blast load analysis of a concre te column … order to do the blast analysis in autodyn, ......

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International Journal of Civil Engineering and Technology (IJCIET)Volume 8, Issue 4, April 2017, pp.

Available online at http://www.iaeme.com/IJCIET/issues.

ISSN Print: 0976-6308 and ISSN Online: 0976

© IAEME Publication

BLAST LOAD

COLUMN EMBEDDED WITH

M.Tech Student,

Mar Athanasius College of Engineering, Kothamangalam, Kerala, India

Associate Professor, Department of Civil Engineering

Toc-H College of Engineering, Ernakulam, Kerala, India

ABSTRACT

This study investigates the behaviour of a concrete column embedded with

pretwisted bars subjected to blast loading. Pretwisted bars are those that are twisted

along its central axis. It has been found out that the buckling capacity of the steel bars

increases as the angle of twist increases. As a preliminary step, we find the optimum

angle of twist for the steel bar by the buckling analysis of the bar over a set of twist

angles. The steel bar twisted at this optimum angle is embedded in the concrete

column. The column is of size 250 mm

ANSYS Workbench 16.0. Later on, it was subjected to Blast Loading. The Blast

analysis of the concrete column was done in ANSYS AUTODYN. The explosive was

placed at a stand-off distance of 1500 mm. The blast analysis was carried out for

5000 kg, 3000 kg, 300 kg and 10 kg of TNT

air box around the column to act as a medium for the blast. In order to reduce the

large time consumption of the blast analysis, a one dimensional wedge which is filled

with AIR and TNT materials was created in AUTODYN and it was remapped into the

3 D Air box. The blast analysis of the column is continued till the column failure or

the kinetic energy absorbed in the column diminishes. The results from blast analysis

shows that the column failed when subjected to

can withstand with minute damages when subjected to 300 kg and 10 kg. The current

study includes only blast analysis of columns that are unloaded.

Key word: Pretwisted bars, Buckling Analysis, Blast Analysis, TNT, AUTODYN, Re

mapping.

Cite this Article: AnuSneha John and Dr. Nivin Philip, Blast Load Analysis of a

Concrete Column with Pretwisted bars

and Technology ,8(4), 201

http://www.iaeme.com/IJCI

IJCIET/index.asp 1768 [email protected]

International Journal of Civil Engineering and Technology (IJCIET) 2017, pp. 1768–1777 Article ID: IJCIET_08_04_200

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=4

6308 and ISSN Online: 0976-6316

Scopus Indexed

LOAD ANALYSIS OF A CONCRE

COLUMN EMBEDDED WITH PRETWISTED

BARS

AnuSneha John

M.Tech Student, Computer Aided Structural Engineering


Dr. Nivin Philip

Associate Professor, Department of Civil Engineering

H College of Engineering, Ernakulam, Kerala, India




ases as the angle of twist increases. As a preliminary step, we find the optimum


bar twisted at this optimum angle is embedded in the concrete

. The column is of size 250 mm×250 mm. The software used for mode



off distance of 1500 mm. The blast analysis was carried out for

, 3000 kg, 300 kg and 10 kg of TNT. For the blast analysis, we created a 3 D


consumption of the blast analysis, a one dimensional wedge which is filled



rgy absorbed in the column diminishes. The results from blast analysis

shows that the column failed when subjected to 5000 kg, 3000 kg TNT blast whereas it


y blast analysis of columns that are unloaded.

Pretwisted bars, Buckling Analysis, Blast Analysis, TNT, AUTODYN, Re

AnuSneha John and Dr. Nivin Philip, Blast Load Analysis of a

Concrete Column with Pretwisted bars. International Journal of Civil Engineering

), 2017, pp. 1768-1777

CIET/issues.asp?JTypeIJCIET&VType=6&IType=

[email protected]

