A 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 non-linear 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.

Priyanka M, PG student (MTech 4th sem Structures), N. Munirudrappa, Professor, Civil Engineering department, Dayananda Sagar College of Engineering, Shavige Malleshewara hills, K.S.Layout, Bengaluru.

Inroduction

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 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.

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.

Review of Literature
Introduction

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 other 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 special shelter 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 [7], 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) 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 [5], studied the effects of mesh size on pressure and impulse distribution of blast loads with the aid of hydrocodes. 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 same street. 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 [1], 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.

Ray Singh Meena in 2009 [8], 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 [11], carried an analytical study on RC column subjected to blast loading and progressive collapse analysis of a mulit-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 [2], 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.

Blast Loading and Its Behaviour

Explosion Science

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 airblast 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 [3].
  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.

Shock Waves or Blast Waves

Blast wave propagation
Figure 1: Blast wave propagation [10]
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 [2]. 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. 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.

Generalized Blast Pressure History
Figure 2: Generalized Blast Pressure History[11]
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- tures. 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. 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 [11].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.

Dynamic Loadings

Blast loads on buildings
Figure 3: Blast loads on buildings [10]
Drag exerted by the blast winds required to form the blast wave [11]. 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. 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.

Effects on Structures

Blast effects on building structures can be classified as primary effects and secondary effects. Primary effects include [8];
  1. Airblast: the blast wave causes a pressure increase of the air surrounding a building structure and also a blast wind.
  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) [9]
Blast Pressure Effects on a Structure
Figure 4: Blast Pressure Effects on a Structure [9]
  • 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.
  • 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.
  • 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.

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

SDOF system Blast loading
Figure 5: (a) SDOF system (b) Blast loading [8]
(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:
  • 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.
  • 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.
  • 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 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 a 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 [6].

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.

Case Studies

Column Subjected to Blast Loading [10]

Simplified blast loading
Figure 6: Simplified blast loading [10]
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. While the axial load capacities of the two columns are still the 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-Dyna 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.

3D model of the column using explicit code LS-Dyna
Figure 7: 3D model of the column using explicit code LS-Dyna [10]
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 loading is very different from the static loading in terms of the dynamic inertia effect and structural response.

Reinforced Concrete Panels [1]

In this case study eight 1000x1000x70 mm reinforced concrete panels were doubly reinforced with welded steel mesh of designation MW 25.8, which has bar cross-section area of 25.8 mm2, mass per unit area of 2.91kg/m2 and center-to-center spacing of 152mm in each direction. The bar yielded stress and ultimate strength are 480MPa and 600MPa respectively. The concrete had an average 28day compressive strength at the age of testing the panels being 42MPa.

3D model of the column using explicit code LS-Dyna
Figure 8: Test specimen geometry and reinforcement details (all dimensions in mm) [1]
The test set-up was commenced by burying the steel box in the ground, with its top being level with the ground surface. Two cables connected the explosion source to the instrumentation bunker located 150 m away. Rubber pads of the same width and length as the steel angle legs were placed between the angles and the test specimen bottom to ensure uniform support conditions. Similar pads were used between the test specimens and the clamps used to prevent the uplift. Subsequently, the tripod holding the charge was centered above the center of the panel and the charge was hung with a wire. The explosive used was ANFO, comprising 5.7% fuel oil and 94.3% ammonium nitrate, shaped into an approximately spherical form. The explosive energy of ANFO is 3717 kJ/kg, which is 82% of the energy of one kilogram of TNT. Fig. 4 shows the test specimen in place and the tripod holding the charge.

The blast tests were conducted on a Canadian Armed Forces Base. The following procedure was typically followed for each test. The test panel was placed in top of the box and the instrumentation was connected. The ground around the specimen was leveled and compacted. The wooden tripod supporting the explosive charge was erected, ensuring a standoff distance of 3 m. Locations of the incident pressure gauges from the charge center were measured and recorded (fig 9). All personnel were evacuated to a safe distance and the explosive charge was initiated at 1.5 km away from ground zero. After the operator called the area clear, the state of the specimen was observed and recorded. Panels CS4 and GSS1 were subjected to the blast load due to the detonation of 22.4 kg of ANFO while the remaining five panels, i.e. CS2, CS3, GSS2, GSS3 and GSS4 were subjected to the load generated by the detonation of 33.4 kg of ANFO. In each case the distance from the center of the charge to the center of the test panel was 3.0 m.

3D model of the column using explicit code LS-Dyna
Figure 9: Locations of the reinforcement and concrete/GFRP surface strain gauges:
(a) Reinforcement strain gauges (6 mm length);
(b) strain gauges on the bottom surface (30mm);
(c) strain gauges on the top surface (30 mm).[1]

