Dynamic response of a space framed structure subjected to blast load

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0Dynamic response of a space framed structure subjected to blast load Jayashree.S.M1, R.Rakul Bharatwaj2, Hele

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0

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

3 – 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.

1 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

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

2 Methodology

The methodology adopted in the investigation is shown in figure 1

Figure 1: Figure showing the methodology adopted

3 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 3 m The column size used in

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

(%)

SIFCON (%)

Modulus of Elasticity(N/mm²)

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, 3 and 4 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 2 : Figure showing the mode shape 1

Figure 3 : Figure showing the Mode shape 2

Figure 4 : Figure showing the Mode shape 3

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

4 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 3 D model of the frame building using SAP

Figure 6 : Figure showing the 3-D Model of the building

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 7 and 8, respectively It is observed that the mode shapes obtained from analysis are same as that of theoretical approach

Figure 7 : Figure showing the typical mode shape for RCC frame

Figure 8 : 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 2 gives the comparison of fundamental frequency

of RCC and SIFCON frames It is found that the results are reasonable and difference is

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)

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

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

5 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

1 The overall dynamic behavior of SIFCON frame is better than that of RCC frame

2 The fundamental frequency of SIFCON frame is about 30% more than that of RCC frame

3 The reduction in displacement of about 25-30 % is achieved using SIFCON

4 Reduction in displacement shows the capacity of SIFCON in resisting blast load than the conventional RCC, thereby minimizes the damage

6 References

1 Nitesh.N Moon., (2009), Prediction of blast loading and its impact on buildings, National Institute of Technology, Rourkela

2 Abdel Hafez.A, and Ahmed S.,(2004) ,Shear behavior of high-strength fiber reinforced RCC beams, Journal of Engineering Science, Assuit University,32, pp 79-96

3 Balaguru.P.M and Shah.S.P.,(1992), Fiber reinforced RCC composites, McGraw-Hill Inc., New York

4 Bill Keane and Philip Esper.,(2009), Forensic investigation of blast damage to british buildings, Proceedings of the ICE - Civil Engineering, pp 4-11

5 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

6 Ronald L.S., (2006), Response of wide flange steel columns subjected to constant axial load and lateral blast load, Civil Engineering Department, Blacksburg, Virginia

7 Ngo.T, Mendis.P Gupta.P and Ramsay.J, (2007), Blast Loading and blast effects on structure, The University of Melbourne, Australia

8 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

9 SAP Theory manual, Version 2000