Blast Protection Techniques: A Review

Abstract

In the past few years, many countries have suffered severely from the attacks in form of blasts and other terrorist activities. These attacks have caused a major loss to infrastructure, human lives as well as economic losses. When the explosive occurs intentionally, it is called blast and if the explosive explodes accidentally then it an explosion. In India, the blast resistant design of structures is categorized as explosion above ground, IS 4991 (Indian standard criteria for blast resistant design of structures for explosions above ground, New Delhi, India, 1969) and underground blasts, IS 6922 (Indian Standard criteria for safety and design of structures subject to underground blast, New Delhi, India, 1973). In addition, different international codes and regulations namely NATO (Department of Defense in Structures to resist the effects of accidental explosions, UFC 3-340-02, DC, United States of America, Washington, 2008), FEMA (NATO in Manual of NATO safety principles for the storage of NATO ammunition and explosives, AC/258-D/258, Bonn, Germany, 1993) and DOD (FEMA 428 in Primer to design safe school projects in case of terrorist attacks, Federal Emergency Management Agency, United States of America, 2003) provide guidelines to mitigate blast induced effects on structures. A brief sketch on the development of blast pressure time history generated from detonation of manmade explosions is discussed. Factors affecting the intensity of blast load and its effects on various structural components both experimentally and analytically are also summarized. Theoretical and empirical approaches proposed by various researchers for the prediction of realistic blast load are also studied. The review also includes blast mitigation strategies proposed by various investigations for an improved structural performance against blast. The studies presented in the literature review are broadly categorized under two sections namely studies on air and surface blast loading and studies on underground mine blast loading.

1 Introduction

In the recent years the probability of accidental explosions has increased along with the threat from the terrorist attacks. These loads must be incorporated in designing important structures such as public buildings, tunnels, roadways, railways, water tanks and pipelines [1,2,3,4,5]. Activities like disintegrating of rocks in mine wells require energy in form of blasting and the required energy is not 100% efficient and some energy escape into atmosphere which generate air blast and ground induced vibrations in surrounding areas. An explosion has various categories of blast injury such as primary, secondary, tertiary and quaternary which cause direct impact, fragment of material, displacement of victim by wind pressure and cause burns, toxin inhalation, exposure to radiation respectively as studied by Shirbate and Goel [6]. Depending upon intensity of explosion, distance of structure from blasting source, soil characteristics and type of the structure the damage may range from heavy (partial collapse, cracking, local fatigue, non-serviceability) to minor (nonstructural member damage). In 1946 Friedrich Gerhart Friedlander developed the simplest form of blast wave known as Friedlander waveform given by Eq. 1. The equation was later modified by W.E. Baker in 1973 by curve fitting the experimental results and is most commonly used to calculate and represent the blast waveform. The free-field pressure–time response from an explosion in air is described by Fig. 1. The modified Friedlander equation depends on the time, t which is measured from time of arrival t = t_o–ta.

$${\text{P}}\left( {\text{t}} \right) = {\text{P}}_{{\text{s}}} {\text{e}}^{{\frac{{ - {\text{t}}}}{{\text{t*}}}}} \left( {1 - \frac{{\text{t}}}{{\text{t*}}}} \right)$$

(1)

Fig. 1

Ideal blast wave resulting from explosion in air. Goel et al. [16]

where P_s is the peak pressure and t* is the time at pressure first crosses the horizontal axis.

$${\text{P}}\left( {\text{t}} \right) = {\text{P}}_{{\text{o}}} + {\text{P}}_{{{\text{pos}}}} {\text{e}}^{{\frac{{ - {\text{bt}}}}{{{\text{t}}_{{{\text{pos}}}} }}}} \left( {1 - \frac{{\text{t}}}{{{\text{t}}_{{{\text{pos}}}} }}} \right)$$

(2)

In the above equation, P_pos is known as peak positive over pressure, t_pos is the duration of peak positive over pressure, b describes the decay of the curve, P_o is the ambient air pressure, t_o is the time at peak positive over pressure and ta is the arrival time. From Fig. 1 it can be observed that a negative phase follows the positive phase, in which the pressure is lower than ambient pressure known as under pressure, P_neg. The duration of peak under pressure is known as negative duration, t_neg. The integrals of over pressure and under pressure curves are known as incident over pressure impulse, I_pos and under pressure impulse, I_neg, respectively. The peak overpressure is the highest pressure the initial blast wave creates over the ambient atmospheric pressure.