The present testing program and its result indicate that assessing the blast response and resistance of reinforced concrete elements by using actual explosives is a complex task. It is well-known that sometimes minor changes in material properties, test set-up and the surrounding environment could produce significantly different responses at close range. Although in this study replicate specimens were used to assess the effect of such variability's, based on the quantitative and qualitative results, it is not possible to arrive at general conclusions regarding the effectiveness of GFRP for blast mitigation. Theoretically, the blast resistance of structures that are loaded in the impulse realm can be effectively increased by increasing their ductility rather than their strength. Since the addition of FRP increases the strength of flexural members, but not their ductility, the case for the use of FRP in such cases is not obvious. On the other hand, structures that are loaded in the pressure realm would benefit most from an increase in strength rather than ductility. The loading realm is determined by the ratio of the positive phase duration to the natural period of the structure. In the current testing program for nominally similar blast scenarios, noticeably different positive phase durations were measured. One of the reasons for the scatter in the results for the larger charge size may be that the selected charge size of 33.4 kg at 3.1 m standoff is too high even for the retrofitted panels to resist. Hence, if the pressure-impulse combination produced by this charge exceeds the resistance of both the control and the retrofitted panels, then it would not be possible to use the results of the test to assess the effectiveness of the GFRP in mitigating blast damage. The results of tests under the smaller charge size indicate that the retrofitted panel performed very well because it suffered only light damage and had a residual strength that was 75% higher than that of the companion control panel. Note that the smaller charge caused noticeable damage in the control panel, including a 2 mm permanent deformation, but the same did not happen in the retrofitted panel.

Reinforced Concrete Slabs [4]

UMR Mine
A series of square RC slabs with nominal dimensions of 1200 x 1200 x 90mm were chosen for the experimental and analytical investigation. Different materials and upgrade schemes were investigated against out-of-plane blast loads. Five RC slabs were built and strengthened with different schemes and different materials such as CFRP or SRP. With the exception of the control slab (1), two slabs (2A and 2B) were strengthened with CFRP laminates, and the other two (3A and 3B), were strengthened with SRP laminates.

The experimental specimens were tested at the experimental mine at the University of Missouri-Rolla. As shown in Figure 10a, it can be seen that the distance from the test specimen to the mine walls and ceiling are far enough apart that open air design methods are applicable within a reasonable degree of accuracy. As shown in Figure 10b, the test specimens were simply supported on steel box beams. The charge was suspended above the test specimens to the specific standoff distance by a wire, which was also used as the circuit to detonate the charge. Each charge was composed of desensitized RDX high explosive.

UMR mine test site
Figure 10: (a) UMR mine test site (b) Test setup.[4]
In this work, the blast charge weights and the standoff distances to impose a desired displacement ductility level were estimated based on the modified DBD method to account for blast effects. The results of field test for the control slab showed that the achieved displacement ductility levels matched closely with the predicted values. Therefore, a primary conclusion drawn from the experimental results is that the charge weight and standoff distance to generate blast loads can be effectively estimated by the DBD method. Furthermore, slabs retrofitted on the bottom side only were severely damaged irrespective of the strengthening material. However, slabs retrofitted on both sides were adequate in resisting the given threat level; but, failure due to the insufficient shear capacity was observed. By comparing the test results of slabs strengthened on the bottom side and on both sides, the main conclusion was that slabs may require retrofitting on both sides in order to make these slabs resistant to blast loads.

Conclusion

It is not economical to design all buildings for blast loading. Public buildings, tall structures and city centers have to be designed against terrorists' attacks and sudden explosions. It is recommended that guidelines on abnormal load cases and provisions on progressive collapse prevention should be included in the current Building Regulations and Design Standards. Requirements on ductility levels will also helps to improve the building performance under severe load conditions. Evaluation of DLF resulting due to blast loading under several conditions have to be included in the design procedure to get into the correct evaluation of the stress characteristics of the material under consideration.

References

  • A.Ghani Razaqpur, Ahmed Tolba and Ettore Constestabile, Blast loaing response of reinforced concrete panels reinforced with externally bonded GFRP 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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The first Sandvik DT922i, a computer controlled fully automatic tunneling jumbo, has been introduced by Sandvik Mining and Rock Technology in India with a vision to achieve high levels of safety, productivity and

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Horizontal Directional Drilling is a technology in pipe and utility installation that allows greater accuracy and flexibility in placement and ends the need for costly digging, large crews, road closures and other

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Special knowledge is required for building tunnels and wind energy plants in order to implement demanding projects. The facilities and manufacturing processes required for such projects can be adjusted well in advance for

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The Large Hadron Collider (LHC) at CERN is the biggest particle accelerator in the world – and growing. To equip the LHC for new, more ambitious experiments in the next decade, the accelerator is currently

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The river has its source at over 3000 meters high in the snow-covered Andes. It reaches the Pacific Ocean after 250 km. On its way, it overcomes a considerable gradient and is, therefore, ideally suited to generate electricity

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Tan Shunhui, Chairman, CREG, discusses the competitive advantages of the company’s tunneling equipment and solutions, emerging opportunities in developing countries, factors driving its growth and success across

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15-meter mega Slurry TBM rolled off the assembly line at CRCHI. The excavation diameter of the equipment is 15.01 m, its total length is about 130 m, total weight is about 4300 t, installed power is about 9755 kW, rated

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Being the longest water delivery line in the history of Jilin province, transferring water from Songhua River into Central Jilin Province will transfer the largest capacity of water and cover the largest areas. The water diversion

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Amit Kaul, BASF India Limited (Construction Chemicals Division), India, tells about the innovative tunnel lining system and related design aspects, and also describes the main advantages of this lining system in comparison

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Underground infrastructures are an integral part of modern cities as they serve to counter problems of surface congestion and environmental degradation. The problems of ground vibrations, vibration induced noise and ground

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