, a drag force, on objects encountered.
The general shape of a pulse shape is shown in Figure 2. Important factors pertinent to burst
pressures include the peak pressure, the duration, the air density behind the shock front, the
velocity of the shock front, and the impulse of the blast pressure.

8
3.1.2 DYNAMIC LOADINGS
Drag exerted by the blast winds required to form the blast wave. These winds push, tumble and
tear objects. Blast pressure can create loads on buildings that are many times greater than
normal design loads (Fig 3), and blast winds can be much more severe than hurricanes. Buildings
with relatively weak curtain walls and interior partitions would probably be gutted very early
during the blast phase, even at low over pressures. Dynamic pressures would then continue to
cause drag loads on the structural frames that is left standing. Slabs over closed basements
would experience the downward thrust of over-pressure, which would be transmitted to
supporting beams girders and columns. Foundation would experience blast induced vertical and
overturning forces.
Fig 3.3: Blast loads on buildings
Failure would occur unless the structural system was designed to resist these large quickly
applied loads. Structures with load bearing walls or curtain walls that not blowout easily could
be completely demolished or toppled by blast loads. Such structures would experience the
combined loading conditions caused by the incident overpressure, the dynamic and highly
transient reflected pressure that develop when the shock waves strikes a surface of the
structure. People in the basement shelters who are protected against catastrophic structural
collapse, high pressure and flying objects would have the greatest possibility of surviving the blast
phase.
3.2 EFFECTS ON STRUCTURES
Blast effects on building structures can be classified as primary effects and secondary effects.
Primary effects include;
1. Air-blast: The blast wave causes a pressure increase of the air surrounding a building structure
and also a blast wind.

9
2. Direct ground shock: An explosive which is buried completely or partly below the ground
surface will cause a ground shock. This is a horizontal (and vertical, depending on the location of
the explosion with regard to the structural foundation) vibration of the ground, similar to an
earthquake but with a different frequency.
3. Heat: A part of the explosive energy is converted to heat. Building materials are weakened at
increased temperature. Heat can cause fire if the temperature is high enough.
4. Primary fragments: Fragments from the explosive source which are thrown into the air at high
velocity (for example wall fragments of an exploded gas tank). Secondary effects can be
fragments hitting people or buildings near the explosion. They are not a direct threat to the
bearing structure of the building, which is usually covered by a facade. However, they may
destroy windows and glass facades and cause victims among inhabitants and passers-by.
Blast loading on structures can be explained by three main loading conditions (figure4)
1. In the first type a relatively large shock wave reaches a structure relatively small enough
that the blast wave encloses the entire structure. The shock wave effectively acts on the
entire structure simultaneously. Additionally, there is a drag force from the rapidly moving
wind behind the blast wave. The structure is, however, massive enough to resist translation.
Fig 3.4: Blast Pressure Effects on a Structure
2. The second condition also involves a relatively large shock wave and a target much smaller
than the previous case. The same phenomena happen during this case, but the target is
sufficiently small enough to be moved by the dynamic, drag pressure.

10
3. In the final case, the shock burst is too small to surround the structure simultaneously and
the structure is too large to be shifted. Instead of simultaneous loading, each component
is affected in succession. For a typical building, the front face is loaded with a reflected
overpressure.
3.3 STRUCTURAL RESPONSE OR ANALYSIS TO BLAST LOADING
Blast loading is a short duration load also called impulsive loading. Mathematically blast loading
is treated as triangular loading. The ductility and natural period of vibration of a structure governs
its response to an explosion.
Ductile elements, such as steel and reinforced concrete, can absorb significant amount of strain
energy, whereas brittle elements, such as timber, masonry, and monolithic glass, fail abruptly. In
the investigation of the dynamic response of a building structure to bomb blast, the following
procedures are followed
(a) The characteristics of the blast wave must be determined; (b) the natural period of response
of the structure (or the structural element) must be determined; (c) The positive phase duration
of the blast wave is then compared with the natural period of response of the structure. Based
on (c) above, the response of the structure can be defined as follows:
i. If the positive phase duration of the blast pressure is shorter than the natural period of
vibration of the structure, the response is described as impulsive. In this case, most of the
deformation of the structure will occur after the blast loading has diminished.
Fig 3.5: (a) SDOF system (b) Blast loading
ii. If the positive phase duration of the blast pressure is longer than the natural period of
vibration of the structure, the response is defined as quasi-static. In this case, the blast will
cause the structure to deform whilst the loading is still being applied.
iii. If the positive phase duration of the blast pressure is close to the natural period of vibration
of the structure, then the response of the structure is referred to as dynamic. In this case, the