The positive phase impulse and negative phase impulse are the area under the pressure curve for the duration of the time period of positive overpressure and negative overpressure respectively. The initial decay rate is a measure of how quickly the pressure returns from the peak over pressure to the ambient pressure. Thus, the behavior of blast wave is a highly complex phenomenon and the present structural engineers require comprehensive expertise for the design of blast resistant structures. Moreover, the protection of existing structures under the threat from the lethal terrorists’ groups also needs serious attention. Unlike seismic and wind loads, blast loads are a short duration phenomenon. Though a blast load occurs for milliseconds it is capable to cause catastrophic damage to structures and human life. The common effects of blast load on structure includes 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. In the next section, a detailed review on the research based on the prediction of blast loading along with experimental and analytical blast effects on the various structural components is presented. The section also summarizes the various blast protection techniques implemented by researchers in protecting various civil engineering structures.

2 Studies on Air and Surface Blast Loading

The last few years have seen some horrendous damages to various civil engineering structures targeted by the unethical groups of the society. The present study enlists the performance of these structures as shown in Fig. 2 subjected to the erratic blast loading before and after the blast exposure. It is thus of utmost important to understand the loading generated by the burst of the chemicals involved in these attacks. A chronological review on the estimation of blast load parameters is therefore presented herein. The review projects the accurate formulations derived by various researchers based on the experimental and analytical studies. The estimation of blast load primarily depends on several factors namely the magnitude of blast load on structure during an explosive detonation namely charge weight, standoff distance, geometric configuration of structure and orientation of structure. The power of the blast primarily depends on the charge weight and standoff distance with the former expressed as the equivalent weight of Trinitrotoluene (TNT) that the building will encounter. Range or stand-off is calculated with reference to the center of gravity of the charge situated in the vehicle or the structural member. The Indian code for air blast load, IS: 4991 [1] covered important terminologies explaining the blast phenomenon. The blast parameters required for estimation of blast load were tabulated for 1000 kg of explosive and standoff distance varying from 15 to 99 m. A concise procedure was explained for calculation of blast parameter for explosives other than 1000 kg. An example on calculation of pressure–time curves on rectangular above ground building is enclosed in the document. It divided the structures based on openings and point of blast and according method has been explained for calculation of pressure versus time. The code also included recommendations for planning resistant buildings. The code also enclosed a procedure for calculation of structural response and included formula for calculation of time period of structural members under blast. The code needs to provide design guidelines to protect structures against blast load along with an approach to calculate to blast load for all types of structures for various blast explosives. Held [7] proposed simple equations for calculating blast load generated due to detonation of explosives. The study also proposed simple relationship to calculate blast impulsive, distance of the explosion and damage number considering the sensitivity of the structure. Dharaneepathy et al. [8] studied the blast load acting on the tall structures and importance of critical ground zero distance in determining the same. The study included cylinders of diameter 5 m and heights 100 m, 200 m and 300 m along with a structure 50 m diameter and 100 m subjected 125 kg of trinitrotoluene (TNT) placed at different distances and height