asp?JType=IJCIET&VType=8&IType=4

ANALYSIS OF A CONCRETE

PRETWISTED

Computer Aided Structural Engineering


H College of Engineering, Ernakulam, Kerala, India




ases as the angle of twist increases. As a preliminary step, we find the optimum


bar twisted at this optimum angle is embedded in the concrete

mm. The software used for modeling was



off distance of 1500 mm. The blast analysis was carried out for

. For the blast analysis, we created a 3 D


consumption of the blast analysis, a one dimensional wedge which is filled



rgy absorbed in the column diminishes. The results from blast analysis

5000 kg, 3000 kg TNT blast whereas it


Pretwisted bars, Buckling Analysis, Blast Analysis, TNT, AUTODYN, Re-

AnuSneha John and Dr. Nivin Philip, Blast Load Analysis of a

International Journal of Civil Engineering

&IType=7

Blast Load Analysis of A Concrete Column Embedded with Pretwisted Bars

http://www.iaeme.com/IJCIET/index.asp 1769 [email protected]

1. INTRODUCTION

Pretwisted bars are the steel bars that are twisted along its central axis. Many researchers have

found out that the buckling capacity of a steel bar increases with the increase of twist angle.

The concept is, an axially loaded compression member buckles about the section with least

resistance and for a pretwisted bar the direction of the least resistance varies along its central

axis. The coupling of the strong and weaker planes results in higher buckling capacity. On the

basis of this fact, we modeled a concrete column embedded with pretwisted bars and it was

subjected to blast loading. The blast loading is done for different explosive capacities. The

explosive capacities differ on the basis of the weight of TNT used.

The steel bar cross section is 20×15 mm. A rectangular bar is taken due to the reason that

the moment of inertia vary at each point along the centroidal axis which results in high

buckling capacity. The modeling and buckling analysis of the steel bar was done in ANSYS

Workbench for different twist angles at an increment of 15°. The boundary condition given

was fixed-fixed and the top face was allowed for axial displacement. A unit load of 1 N was

applied on top and the bar was analysed for buckling. The optimum angle of twist was found

to be 165° (Refer Figure 1).

Figure 1 Variation of Buckling Capacity of Steel bar with Angle of twist

The concrete column which is subjected to blast loading is modeled by embedding the

steel bars that are pretwisted at the optimum angle of twist 165°.

2. BLAST PHENOMENON

When an explosion occurs, large amount of pressure are released out. The pressure

propagates to the surrounding in the form of waves. These shock waves are the blast waves.

Figure 2 is the idealized profile of pressure during a free air blast. The profile shows both

positive and negative phase of the blast wave propagation. Once the peak value is attained,

the pressure decreases in an exponential manner until it reaches its ambient pressure value at

tA+ t0. This is known as the positive phase duration. When we consider the extend of damages

caused by the blast waves, the effect of negative phase is neglected since it does not cause

any significant damage compare to the positive phase.[4]

4.2

4.4

4.6

4.8

5

5.2

5.4

0 30 60 90 120 150 180

Bu

ckli

ng

Lo

ad

Ca

pa

city

(k

N)

Angle of Twist (degrees)

Series 1

AnuSneha John and Dr. Nivin Philip


Figure 2 Idealized Pressure Wave Profile [4]

3. METHODOLOGY

The concrete column is modeled in ANSYS Workbench 16.0. In order to do the blast analysis

in AUTODYN, the column has to be modelled in Explicit Dynamics. The materials of the

model should be explicit materials, otherwise AUTODYN solver cannot analyse the model.

3.1 Material Modelling

The material models for the Explicit dynamics materials are different. The concrete material

from explicit dynamics is M35 CONCRETE and its material model is a Riedel-Heirmaier-

Thoma (RHT) model. For the steel 4340, the material model is Johnson Cook (J-C) model.

Therefore, we have inputted the parameters of each model for the corresponding material.

The RHT (Riedel-Hiermaier-Thoma) model is very helpful in modelling concrete for

dynamic loading. It is a combination of both plasticity and shear damage model. The equation

for the RHT model is given in Equation 1. [4]

3( ( . ) ) ( ) ( ) (1)NHTL

fail c rate rate

c c

ppY f A F R F

f fθ ε= −

Where c

f is the compressive strength,HTL

p is the tensile strength, p is the hydrostatic pressure,

rateF is the strain rate factor, 3( )R θ is the internal resistance force and A , N are the constants

Johnson Cook (J-C) model is used for the material modelling of the steel reinforcement.