11
deformation of the structure is a function of time and the response is determined by solving
the equation of motion of the structural system.
Equation of motion for an undamped forced system is given by
MŸ (t) + KÝ (t) = F (t) - - - - - - - - - - (a)
The force is given by
F (t) = F0 (1- T / td) - - - - - - - - - - (b)
Initial conditions for triangular pulse is Y0=0, V0= 0
The total displacement of an un-damped SDOF system is given by
Y (t) = Y0 cosωt + (V0 /ω) sinωt + 1/mω∫t0
F (t) sinω (t-T) dt- - - - - - (c)
Displacement
Y (t) = Fm/K (1-cosωt) + Fm/ktd ((sinωt/ω) –t) - - - - - - - - - - - - - - (d)
Velocity
Ý (t) =dy/dt= Fm/K [ωsinωt+1/td (cosωt-1)] - - - - - - - - - - - - - - - - (e)
In which ω is the natural circular frequency of vibration of the structure and T is the natural period
of vibration of the structure which is given by equation
ω = 2π/T √=K/M - - - - - - - - - - - - (f)
The maximum response is defined by the maximum dynamic deflection Ym which occurs at time
tm. The maximum dynamic deflection Ym can be evaluated by setting dy/dt in Equation (c) equal
to zero, i.e. when the structural velocity is zero. The dynamic load factor, DLF, is defined as the
ratio of the maximum dynamic deflection Ym to the static deflection Yst which would have
resulted from the static application of the peak load Fm, which is shown as follows:
DLF=Ym / Yst - - - - - - - - - - - - - - (g)
DLF=1/ (2πtd/T) {sin2π (t/T) - sin2π (t/T - td/T)} - cos2π t/T - - - - - - (h)
The dynamic load factor of blast loading is given by equation (h) to be considered in evaluating
the correctness of evaluating the dynamic stresses.

12
CHAPTER 4
CASE STUDIES
4.1 COLUMN SUBJECTED TO BLAST LOADING
A ground floor column of a multi-storey building was analyzed. The parameters considered were
the concrete strength (40MPa for NSC column and 80 MPa HSC column) and spacing of
ligatures (400mm for ordinary detailing-OMRF (ordinary moment resisting frame) and 100mm
for special seismic detailing-SMRF (seismic moment resisting frame)). It has been found that with
increasing concrete compressive strength, the column size can be effectively reduced. In this case
the column size was reduced from 500 x 900 mm for the NSC column down to 350 x 750 for the
HSC column.
Fig 4.1: Simplified blast loading
While the axial load capacities of the two columns are stillthe same. The blast load was calculated
based on data from the Oklahoma bombing report (ASCE 1996) with a standoff distance of 11.2m.
The simplified triangle shape of the blast load profile was used (fig 6). The duration of the positive
phase of the blast is 1.3 milliseconds. The 3D model of the column was analyzed using the
nonlinear explicit code LS-Dina 3D (fig 7) (2002) which takes into account both material
nonlinearity and geometric nonlinearity. The strain rate- dependent constitutive model proposed
in the previous section was adopted. The effects of the blast loading were modeled in the
dynamic analysis to obtain the deflection time history of the column.
From this case study on the response of HSC and NSC columns subjected to bomb blast a strain-
rate dependent constitutive model for concrete is proposed which is applicable to both normal
strength and high strength concretes. It was found that shear failure was the dominant modes of
failures for close-range explosion. HSC columns were shown to perform better than NCS columns
(with the same axial load capacity) when subjected to extreme impulsive loading, they also
had higher energy absorption capacity. Results from the study concluded that the impulsive

13
loading is very different from the static loading in terms of the dynamic inertia effect and
structural response.
Fig 4.2: 3D model of the column using explicit code LS-Dina
4.2 REINFORCED CONCRETE PANELS
4.2.1 TEST SPECIMENS
As shown in Figure 2, nine 1000 × 1000 × 150 mm reinforced concrete slab specimens with double
lattice reinforcements with 12-D10 steel bars with 82mm regular spacing in each direction were
made for the experiment. The bar yield strength and ultimate strength are 400 MPa and 600
MPa, respectively. The average 28day concrete compressive strength is 24 MPa and the average
compressive strength at the age of testing is 25.6 MPa. The mix proportion of concrete slab
specimens is shown in Table 1. The specimens include NSC (normal strength concrete; the control
specimen), CFRP (carbon fiber reinforced polymer), polyurea (same thickness as CFRP specimen),
BFRP (basalt fiber reinforced polymer), and CFRP with polyurea specimen. CFRP and BFRP
specimens were retrofitted to concrete on one side with 2 layers. And polyurea are sprayed to
concrete with same thickness as CFRP.
Also, total retrofitted thickness of CFRP with polyurea specimens is same as the other retrofitted
specimens with sprayed on the specimen with one layer of CFRP sheet. Two specimens are made
for each case except for the BFRP specimen.