of burst 2 m. The responses of the structures were tabulated and presented graphically using software package BLAST and finite element code OSTA. In addition, a 200 m chimney along with a 120 m hyperboloid cooling were analyzed using the proposed method and results for top displacement were compared with the IS 4991 [1] and ASCE, design of structures to resist nuclear weapon effects. The study recommended that the critical ground zero distance should be used as a design distance instead of any arbitrary distance for blast resistant design of structures. Smith and Hetherington [9] discussed the classification of explosives along with chemical formulations and thermodynamics of explosion. The study overviewed the blast wave parameters and its effects on structures. Different types of explosions namely internal, underwater and external explosions were investigated in depth and the after effects of blasts in form of stress waves and aftershocks were also illustrated with examples. The structural response subjected to the impulse loading was also elaborated with the concept of the incremental solutions to the equations of motion. The study concluded with the commonly adopted armor techniques to protect systems subjected to ballistic loading, performance of buried structures against blast and methodologies to be practiced for blast resistant design of reinforced and steel structures. Chock [10] reviewed blast pressure profiles prescribed by Kingery and Bulmash and Baker and the U.S. Army’s Explosions in Air methods to develop a design code BLAST. F to help engineers in the study of structures subjected to air blast loading. The study demonstrated the accuracy of the tool with a finite element code, NASTRAN in the field of aerospace industry. The versatility of the code was proved by applying a blast load to an aero elastic wing model with the results showing good agreement with other finite element models. Lam et al. [11] carried out a parametric study on two rectangular wall panels 3 m high and 1 m wide subjected to hemispherical blast pressure having charge weight (W) equal to 125 kg and 1000 kg of TNT equivalence at a standoff distance (R) of 10 m and 20 m respectively. A detailed procedure for determination of response spectrum was illustrated with an example of a cantilevered wall panel having total mass of 2.26 tonnes and an effective natural period of 0.05 s subjected to charge weight of 500 kg of TNT equivalence detonated at a minimum standoff distance of 12 m from the wall panel. Ngo et al. [12] outlined different methods to evaluate blast loads and its effect on structures. A detailed investigation on blast phenomenon was also presented. In addition to the blast due to air explosion in case of chemical explosions, internal domestic gas explosion and its effect on structures was also discussed. The study reviewed case studies on RC column subjected to blast load and progressive collapse analysis. The study concluded with the effects of building façade under blast load. Bajic et al. [13] proposed a method for obtaining trinitrotoluene (TNT) equivalent of high explosives used in explosive ordnance of Serbian armed forces. The six primary blast wave parameters such positive overpressure, positive phase duration, positive impulse and their negative counterparts were evaluated using modified Sadovskiy equations and the modified Kingery-Bulmash equations. A comparison between these parameters by the mentioned equations was also drawn for different explosive ordnance. It was concluded that a difference between blast wave parameters was observed taking into account TNT equivalent which from explosion safety point of view is of utmost important. Hussein [14] studied the nonlinear analysis of single degree of freedom (SDOF) system subjected to blast loading using computer program NON-SDOF. The program was provided with input data of mass, primary and secondary stiffness, blast force, critical damping, yield strength, etc. Two blast pulses in form of simple pulse and bilinear pulse were applied to SDOF system. Results were obtained for displacement, velocity and acceleration time histories along with energy and hysteresis results for two pulses. Sochet et