The coefficients of the J-C model improves the predictability of the response of the material

and also the strain rate effect on the stress flow grew higher. The mathematical expression for

the stress flow is given by the Equation 2. [4]

( )[ ( ) ][1 ln ][1 { } ] 2p n p mroom

flow

melt room

T TA B C

T Tσ ε ε

−= + + −

−

Where flowσ is the stress flow,

pε is the effective plastic strain,T is the actual temperature,

roomT is the room temperature, meltT is the melting temperature and A , B and C are constants

3.2. Meshing

After the modelling of the column, it is meshed. The mesh size for the concrete part is 25 mm

and that of the steel reinforcement is 4 mm.



3.3. Modeling and Analysis of One Dimensional Wedge

One dimensional wedge filled with Air and TNT is modeled in AUTODYN. The wedge

length depends upon the distance of column from detonation point. Therefore, it is important

to note that the wedge length must be greater than the distance of column from the detonation

point.

In AUTODYN, the Air model is an Ideal gas and the TNT model is JWL EOS. Figure 3,4

and 5 shows the one dimensional wedge for 5000 kg TNT, 3000 kg TNT, 300 kg TNT and 10

kg TNT. All the wedge models were created with a flow out boundary at its end in order to

prevent the reflection of the blast waves when it comes in contact with the end or the

boundary of the wedge.

Figure 3 One dime nsional wedge for 5000 kg TNT

Figure 4 One dimensional wedge for 3000 kg TNT

Figure 5 One dimensional wedge for 300 kg TNT and 10 kg TNT

From the ideal blast wave characteristics we know that the negative pressure does not

create structural damages compared to the positive phase and hence we run the one

dimensional analysis of the wedge model till the wave reaches the end of the wedge. That is

the blast wave would be within the end of the wedge so that the negative pressure in the ideal

blast pressure curve is ignored in the analysis. The end time of the analysis is noted. Figure 6

shows the schematic diagram of the blast wave propagation in one dimensional wedge.

Figure 6 Blast wave propagation through the wedge



3.4. Modeling Air box and Remapping

We import the column model into AUTODYN by linking the AUTODYN with the Explicit

Dynamics. In AUTODYN, an air box was created around the column to act as the medium to

transport the blast wave to the column. The size of the air box is 2000 mm width, 2000 mm

depth and 4000 mm height. The box is placed in such a way that the column is located at the

centre of the box. The analysed one dimensional wedge is remapped to one of the vertex of

the air box. And thus the one dimensional air and blast wave get transformed into three

dimension. The blast analysis starts from the end time of the one dimensional analysis of

wedge.

Column is a Lagrangian meshed element whereas Air is a Euler domain. Therefore,

Lagrangian – Euler interactions has to be provided which is done by defining the fully

coupled option between the air domain and the column. This makes the Lagrangian mesh to

interact dynamically with the Euler mesh. Also the cycle limit as well as the time limit should

be defined to set the analysis limit.

3.5. Boundary Conditions

The boundary condition provided for the column was fixed-fixed end condition. Therefore,

both the ends are restricted to rotations and translations. This is done by arresting the velocity

in the axial direction, so that the column does not move in the axial direction. Around the air

domain a boundary condition of flow out has been provided to prevent the blast wave

reflections when they come in contact with the end faces.

Figure 7 (a) Fixed End Condition of Column and (b) Flowout Boundary of Air box

3.6. Blast Analysis

When the blast energy got filled in the entire air box, the kinetic energy of the air in the box

diminishes to zero. The analysis was continued till the column fails or the kinetic energy

absorbed in the column diminishes. Once the absorbed energy in the column started falling,

there won’t be much distinguishable damages to the column.

4. RESULTS

4.1. 5000 kg TNT Blast

In this case, the weight of the TNT is 5000 kg. Figure 8 shows the kinetic energy variation

with respect to time of the air medium and it resembles ideal blast curve.



Figure 8 Kinetic Energy – Time Graph of Air Domain for 5000 kg TNT

The maximum kinetic energy of concrete is 1.5×1011

Jµ at 1.5 ms.The kinetic energy

absorbed by concrete at 0.45 ms is nearly 0.25×1010

Jµ (From Figure 9). After 7.1 ms, it

drops and thereafter there is not much effect of kinetic energy on the column.The highest

amount of kinetic energy absorbed by the steel bars is 1.3×1010

Jµ at 5 ms (From Figure 9).