14
Fig 4.3: Test specimen geometry
Table 4.1. Mixed proportion of concrete slab specimen
Table 4.2. Material properties of retrofitted materials

15
4.2.2 TEST SETTING
As shown in Figure 3, the test specimen is placed on a buried steel frame box in the ground. This
setting system can eliminate the clearing effect by the ground surface acting as a target of infinite
dimensions with the test specimen being part of this unlimited surface (Razaqpur et al. 2007).
The specimen placed on the buried steel box is fixed by clamps used to prevent uplift and keep
the consistent boundary condition. Rubber pads of the same width and length as the steel angle
legs were placed between the angles and test specimen to ensure uniform support conditions.
The explosive used for the test was spherical ANFO, which was held by wood horizontal bar.
Figure 4 shows the test specimen setup with the 35lbs ANFO (28.7lbs TNT) explosive charge. The
1.5m standoff from specimens to explosive middle point is consistently maintained.
Fig 4.4: Overview of supporting steel box Fig 4.5: Test setup
4.2.3 MEASUREMENT OUTLINE
The free field incident pressure was measured at 5m from the center of the test slab specimens
as shown in Figure 3 where reflected pressure on concrete specimen was measure at the center
of the top surface of the specimen and 230mm from the center between the center and 1/3 point
of specimen diagonal length. To measure strain, 6mm strain gauges are attached on tensile part
of steel and 30mm strain gauges are attached on concrete upper and lower surfaces as shown
Figure 5. In case of retrofitted specimen,FRP strain gauges are attached instead of concrete strain
gauges on bottom surface. Also, LVDTs on the specimen center are used to measure the
maximum and residual displacements. The data acquisition systems used are Dewe 2010 and
Dewe 5000.

16
(a) Steel strain (b) concrete strain
Fig 4.6: Sensor location of the concrete specimen
4.2.4 TEST RESULTS
The measured free field pressure for the test of CFRP specimen is similar with the analysis blast
pressure results obtained using ConWEP as shown in Figure 6(a). In case of reflected pressure on
concrete surface, specimen (Figure 6(b)) was applied with a secondary pressure due to sequential
detonation of ANFO charge. But, this type of sequential pressure load was observed in the
simulation using ConWEP. The duration of reflected pressure is less than 0.5msec, which shows
that the high pressure was applied to the specimen for a very short duration. The applied impulse
which is constituted by pressure and duration is shown in Table 1 and the reflected impulse is
generally 10 times greater than the free field pressure in this test.
(a) Free field pressure on CFRP (b) Reflect pressure on CFRP
Fig 4.7: Blast pressure on concrete specimen

17
Although NSC 1 was charged with TNT 35 lbs. for preliminary test, the reflected pressure on
specimen did not recognize the signal. Therefore, based on the preliminary test, ANFO 35lbs was
selected as an appropriate charge.
In Table 1, the maximum strains of steel rebar and concrete/FRP obtained at the center of the
specimens are tabulated. The results showed that all of bottom steels have yielded where as
some have and have not yielded in top steels. The strain gauges for concrete and FRP attached
at the top and bottom surfaces showed large strains, which means the specimens have gone
through significant damages and plastic deformations, generally exceeding 10,000 με. Even
though the concrete strain has exceeded allowable strain, the residual strain was nearly 1,000
~5,000 με, showing great recovery. From the test results, it can be concluded that the retrofitted
FRP concrete specimen has a great ability to recover strain and a sufficient ductility.
Table 4.3: Blast test results
The maximum and residual displacements of each specimen were measured at the center of the
specimen. To measure specimen behavior under blast loading as shown in Figure 3, LVDT is
connected to the specimen. From this time-displacement relationship, frequency of the
specimens under blast load is measured and compared to the analytical results. As shown in
Figure 4, the results vary significantly from a specimen to a specimen due to variabilities in
environmental conditions and explosive charge shape. Therefore, the average displacements are
compared. In case of maximum displacement, CFRP with polyurea (CPU) specimen shows
smallest displacement due to combination highly ductile material of polyurea and highly stiff and
strong material of CFRP. Polyurea specimen does not have sufficient stiffness compared to CFRP
specimen. The maximum displacement of BFRP specimen is similar to CFRP specimen, but its
residual displacement is nearly large as the NSC specimen. It seems that the BFRP is an