al. [15] overviewed the available formulations to determine the blast wave loading. The blast load parameters namely peak positive overpressure, positive duration and blast impulse were calculated for three explosives such as trinitrotoluene (TNT), Pentaerythritol TetraNitrate (PETN) and Ammonium Nitrate/Fuel Oil (ANFO). The study compared the blast wave characteristics using blast software tools such as AUTODYN and CONWEP and ASIDE. The results were presented by two approaches namely the reduced distance was expressed in terms of characteristic quantities of the explosive and the quantities of explosive were expressed in terms of reduced distance. The study expressed the TNT equivalents in terms of pressure and impulse for the comparisons of ANFO and PETN. Goel et al. [16] congregated the various blast wave parameters by different researchers which would enable the designers to understand compare and then compute these parameters. Moreover, the expressions presented in the present study were obtained on the analysis of spherical charge and to obtain the parameters for hemispherical charge a factor of 1.8 could be applied directly. In addition to peak positive overpressure and positive over pressure duration the study also concenters other parameters like positive impulse, under pressure phase and wave decay. The study also addressed the dilemma faced by the designers in selecting the method for determining the blast load. A summary of the blast load parameters recommended by various researchers to accurately perceive the blast phenomenon has been listed in Table 1. It was observed in various studies, that the empirical formulations proposed by Kinney and Grahm’s [17] to calculate the blast load are the most commonly used by researchers due to their close agreement with the experiments. Karlos and Solomos [18] elaborated the blast wave characteristics essential for the design of new or the retrofitting of existing structures so as to be able to withstand the effects of explosive loads. The study outlines the procedure to calculate the blast loads with the help of examples considering the intensity of blast load and location of blast. The procedure to calculate blast pressure histories for the design of structural and non-structural elements was also presented. Rigby et al. [19] emphasized the importance of negative phase in the calculation of blast pressure histories. The study discussed the modified equations to calculate the negative blast phase and validated the results for hemispherical charges subjected to concrete external wall. A single degree of freedom model was also validated experimentally along with the proposed equations to calculate the blast load. The study also included the finite element analysis to study the failure patterns of the materials to highlight the importance of negative phase and it was concluded that the effect of the negative phase was underestimated and cannot be neglected. Ullah et al. [20] reviewed the blast wave parameters for air and ground blasts considering the Unified Facilities Criteria (UFC 3-340-02). The study also recommended empirical and analytical equations for accurate calculations of blast pressures required for the dynamic analysis of structures. The guidelines formulated to estimate the blast load is applied analytically in the next section to evaluate the performance of structural components and global responses of structures. The present study also details the performance of Murray Federal Building damaged during the Oklahoma City bombing, 4^th April 1995. The illustrations shown in Fig. 3 animates the stages of damages experienced by the building housing offices for the United States security administration, military, kids and child care with nearly five hundred employees. The tragic incident left one hundred and sixty-seven men, women and children killed and injured eight hundred and fifty-three others. The blast led to a damage of three hundred and twenty-four surrounding buildings, overturned and burned around eighty-six automobiles and, blew out windows and doors and caused property damage worth six hundred and fifty- two million dollars. The schematic representation is elaborated in to following six stages.

