Effect of Blast Loading on Columns- A Critical Review

IJSTE - International Journal of Science Technology & Engineering | Volume 3 | Issue 09 | March 2017 ISSN (online): 2349-784X

Effect of Blast Loading on Columns: A Critical Review Anusree A Assistant Professor Department of Civil Engineering Muthoot Institute of Technology and Science, India

Ann M Biju UG Student Department of Civil Engineering Muthoot Institute of Technology and Science, India

Athulya Sreesa E K UG Student Department of Civil Engineering Muthoot Institute of Technology and Science, India

Devika Remesh UG Student Department of Civil Engineering Muthoot Institute of Technology and Science, India

Rinsa K UG Student Department of Civil Engineering Muthoot Institute of Technology and Science, India

Abstract An explosion within or nearby a building can cause catastrophic damage to the building. The blast explosion is due to pressure or vehicle bomb or quarry blasting. Lifeline structures like buildings, bridges, pipelines, industrial plants, dams etc. plays an important role in the economy of our country so that they have to be protected from these explosions. Because of the nonlinear behavior of the material, the dynamic response of the structures under blast loading is complex to analyze. Explosion will generate large dynamic loads which would be greater than original loads for which the structure is designed and analyzed. Normally, the conventional structures are not designed to resist blast loads because its magnitude is very higher than the normal design loads. So the engineers are seeking solutions to potential blast situations. Column is the most important load bearing structure in a building.

So it is necessary to study the effect of blast loading on columns. This study focuses to enhance the understanding of the behavior of columns when subjected to blast load and also the effect of various parameters like standoff distance, aspect ratio etc. are discussed.

Keywords: Aspect Ratio, Axial Load, Blast Load, Column, Stand-Off Distance ________________________________________________________________________________________________________ I.

INTRODUCTION

In the past decades, considerable importance has been given to blast load because of accidental or intentional activities related to important structures. The threat of terrorism is in its peak in the entire world. Thus it is necessary to protect the civil infrastructures like embassies, nuclear energy plants, government buildings, power stations etc. from the blast shock. Many researches are going throughout the world for studying the effect of blast loading on various structures.

An explosion is defined as a large scale, rapid and sudden release of energy. According to the nature, explosions can be classified as physical, nuclear or chemical events. A blast explosion nearby or within a structure is due to various reasons like vehicle bomb, pressure or quarry blasting, This can produce external as well as internal damages to the building. This may result in collapsing of walls, windows and damage of lifeline structures.

Conventional structures are not designed to resist blast load. Since the magnitude of design load is very less, the structure is susceptible to collapse under blast. While considering the effect of explosion on structures, there are some fundamental characteristic which are to be considered irrespective of the source. There will be tremendous damage and loss of life if the explosion occurs in an urban area. Immediately after explosion, pressure waves are propagating through the atmosphere and it is the main cause for the damage of the structure. These waves travel faster than speed of sound and collide with the structure.

Fig.1 shows a typical blast pressure profile. At the arrival time tA, following the explosion, pressure at that position suddenly increases to a peak value of overpressure, Pso, over the ambient pressure, Po. The pressure then decays to ambient level at time td, then decays further to an under pressure P so (creating a partial vacuum) before eventually returning to ambient conditions at time td+ and td„. The quantity Pso is usually referred to as the peak side-on overpressure, incident peak over pressure or merely peak over pressure. The incident peak over pressures Pso are amplified by a reflection factor as the shockwave encounters an object or structure in its path. Reflection factors depend on the intensity of the shock wave, and for large explosives at normal incidence these reflection factors may enhance the incident pressures by as much as an order of magnitude. Throughout the pressure-time profile, two main phases can be observed; portion above ambient is called positive phase of duration td, while that below ambient is called negative phase of duration, td„. The negative phase is of a longer duration and a lower intensity than the positive duration. As the All rights reserved by www.ijste.org

284

Effect of Blast Loading on Columns: A Critical Review (IJSTE/ Volume 3 / Issue 09 / 058)

stand-off distance increases, the duration of the positive-phase blast wave increases resulting in a lower-amplitude, longer-duration shock pulse. Charges situated extremely close to a target structure impose a highly impulsive, high intensity pressure load over a localized region of the structure; charges situated further away produce a lower-intensity, longer-duration uniform pressure distribution over the entire structure. Eventually, the entire structure is engulfed in the shock wave, with reflection and diffraction effects creating focusing and shadow zones in a complex pattern around the structure. During the negative phase, the weakened structure may be subjected to impact by debris that may cause additional damage.