18
inappropriate blast strengthening material if residual displacement is an important parameter.
In case of maximum displacement, CFRP, polyurea, CPU and BFRP have retrofitted effect of
21.4%, 15.7%, 37.4% and 19.8%, respectively. And for residual displacement,CFRP,polyurea,CPU
and BFRP with respect to NSC have retrofitted effects of 67.4%, 43.0%, 63.7% and -11.3%,
respectively.
Fig 4.8: Behavior of conc. specimen (NSC, CFRP) Fig 4.9: Error bar of disp. under blast loading
Even though the displacement is one of the estimating parameter for retrofit effect, significant
variations and uncertainties can exist. To assess the effectiveness of each strengthening method,
the specimens were inspected for cracking, spalling, and delamination. The crack pattern of NSC
specimen of bottom surface is shown in Figure 5(a). The damages are concentrated at the center
and the overall crack pattern follow concrete yield line. Also, the NSC specimens show a shear
failure compared to retrofitted specimens, which show a bending failure. The retrofitted
specimens show de-bonding failure in interface of FRP and concrete surface. Also, local concrete
crushing has been observed around supported edges in FRP strengthened specimen due to more
rigidity.
(a) Normal strength concrete specimen (b) Failure of each FRP specimen
Fig 4.10: Failure mode each specimen under blast loading

19
However, the retrofitted specimens generally suffered less damage than the control specimens
from the perspective of cracking and overall concrete failure. It can be safely concluded that
retrofitted FRP can sufficiently absorb the energy due to blast load.

20
CHAPTER 5
CONCLUSION AND FUTURE SCOPE
5.1 CONCLUSIONS
From this study, various externally bonded strengthened RC slabs’ response induced by explosive
blast wave pressure are evaluated to understand the retrofit effect. The reflected blast pressure
and impulse values calculated using the ConWEP were in reasonable agreement with the
experimental data. The performance of retrofitted specimens compared to control specimens
when subjected to blast loads of ANFO 35 lbs. has shown the retrofitted effect about 15~38% for
maximum displacement. An average of retrofitted residual displacements was higher than
normal strength concrete specimen’s residual displacement, even though there was no
consistent trend due to various environmental conditions. Therefore, to evaluate the damage
under blast load, failure mode must be considered. From the test results, the retrofitted FRP
specimen has shown bending failure proving that retrofitted FRPs can be used for structural blast
strengthening.
5.2 FUTURE SCOPE OF STUDY
1. Cases in which the axial load does not remain constant during the column response time are
possible. These include situations where the bomb is located within the structure and the
blast excites the girders connected to the column. The effect of this time-varying axial load
should be studied.
2. Cases should be studied when the explosions within a structure can cause failure of interior
girders, beams and floor slabs.
3. Tests and evaluation of connections under direct blast loads.
4. Tests and design recommendations for base plate configurations and designs to resist direct
shear failure at column bases.

21
REFERENCES
 A.Ghani Razaqpur, Ahmed Tolba and Ettore Constestabile, Blast loading response of
reinforced concrete panels reinforced with externally bonded GRPF laminates. Science
direct, Composites: Part B 38 (2007) 535-546.
 Alok Goyal, Blast resistant design: Critical issues, proceedings of the sixth structural
engineering convection, pp IPXI-1-10, Dec 2008
 Anatol Longinow A, and Mniszewski KR., "Protecting buildings against vehicle bomb attacks,"
Practice Periodical on Structural Design and Construction, ASCE, New York, pp. 51-54, 1996.
 B. Lu, P. Silva, A. Nanni, and J. Baird, Retrofit for Blast-Resistant RC Slabs with Composite
Materials, University of Missouri–Rolla, SP-230—76
 B.M. Luccioni1, R. D. Ambrosini & R.F. Danesi1, Assessment of blast loads on structures,
WIT Transactions on Engineering Sciences, Vol 49, 2005.
 Mario Paz, Structural dynamics, second edition, CBS publishers and distributors, 2004.
 Philip Esper, investigation of damage to buildings under blast loading and recommended
protection measures, 9th International Structural Engineering Conference, Abu Dhabi,
November 2003.
 Ray Singh Meena, BE thesis report, National Institute of Technology, Rourkela (2009).
 Smith and Hetherington (1994) Blast and ballistic loading of structures, Oxford
Butterworth- Heinemann.
 T. Ngo, P. Mendis, A. Gupta & J. Ramsay, “Blast Loading and Blast Effects on Structures – An
Overview", The University of Melbourne, Australia, EJSE Special Issue: Loading on Structures
(2007).
 T.D. Ngo, P.A. Mendis, & G. Kusuma, "Behavior of high-strength concrete columns subjected
to blast loading, The University of Melbourne, Australia (2002).

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