• 1. Stage 1: The Murray Federal Building is under threat and targeted by the unethical group of the society acquiring a domestic truck for bombing and parked it at a distance less than 4.3 m from the building face.

• 2. Stage 2: The bomb is estimated to have contained the explosive equivalent of 1,820 kg of TNT. The threat in form of explosives is equipped in the vehicle and detonated.

• 3. Stage 3: The blast wave soon surrounds the building causing damage to window panels, exterior walls are blown with some columns also damaged.

• 4. Stage 4: The blast wave forces floors upward.

• 5. Stage 5: The blast damage resulting in the progressive collapse.

• 6. Stage 6: More than one third (approximately 42%) of the original building floor area was destroyed by blast-induced damage or the resulting progressive collapse. It was estimated that 10% of the damage was blast induced while 90% of the damage was progressive collapse-induced.

Fig. 2

Performance of various civil engineering structures before and after blast attacks

Table 1 Blast parameters proposed by various researchers

Fig. 3

Schematic representation of damaged Murray Federal Building during Oklahoma City bombing

2.1 Experimental and Analytical Blast Studies

Due to the hazardous and life-threatening effects of blast loads, the experimental evaluation of structural elements is limited. Many researchers have preferred to perform analytical tests to investigate the performance of various materials and structural components subjected to blast load. The accurate determination of analytical blast load history becomes critical for such an investigation. Krauthammer and Altenberg [21] evaluated the performance of glass panels modeled as a single degree of freedom system neglecting the damping and subjected to blast loads of 10 kg and 100 kg TNT. The numerical simulation was carried out using Mathematica software. The blast load was modeled linearly considering both positive and negative phase blast effects and the results were compared with that obtained from positive phase blast loading only. It was observed that the negative phase blast loading led to large displacement motions and in case of glass panels has relatively larger importance. The effect of scaled distance and charge weight on the failure mode of glass panels is also assessed. Iqbal [22] studied the performance of concrete containment of nuclear power plant when subjected to external blast loading. The empirical formulations proposed by various researchers to calculate blast pressure were compared with the experimental determination of air blast parameters. The study also performed dynamic analysis of full-scale typical reactor containment using SAP2000 subjected to external blast loads varying from 30 to 160 tons of Trinitrotoluene (TNT) at a detonation distance of 50–200 m. It was observed failure distances were observed at 110 to 200 m for the blast charges of 40, 60, 80, 100, 120, 140 and 160 tons of TNT. Jayasooriya et al. [23] investigated the blast response two dimensional reinforced concrete frames (two storeys) subjected to external blast using LS DYNA software package. Damage assessment of front column was also evaluated. The column and beam dimensions were taken as 400 mm × 400 mm and 300 mm × 450 mm respectively. The material properties of concrete and yield strength of steel along with the transverse reinforcements in these structural components were also specified. MAT 72 REL 3 was used to model concrete. Structural analysis was conducted for six blast loading cases with charge weights of 150, 350, 420,500, 650 and 700 kg of TNT and constant standoff distance of 10 m. The software also examined material behavior under rapid strain rate. Response of the structure in form of strain developed was recorded before blast, after blast at 75 ms (ms), 80 ms and 200 ms respectively. Moreover, the residual displacement was also recorded at first floor. It was observed that displacement varied in non-linear manner and increasing gradient, with blast pressure for a constant standoff distance. Finally, the study illustrated an expression for evaluation of vertical damage index of ground column. Moon [24] reviewed different parameters of blast loading. The main objective of the study was divided into two parts; firstly, to compute the blast loading for closed and open rectangular structures along with open frame structures, secondly 6.4 m high RC columns subjected to constant axial loads and lateral blast loads were examined. The finite element package ANSYS was used to model RC columns using Solid 65 element for concrete and Link 8 element was used to model the steel in reinforced concrete. The columns were modeled for the concrete strength of 40 MPa for Normal strength column (NSC) and 80 MPa for High strength column (HSC) and stirrups spacing 400 mm for ordinary detailing and 100 mm for special seismic detailing. The column size for NSC was kept 500 × 900 mm and for HSC was kept 350 × 750 mm. The blast load was calculated for a 5 m standoff distance using Oklahoma bombing report. Step wise procedure for calculation of blast load was also discussed. The lateral deflection at midpoint for the above-mentioned columns were also tabulated and was observed that the critical impulse for the higher strength column case was significantly higher. Hwang [25] performed linear and nonlinear dynamic analysis of three storeys and ten storeys steel frame building using explosive charge of 1000 lb TNT at standoff distance of 15 ft, 30 ft, 50 ft and 100 ft. The study reported