Dynamic response of a space framed structure subjected to blast load

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 4, No 1, 2013 © Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN 0976 – 4399 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M1, R.Rakul Bharatwaj2, Helen Santhi.M3 1- PG Student, School of Mechanical and Building Science, VIT University Chennai Campus 2- PG Student, Civil Engineering Department, NIT Warangal, Andhra Pradesh – Professor, School of Mechanical and Building Science, VIT University Chennai Campus jayashree.1610@gmail.com doi: 10.6088/ijcser.201304010010 ABSTRACT This paper investigates the dynamic response of a space framed structure due to blast load An explosion due to blast load can cause devastating damage on the building causing collapse of entire structure and loss of human life Studies have been conducted on the dynamic response of structures to blast load These studies generally help in enhancing the understanding of structural behavior to blast load In this paper an attempt has been made to use Slurry Infiltrated Fiber Reinforced Concrete (SIFCON), a type of FRC with high fiber content as an alternative material to Reinforced Cement Concrete (RCC) SIFCON has high energy absorption capacity, higher strength and it is highly ductile Space framed models are developed and time history analysis is carried out for blast load using the software package SAP 2000 The properties of SIFCON and RCC are derived from the experiments The dynamic characteristics such as fundamental frequency, mode shapes are evaluated The displacement time history response of frames with SIFCON and RCC due to blast load is compared The results showed that the reduction in the displacement of about 25-30 % is achieved using SIFCON Keyword: Blast load, dynamic Response, RCC, SIFCON, SAP2000, Time History Analysis Introduction In the past few years, a structure subjected to blast load gained importance due to accidental events or natural events Generally conventional structures are not designed for blast load due to the reason that the magnitude of load caused by blast is huge and, the cost of design and construction is very high As a result, the structure is susceptible to damage from blast load 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 Concrete is the prime building material in construction industry In the foreseeable future, there seems to be no alternative to concrete as a construction material Although strength of concrete is most important, it is also necessary that the concrete is durable, workable and provide a good service life For example, when a structure is subjected to blast load, the structure should be good enough to protect the building from damage and occupants from death This made the engineers to think seriously and to find out the appropriate technology for improving the performance of concrete subjected to blast load Increase in demand and decrease in supply of aggregates for the production of concrete results in the need to identify new sources of aggregates SIFCON gains importance because it eliminates the use of coarse aggregate SIFCON gained its importance because it is perfect for protecting structures from explosions, like bombs or industrial accidents, and preventing projectiles from damaging the structure To protect the structure from blasting and explosion, SIFCON could be the one Received on June, 2013 Published on August 2013 98 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M better solution In 1960's, the analysis of blast load in design of structures was initiated The U.S army released a publications for the structures designed to resist accidental explosion Philip Esper in 2003 investigated the protection against blast for existing and future structures Tests done on the site and numerical analysis were the highlights He concluded from his studies that ductile materials absorb strain energy to a significant extent but brittle materials fail abruptly Ghani Razagpur et al in 2006 analyzed the RCC panel behavior exposed to blast load These panels were retrofitted with glass fiber reinforced polymers The results indicated that the glass fiber reinforced polymer is not suitable under all condition and experimental works can ascertain the strengthening effects better than theoretical study Ronald in 2006 investigated the behavior of steel columns subjected to axial and lateral blast load The finite element package ABAQUS was used for modeling the structure with different slenderness and boundary conditions The blast load applied to the structure was not uniform The changes in the displacement time histories and formation of plastic hinge were noticed This is due to the changing of axial load [6] Ngo in 2007 has given an overall view on the analysis and design of buildings subjected to blast load This was done to get better knowledge about blast loads and the dynamic response of the different structural components With this study, he concluded the design considerations for adverse events like bomb blasts or impacts with high velocity Methodology The methodology adopted in the investigation is shown in figure Figure 1: Figure showing the methodology adopted Theoretical investigation A three storey space framed building using conventional RCC (M25) and SIFCON is considered for the investigation The height of each storey is m The column size used in 99 International Journal of Civil and Structural Engineering Volume Issue 2013 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M the building is 300 x 300 mm The size of the beam is 250 x 450 mm Slab thickness is 100 mm and thickness of the wall is 230 mm Seismic weight of the building at each floor is computed with a live load of 3kN/m2 Considering the building with rigid beams, the spring constant, k is calculated using the formula 12EI/h3 The material properties are obtained from the experimental results The modulus of elasticity of beam specimens with RCC and different percentages of SIFCON is found individually from the Stress Vs Strain behaviour Table 1presents the modulus of elasticity of the beams with various SIFCON ratio Based on the flexural behaviour beams, it is found that the 40 % SIFCON showed high flexural strength and modulus of elasticity [8] Table 1: Table showing the modulus of elasticity of various beams SL.NO RCC (%) SIFCON Modulus of (%) Elasticity(N/mm²) 100 - 23158 80 70 60 100 20 30 40 20953 21349 32633 39067 50 50 28282 Modal analysis has been done manually using the spring constant and seismic mass of each floor Eigen values are found for both RCC and SIFCON frames