Fig. 1: Variation of overpressure with distance at a given time from center of explosion.

Dynamic response of structures subjected to blast loads is complex to analyze because of the non-linear behavior of the material.

Blast analysis may be performed using a time-history analysis. Various software are available for the analysis of structures subjected to blast loads.

II. LITERATURE REVIEW Several works were reported on blast analysis of structural members. This paper focuses to understand the effect of blast loading on columns. Columns are structural elements which transmit the weight of the structure above it to other structural elements below.

It transmits compressive force from all other structural members to the footing. In terms of strength and stability, they should be the second strongest structural component next to the underlying foundation. Blast load though often ignored, have proved to be of vital importance in the present day world.

Blast load can be simulated by using different mechanisms and the effects of explosion on structures are overviewed by Ngo et al. The nature of explosion by the mechanism of blast waves in free air was explained. Different methods were proposed to estimate blast loads and response of structures subjected to blast loading were also studied [1].

Shape of the column plays a vital role in its behavior when subjected to blast loads. Column may be circular, square or rectangular in cross section. Elsanadedy et al. [2011] investigated the effect of blast load generated on the existing exterior RC circular columns. The effect of stand-off distance, charge weight and the pressure CFRP retrofitting were studied through parametric studies. Using the software LS-DYNA, the non-linear finite element analysis was carried out. It was observed that CFRP strengthening could be an effective solution to limit the damage caused by moderate explosions [2]. Abbas et al presented the effect of blast load on an existing exterior RC circular column of a typical building. The effects of standoff distance, charge weight and presence of CFRP retrofitting were examined. The software used for analysis was LS-DYNA. The results obtained indicated that CFRP strengthening could be an effective solution to reduce the damage due to blasting [3].

The response of simple RC columns subjected to constant axial loads and lateral blast loads was examined. Finite element modeling software ANSYS was used to model RC column with different boundary conditions. For the response calculations, a constant axial force was first applied to the column and the equilibrium state was determined. Next, a short duration lateral blast load was applied and the response time history was calculated [4]. Moment- curvature behavior of reinforced concrete column with constant axial load was determined using finite element method and introduced to a model having single degree of freedom.

Using this SDOF model, dynamic response of RC columns subjected to blast load was estimated. The accuracy of the SDOF model was determined using the obtained model [5]. Another important parameter used to find the effect of blast load on columns is the aspect ratio. Samir performed numerical simulations to find the dynamic response and residual strength of RC columns subjected to various blast loads using the finite element software ABAQUS. The effects of transverse and longitudinal reinforcement ratios, charge weight and column aspect ratio were the parameters considered. He concluded that the residual displacement become more significant after a certain charge weight [6]. The effect of blast load on three structures with different aspect ratio was investigated

by Shallan et al. through numerical simulations. Modeling was done using the software AUTODYN. It was found that the effect

All rights reserved by www.ijste.org

285

Effect of Blast Loading on Columns: A Critical Review (IJSTE/ Volume 3 / Issue 09 / 058)

of blast load decrease with increase in standoff distance. Also they concluded that there is no variation in the displacement of column in the phase of blast load with variation of aspect ratio [7].

Load bearing capacity of columns can be improved by varying the type of concrete and also by the addition of materials which impart additional strength to the concrete. Nikl et al. conducted 9 tests to assess the as-built performance of column and the effectiveness of carbon fiber jackets in improving the protection of critical elements under blast load. They developed a quasistatic load protocol to replicate the damage pattern observed in blast test in the field. Jacket strain was measured and the load deflection curves were plotted [8]. Auode et al. studied the blast load performance of ultra-high performance fiber reinforced concrete columns. The parameters considered were the type of concrete, fiber properties, fiber content, transverse reinforcement, spacing and longitudinal reinforcement ratio. They concluded that Ultra High Performance Fiber Reinforced Concrete (UHPFRC) improves the blast performance of RC columns. They also reduce the maximum and residual displacement enhancing the damage tolerance [9]. A series of residual load carrying test on post-blast ultra-high performance concrete was studied by Ki et al. Newly developed Ultra High Performance Concrete (UHPC) material with ultra-high strength and ductility was used to make the RC columns and concluded that UHPC columns have high load carrying capacities than conventional concrete columns [10].