displacement responses, storey drift, capacity ratio and inelastic demands of the considered structures using finite element software SAP2000. The default plastic hinge properties are considered for nonlinear analysis. The blast load was calculated using CONWEP program based on TM 5–855 and applied to the structure. Goel et al. [26] examined the effect of various stiffener configurations on the response of rectangular plate modelled as shell element subjected to air blast loading using ABAQUS software. The study also compared the dynamic performance of plate with stiffener, in comparison with equivalently thickened unstiffened plate. It was observed that the equivalently thickened unstiffened plates exhibit higher peak displacement as compared to the stiffened plate, signifying the importance of the stiffeners placed strategically. It is concluded that the stiffener layout and strain rate consideration govern the dynamic response of the plates subjected to small duration blast loading. Raparla and Kumar [27] studied the linear responses of different building models namely single storey one bay, three storey one bay, five storey one bay and ten storey three bay subjected to blast loading calculated using TM 5–1300. The charge weights were assumed to be 500 kg and 1500 kg of TNT at a standoff distance of 10 m, 2500 kg of TNT at standoff distances of 10 m and 40 m along with the charge weight of 1500 kg of TNT at 25 m range. The analysis was carried out using finite element software SAP2000. The study concluded that the responses obtained from the analysis of heavy structures are low in comparison with lighter structures. The presence of damping also decayed the obtained responses after some time. Anandavalli et al. [28] experimentally demonstrated the importance of laced reinforced concrete structures in comparison with reinforced concrete structures to minimize the separation distance between two explosive storage structures for a charge weight of 75 T (NEC). The separation distance was reduced nearly 101 m to 30 m. It was also observed that the storage structures were found to be in re-usable state after blast trial. Draganic and Sigmund [29] illustrated steps to determine blast loading using Unified Facilities Criteria (UFC), Structures to Resist the Effects of Accidental Explosions, U. S. Army Corps of Engineers, Naval Facilities Engineering Command, Air Force Civil Engineer Support Agency, UFC 3-340-02, 5 December 2008. A multi storied building 16 m long, 17 m wide and 24 m height was subjected to charge weights of 1 kg, 10 kg and 100 kg of trinitrotoluene (TNT) explosive. SAP 2000v14 software package was used to model the 3D model of structure. Beam elements were defined to beams and columns and shell elements for slab and walls. Material nonlinearity was incorporated into the structure for steel and concrete. The analysis included the study of plastic behavior of structure. The explosive charge was applied as a pressure time history very close to the structure. The results for the nodal displacements on the front middle column for three charge weights were represented. Kadid et al. [30] carried out a numerical investigation of reinforced concrete simply supported slabs having plan area 120 m² subjected to uniform blast loading. Nonlinear finite element analysis is carried out using ABAQUS software considering both positive and negative phase blast effects. It was observed that the aspect ratio of slab affects the performance under blast loading. The blast charge, time duration and standoff distance are critical parameters affecting the performance of reinforced concrete slabs described in terms of displacement and propagation of damage. Matsagar [31] investigated the performance of sacrificial blast walls made of steel plates and reinforced and unreinforced concrete slabs subjected to blast loading. The study performed three-dimensional nonlinear dynamic analysis using ABAQUS software. The blast load was calculated using TM5-1300 and modified Friedlander’s equations. The thickness of the steel slab was assumed to be 20 mm whereas the thicknesses of plain concrete, reinforced concrete and steel fiber reinforced concrete were assumed to be 100 mm, 150 mm and 200 mm. The study concluded that the steel fiber reinforced concrete slab was found to be the most effective in mitigating the blast loading. Jain et al. [32] modelled a three-dimensional reinforced concrete wall using shell elements in finite element software ABAQUS subjected to triangular blast wave loading. The blast loading parameters is calculated from TM5-1300 and UFC 3-340-02. The steel reinforced was also modelled using the rebar option for four different thicknesses of wall with bar diameter varying from 8 to 12 mm. The material properties are assumed to be M25 grade concrete and Fe415 steel considering the minimum percentage of steel requirement as per IS 456:2000. It was observed that thickness of the RC wall and grade of concrete govern the blast response predominantly as compared to the percentage of reinforcement, diameter of the rebar and grade of steel. Xu et al. [33] experimentally investigated the performance of columns subjected to different charge weights at a fixed standoff distance of 1.5 m. Total eight column specimens having size 0.2 m × 0.2 m × 2.5 m were tested, four specimens each of high strength reinforced concrete (HSRC) and ultra-high-performance fiber reinforced concrete (UHPFRC). The mix proportions of mix design of UHPFRC and HSRC are same except that UHPFRC contained 2.5% steel fiber.