The column fails at 7.3 ms as shown in Figure 10.

Figure 9 Absorbed Kinetic Energy in Concrete and Steel

Figure 10 Failure Pattern for 5000 kg TNT at 7.3 ms


The weight of explosive is 3000 kg. The maximum kinetic energy in the air domain is

1.3×1014

Jµ at 0.2 ms (Refer Figure 11).The maximum kinetic energy absorbed in concrete is

8×1010

Jµ at 2 ms. The maximum absorbed kinetic energy is 7.2×109

Jµ at 6 ms (Refer

Figure 12).



Figure 11 Kinetic Energy curve of Air for 3000 kg TNT

Figure 12 Absorbed Kinetic energy in Concrete and Steel for 3000 kg TNT

The column fails at 14.35 ms as shown in Figure 13.

Figure 13 Failure pattern of Column for 3000 kg TNT at 14.35 ms


The weight of explosive is 300 kg. From Figure 14, the maximum kinetic energy was 2×1013

Jµ at 0.2 ms. The curve reaches zero at 4.5 ms. The maximum kinetic energy absorbed in

concrete is 1.45×109

Jµ at 1.25 ms and the maximum kinetic energy absorbed in steel is

1.22×108

Jµ at 1.25 ms (Refer Figure 15).



Figure 14 Kinetic Energy of Air for 300 kg TNT

The concrete column at 15.48 ms is shown in Figure 16. Since the kinetic energy absorbed is

diminished, further damage of the column is not possible. Therefore, the column survives the

300 kg blast with minimum damage.

Figure 15 Kinetic energy absorbed in Concrete and Steel for 300 kg TNT

Figure 16 Column at 15.48 ms

4.4. 10 kg TNT blast

The maximum kinetic energy was 16×1010

Jµ at 0.2 ms. The curve reaches zero at 8 ms. The

maximum kinetic energy absorbed in concrete is 1.4×107

Jµ at 3 ms and the maximum

absorbed kinetic energy in steel is 1.22×106

Jµ at 3 ms (Refer Figure 18). The column at

10.25 ms is shown in Figure 19. Thus, the column can withstand the 10 kg TNT blast.



Figure 17 Kinetic Energy of Air for 10 kg TNT

Figure 18 Kinetic energy absorbed in Concrete and Steel for 10 kg TNT

Figure 19 Column at 10.25 ms

5. CONCLUSIONS

From the results of blast analysis we can conclude that the pretwisted bar embedded concrete

column for 5000 kg TNT does not show any kind of resistance against the force. The failure

of the column was observed at 7.3ms. 3000 kg TNT blast on the concrete column with

pretwisted bars also resulted in the failure of the column at a very fast rate. The failure of the

column was at 14.35ms. The column responded differently to the 300 kg TNT blast. The



kinetic energy that was absorbed by the column get diminished so fast, so that the column

does not fail under 300 kg TNT. For the 10 kg TNT blast, the column was found to be safe,

does not even yielded. The kinetic energy that was absorbed by the column was diminished

fastly.

Therefore we can say the concrete column with pretwisted bars are safe under 300 kg and

below TNT blast loads. As the weight of TNT increases, the time for total damage decreases.

The study considered only unloaded column for blast analysis.

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[1] Samer A. Barakatand Farid H. Abed.Experimental Investigation of the Axial capacity of

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Element Analysis of Buckling capacity of pretwisted steel bars. Journal of Engineering

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[3] Ahmed Samir Eisa. Finite Element Analysis of Reinforced Concrete Columns under

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[4] Mohammed Alias Yusof, RafikaNorhidayuRosdi, NorazmanMohamadNor, Ariffin

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[6] Amr A. Nassr, A. GhaniRazaqpur, Michael J. Tait, Manuel Campidelli and Simon Foo.

Experimental Performance of Steel Beams under Blast Loading. Journal of Performance

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[11] Chengqing Wu and Hong Hao. Modeling of Simultaneous Ground Shock and Air Blast

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blast load analysis of a concre te column … order to do the blast analysis in autodyn, ......

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