Various studies were also conducted on the effect of blast loading on bridge columns. Williamson et al. studied the response of reinforced concrete bridge column subjected to blast load. They identified the design parameters influencing the performance of reinforced concrete bridges. Three separate blast design categories were recommended to assess the threat severity using stand-off distance as primary variable [11]. The mechanism leading to the response of RC bridge column subjected to blast loads was explained by Williams and Williamson using the results of detailed numerical models. They concluded that column should have adequate capacity to sustain gravity loads followed by a blast event while designing it. They also used finite element model to explain the cross sectional response mechanism which cause spalling of concrete [12]. A study was conducted by Zheng et al. focusing on the effectiveness of FRP retrofitting in the dynamic response of RC bridge columns subjected to blast loads. A finite element model was developed and validated. A parametric study was also conducted using the proposed model. They concluded that the ratio of diameter to thickness, reinforcement ratio, column length and damping ratio can affect the blast resistance of the bridge [13].

Standoff distance has an important role in determining the blast impact. Fang et al. analytically examined the effect of blast load occurred on column within a short distance. They analyzed the model to evaluate damage levels and defined the relation among taken variables with respect to behavior after blast [14].

Park et al. studied the failure mechanism of RC columns under close-in explosion using a fluid –solid coupling numerical method. They found that the chance of local failure is more when subjected to close-in blast loads. Rapid damage assessment method was validated [15]. A fluid-solid coupling numerical method was proposed and used to investigate the failure mechanism of RC columns. When subjected to close-in blast loads local failures such as crushing, cratering and spalling occurs. Parametric studies to understand the effect of column dimensions and reinforcement ratio on the damage degree were conducted [16].

The material of column is also important to study the effect of blast loading on columns. They presented the model analysis of steel column of a large building frame subjected to blast loading. Using finite element modeling software ANSYS a numerical model was prepared to do the analysis of an implicit model [17]. Omidinasab et al. modeled concrete filled steel square columns and blast load was applied to the model using finite element software ABAQUS. They found that inner layer of concrete was seriously damaged even though concrete deformation is restricted using steel tube. Also, it was found that column displacement can be decreased by increasing scaled distance. They concluded that concrete-filled steel columns have high ductility and resistance to blast [18].

Columns can be classified as long columns and short columns. When the ratio of effective length to the least lateral dimensions of the column is less than 12, then it is called a short column. When the ratio is greater than 12, then it is called a long column. The comparison of maximum stress distribution of long and short columns due to blast loading was studied by Wakchaure and Borole and enhanced the understanding of the role that structural details play in affecting the behavior of column. The finite element package ANSYS was used to analyze RC column subjected to blast loading [19]. Used finite element method to develop an Hshaped steel frame column subjected to blast load under different conditions. Blast load was applied to the bottom column of a steel frame structure by varying the stand-off distance, weight of explosive and height of column to simulate the response of column [20].

The blast response and behavior of multi hazard-resistant bridge piers having circular shaped concrete filled steel tube column were analyzed by Fujikura and Bruneau. The result showed that under blast loading high frequency modes of vibrations influenced the structural response of column. Also, along with assumptions and conditions behind different analytical methods different values of shape factors must be used [21]. Kumar and Kumar examined the response of steel frame building subjected to blast loading.

Response of steel column subjected to pressure by varying charge weight and at different stand-off distance was examined. Also the progressive energy collapse of steel column was examined using ANSYS explicit dynamic AUTODYN [22].

III. CONCLUSION Effect of blast loading on columns was reviewed and importance of blast resistant design of the structure was discussed. Blast loads are transient, vigorous and unpredictable. The extent of damage on each structure should be carefully studied to avoid further damages. Response of columns subjected to blast load will vary depending upon various factors like stand-off distance, aspect ratio, charge of explosive etc.

All rights reserved by www.ijste.org

286

Effect of Blast Loading on Columns: A Critical Review (IJSTE/ Volume 3 / Issue 09 / 058)

From the review conducted on previous literature following conclusions can be drawn.

1) Charges situated extremely close to a target structure impose a highly impulsive, high intensity pressure load over a localized region of the structure.

2) The effect of blast load decreases with increase in standoff distance.

3) Aspect ratio was seen to have no effect on displacement.

4) CFRP strengthening and UHPFRC could be an effective solution to limit the damage caused by moderate explosions.

5) UHPFRC reduces the maximum and residual displacement in columns, enhancing damage tolerance during blast loading.

6) UHPC columns showcased the highest load carrying capacity when compared to conventional concrete columns.