Using the Eigen values, natural frequency, circular frequency and the time period are calculated for both the frames Figure 2, and show the comparison of first three modes obtained for RCC and SIFCON It can be seen that the mode shapes of both RCC and SIFCON frames show similar behavior Figure : Figure showing the mode shape Figure : Figure showing the Mode shape 100 International Journal of Civil and Structural Engineering Volume Issue 2013 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M Figure : Figure showing the Mode shape Figure.5 shows the comparison of natural frequencies obtained for RCC and SIFCON It can be seen that the fundamental frequency of SIFCON is about 30 % more than that of RCC This depicts the strength and stiffness characteristics of SIFCON frame over RCC Figure 5: Figure showing the Comparison of frequencies Analytical investigation 4.1 Modeling of frame The space frame building is modeled in SAP 2000 The beams and columns are modeled as frame elements and the slab is modeled as a shell element The wall load is assigned on the beams The bottom of the frame is fixed The diaphragm action is considered at every floor level The beams and columns are properly connected using the end offsets provision Figure.6 shows the D model of the frame building using SAP Figure : Figure showing the 3-D Model of the building 101 International Journal of Civil and Structural Engineering Volume Issue 2013 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M 4.2 Modal analysis using SAP The seismic weight is lumped at centre of mass at each floor level Modal analysis has been carried out in SAP for both RCC and SIFCON frames The mode shapes and frequencies of the frames are found out Typical mode shapes of RCC and SIFCON frames are shown in Figure and 8, respectively It is observed that the mode shapes obtained from analysis are same as that of theoretical approach Figure : Figure showing the typical mode shape for RCC frame Figure : Figure showing the typical mode shape for SIFCON frame Figure.9 shows the comparison of natural frequencies obtained for RCC and SIFCON frames analytically It can be seen that the fundamental frequency of SIFCON is 30 % more than that of RCC in analytical approach also Table gives the comparison of fundamental frequency of RCC and SIFCON frames It is found that the results are reasonable and difference is 102 International Journal of Civil and Structural Engineering Volume Issue 2013 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M around 10 % The difference may be due to the meshing size of the slab generated in the model Figure 9: Figure showing the Comparison of frequencies Table 2: Table showing Comparison of natural frequencies Fundamental Frequency (Hz) RCC SIFCON Theoretical 1.52 1.96 Analytical 1.76 1.6 4.3 Time History Analysis After modal analysis, a short duration blast load is applied to the frames and the displacement response is studied The air pressure wave caused by the blast as shown in Figure.10 and it is given as input for non-linear time history analysis in SAP Figure 10 :Figure showing the Air pressure wave Non-linear time history analysis is performed for both RCC and SIFCON frames to determine the dynamic response The dynamic response is measured in terms of displacement time history at each floor level of the frames Figures 11 and 12 depicts the displacement time history of RCC and SIFCON frames, respectively 103 International Journal of Civil and Structural Engineering Volume Issue 2013 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M Figure 11: Figure showing the Displacement time history of RCC frame Figure 12 : Figure showing the Displacement time history of SIFCON frame The comparison of maximum displacement at each floor level is shown in Figure.13 It is observed that the displacement of SIFCON frame is about 25-30 % less than that of RCC frame This reveals that SIFCON has the capacity to resist blast load Figure 13 : Figure showing the Comparison of Displacement 104 International Journal of Civil and Structural Engineering Volume Issue 2013 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M Conclusion This study deals with the theoretical and analytical investigation on dynamic response of a structure subjected to blast loading Based on the results the following conclusions are drawn The overall dynamic behavior of SIFCON frame is better than that of RCC frame The fundamental frequency of SIFCON frame is about 30% more than that of RCC frame The reduction in displacement of about 25-30 % is achieved using SIFCON Reduction in displacement shows the capacity of SIFCON in resisting blast load than the conventional RCC, thereby minimizes the damage References Nitesh.N Moon., (2009), Prediction of blast loading and its impact on buildings, National Institute of Technology, Rourkela Abdel Hafez.A, and Ahmed S.,(2004) ,Shear behavior of high-strength fiber reinforced RCC beams, Journal of Engineering Science, Assuit University,32, pp 7996 Balaguru.P.M and Shah.S.P.,(1992), Fiber reinforced RCC composites, McGraw-Hill Inc., New York Bill Keane and Philip Esper.,(2009), Forensic investigation of blast damage to british buildings, Proceedings of the ICE - Civil Engineering, pp 4-11 Ghani Razaqpur.A, Ahmed Tolba, Ettore Contestabile ,(2007), Blast loading response of reinforced concrete panels reinforced with externally bonded GFRP laminates, Composites: Part B, 38, pp 535–546 Ronald L.S., (2006), Response of wide flange steel columns subjected to constant axial load and lateral blast load, Civil Engineering Department, Blacksburg, Virginia Ngo.T, Mendis.P Gupta.P and Ramsay.J, (2007), Blast Loading and blast effects on structure, The University of Melbourne, Australia Jayashree.S.M, Rakul Bharatwaj.R and Dr.Helen Santhi.M, (2013), Flexural behaviour of SIFCON Beams, International Journal of Engineering Research and Technology, 2(2), pp 1-7 SAP Theory manual, Version 2000 105 International Journal of Civil and Structural Engineering Volume Issue 2013 .. .Dynamic response of a space framed structure subjected to blast load Jayashree.S.M better solution In 1960's, the analysis of blast load in design of structures was initiated The U.S army... 2013 Dynamic response of a space framed structure subjected to blast load Jayashree.S.M 4.2 Modal analysis using SAP The seismic weight is lumped at centre of mass at each floor level Modal analysis... structure subjected to blast load Jayashree.S.M Conclusion This study deals with the theoretical and analytical investigation on dynamic response of a structure subjected to blast loading Based on