The observations noted from the failure and damage modes of the columns suggested that the ultra-high performance fiber reinforced concrete performed superior in blast loading resistance as compared with high strength reinforced concrete columns. Hashemi et al. [34] detailed a three-dimensional finite element model of a cable-stayed bridge subjected to blast loading using LS-DYNA software. The dynamic response of the bridge pylons was investigated under different blast loads with small to large explosions detonated at different locations above the deck. The blast load modelled using finite element software was validated using US army manual UFC 3–340-02. The bridge was designed using Australian Standard and it was observed that on application of blast load, the damages incurred to the deck and pylons of the bridge does not result in global progressive failure of the deck or pylon in any of the blast scenarios investigated. For large scale explosions, detachment of cables was observed particularly when the explosion that took place close to the end supports of the deck. It was finally concluded that pylons having a modified octagonal cross-section have superior blast performance when compared to conventional rectangular hollow box sections.

Pourasil et al. [35] carried out progressive collapse analysis of buildings which is based on the alternative path method and the sudden removal of one or several columns. The study developed a procedure for progressive collapse analysis of common steel building structures subject to blast loading. A three-dimensional, seven storey building model subjected to direct blast loading of 1 ton TNT equivalent at a standoff distance of 4 m was simulated using SAP2000 and ABAQUS software. The blast load time history was calculated using ATBLAST tool. The criteria used to determine the behavior of structures against progressive collapse were demand to capacity ratio, plasticity index, and rotation of the members. The study concluded that the internal column of the building reaches the plastic stage later than columns exposed to blast and was a key member to prevent progressive collapse of a building subjected to blast loading. Shin and Lee [36] performed the finite element analysis of single degree of freedom (SDOF) system of steel components subjected to blast loading. The study reviewed the applicability of design charts for the maximum responses of the elastic plastic SDOF system detailed in UFC 3-340-02 using LS-DYNA. The study was based on the assumption that only the positive phase of the blast loading is considered noting that the negative phase has been known to have an insignificant effect on the structural responses. It was concluded that the elastic–plastic SDOF charts presented in UFC 3-340-02 are insufficient to obtain the responses for near-field detonation. Thus, an extensive literature is available investigating the structural performance against the surface and air blast phenomenon. The present review highlights some important experimental and analytical results obtained from the past investigations. A compressive performance review of various materials and components tested against the air and surface blast loading are also tabulated in Table 2 along with the recommendations to improve the structural performance are also presented. Thus, future research supervising the performance of structural materials and components tested against the blast loads at a controlled facility is essential. A numerical study investigating the scaled models of concrete, steel and glass panels subjected a scaled blast load (very low charge blast weight in form of illuminations) experimentally and validating the same with analytical software-based models is also important in establishing the relationship with analytical and experimental testing of structures subjected to such a complex phenomenon.

Table 2 Structural performance assessment with recommendations against blast loading