7) The reinforcement ratio, column length and damping ratio can affect the blast resistance of bridge columns.

8) Concrete-filled steel columns have high ductility and resistance to blast load.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

T. Ngo, P. Mendis, A. Gupta & J. Ramsay, “Blast Loading and Blast Effects on Structures – An Overview”, EJSE Special Issue: Loading on Structures (2007) H.M. Elsanadedy; T.H. Almusallam; H. Abbas*; Y.A. Al-Salloum; S.H. Alsayed, “Effect of blast loading on CFRP-Retrofitted RC columns – a numerical study”, Lat. Am. j. solids struct. (Online) vol.8 no.1 Rio de Janeiro (2011) H. Abbas, Y.A. Al-Salloum, S.H. Alsayed, H.M. Elsanadedy and Rizwan I. Iqbal, “Numerical study for retrofitting of RC columns for blast loading.” (2011) Ramezan Ali Izadifard1*, Somayeh Mollaei2 and Mohammad Esmaeil Nia Omran, “Preparing Pressure-Impulse Diagrams for Reinforced Concrete Columns with Constant Axial Load using Single Degree of Freedom Approach”, International Journal of Advancements in Technology (2016) Nitesh Moon, “Prediction of blast loading and its impact on buildings”, Thesis (2009) Eisa Ahmed Samir ,“Finite element analysis of reinforced concrete columns under different range of blast loads”, International Journal of Civil & Structural Engineering Year : 2014, Volume : 5, Issue : 2 (2014) Osman Shallan, Atef Eraky, Tharwat Sakr, Shimaa Emad, “Response of building structures to blast effects”, International Journal of Engineering and Innovative Technology Volume 4, Issue 2, August 2014 (2014) Tonatiuh Rodriguez-Nikl, Chung-Sheng Lee, Gilbert A. Hegemier, Frieder Seible, “Experimental performance of concrete columns with composite jackets under blast loading”, ASCE (2012) Hassan Auode, Frederic p Dagenais, Russell p Burrell, Murat Saatcoiglu, “Behavior of ultra-high performance fiber reinforced concrete columns under blast loading”, International Journal of Impact Engineering Volume 80, June 2015, Pages 185–202 (2015) Jun Li, Chengqing Wu, Hong Hao, “Post-blast residual loading capacity of ultra-high performance concrete columns”, (2016) Eric B Williamson , Oguzhan Bayrak, Carrie Davis, G. Daniel Williams “Performance of bridge columns subjected to blast loads”, Journal of Bridge Engineering, ASCE (2011) G. Daniel Williams and Eric B. Williamson, “Response of reinforced concrete bridge columns subjected to blast loads”, Journal of structural engineering, 137(9), 903-913, ASCE, (2011) R. Zheng, P. Zohrevand, H. Erdogan & A. Mirmiran, “Performance of FRP retrofitted concrete bridge columns under blast loading”, International Journal of Computational Methods and Experimental Measurements, Volume 2 (2014), Issue 4 (2014) Fang J. Q, Chung p, Wolfe R. W “Analysis of blast loaded protective wall for bridge columns” Bridge Structures, vol. 4, no. 3, 4, pp. 135-141, (2008) Park et al , “ Analytical study of reinforced concrete columns under blast loading”, (2014) Jian Cui, Yanchao Shi, Zhong-Xian Li, Li Chen, “Failure analysis and damage assessment of RC columns under close-in explosions”, ASCE Journal (2015) Ashish Kumar Tiwary ,Aditya Kumar Tiwary ,Anil Kumar, “Blast loading effects on steel columns”, IOSR journal of mechanical and civil engineering (2015) Fereydoon Omidinasab , Peyman Beiranvand , Saeideh Sadeghi Golmakani , Mohammad Zarei, “The finite element analysis for concrete filled steel columns under blast load” (2016) Wakchaure and Borole , “Comparison of maximum stress distribution of ling and short side columns due to blast loading”, International journal of modern engineering research (2013) B. X. Qi, S. Yan, W. Li , X.J Li , “Numerical simulation on dynamic responses and damage of steel frame structures column under blast loads”, ASCE (2010) Fujikura and Bruneau, “ Dynamic analysis of multi hazard resistant bridge piers having concrete filled steel tube under blast loading” ,ASCE (2012) Anuj Kumar and Gaurav Kumar, “Effect of blast wave on steel structures”, ISSN(2013)

All rights reserved by www.ijste.org

287