2.2 Blast Resistant Techniques

Since many centuries, the earthquake loads have posed challenges for structural engineers to develop concepts and techniques to protect structures against the natural dynamic phenomena. At construction sites also practices like pile driving, dynamic compaction, operation of heavy equipment give rise to ground vibration causing damage to neighboring buildings Significant contributions have been made by various researchers all over the world in the form of development of concept of earthquake resistant structures and various control devices has helped the civil engineering community to protect and avoid catastrophic damages incurred to many buildings, bridges, dams, nuclear plants, water retaining structures and other manmade structures. The present challenge is in the form of a man-made generated activity known as blast which is a highly impulsive dynamic load and acts within milliseconds as compared to earthquake load (cause vibration to ground surface i.e., seismic force) which acts over a few seconds. Explosive loads directly attack on the weak part of the structure and earthquake attacks on the foundation of structure. Explosion and earthquake cause local damage and global damage to the structure respectively. Mass plays an important role to resist explosive loads on the other hand mass worsen the response during an earthquake. The other significant contributions developed by various researches in protecting and preventing hazardous damages to civil engineering structures are as also discussed in the present review. Sakula [37] proposed the installation of building facades in form of glazing, framing, cladding and fixing to minimize the damages incurred to structures subjected to blast loading. The study provided guidelines to assess the nature of bomb blast and identify the damage and injury caused due the detonation of explosives. The study advised to model blast using computer program such as DYNA -3D to predict the behavior of frame structure taking the nonlinear behavior of materials in consideration. The study also emphasized on finite element analysis of different glazing systems to be able to withstand the dynamic loadings. Miyamoto and Taylor [38] controlled the performance of special moment resisting frame (SMRF) building designed using Uniform Building Code with fluid viscous dampers. Three different building models namely bare frame, fluid viscous damper (FVDs) equipped frame and shear wall frames were selected to compare the performance of building subjected to air blast loading. It was observed that SMRF buildings installed with FVDs improved the performance of building subjected to large blast loading. The FVDs were found to eliminate the inelastic demand in frames by reducing the displacement and plastic hinge rotation of lateral load resisting frames under blast loading. Hayes Jr. et al. [39] proposed three methods namely pier-spandrel, special moment frame and internal shear walls to improve the blast performance of the Alfred P. Murray building that was severely damaged during the 1995 bombing in the Oklahoma City of United States of America. The study overviewed the performance of the nine storied ordinary moment frame building designed for gravity load and damaged by 1820 kg of explosives (TNT). The structure was strengthened using the ASCE-31–02 (Seismic Evaluation of Existing Buildings) with an assumption that the structure is located in high potential seismic zone of San Francisco, USA. The blast response of the structure using the above-mentioned methods were presented using the software packages developed by U.S. Army Corps of Engineers. The study concluded that seismic detailing of structure improves the blast response of structures. Moreover pier-spandrel and special moment frame significantly improved the performance of building as compared to internal shear walls. Koccaz et al. [40] discussed the blast resistant design of buildings from both architectural and structural design approach. The study was restricted to buildings under the effect of different types of high explosives. The study emphasized the role of architectural planning in reducing the loss of life and damage during blast and for existing structures, location of proper obstructions can mitigate the effects of blast. It was also observed that from structural point of view buildings with domes and arches reduce blast effect better as compared to cubical form. In addition, it was also noted that complex building plans should be avoided and single storey structures behaved better as compared to multi storey structures. Proper layout of building components and their placement to reduce the blast damage were also suggested. Provision for bomb shelters in buildings should also be given importance so that occupants can retire and protect themselves during the bomb attack. Moreover, brief recommendation on cladding, glazing and installation of non-structural component location was also proposed. The study asserted the importance of collapse limit state design and functionality limit design approach. Lastly the beam column connection failure in steel and reinforced concrete structures were presented along with the measures to enhance the behavior under blast load. Smith [41] reviewed the performance of blast walls against the damaging effects of blast waves in reference to the importance of the blast wall’s location relative to the threat and the asset that is to be protected. The study also outlined the research targeted to develop advanced materials used in the construction of these walls. The paper also discussed the recent and ongoing research and development in the design, construction and performance of blast walls. Huang and Whittaker [42] assessed the performance of a conventional and base isolated nuclear power plant (NPP). The diameter of containment vessel was 42 m and the height was 61 m, moreover the height of the internal structure was 39 m. The total weight of the building of NPP was 75,000 tons. The performance of the building was studied for a threat of 2000 kg of trinitrotoluene (TNT) at a standoff distance of 10 m from NPP building detonated on hard rock. Both air and surface blast shocks were studied for performance assessment of structure with the former computed using a computational fluid dynamics code (Air3D) and the later generated using an attenuation model for rock response developed by Wu and Hao [43]. The finite element model for conventional and base isolated NPP was modeled and analyzed using LSDYNA (LSTC 2003). The internal structure was modelled as lumped –mass stick. The base isolated reactor building was developed by placing bilinear springs beneath the base slab of the conventional reactor building and was assigned the properties of lead rubber bearing. A comparative study of two NPP was also conducted and base shear, drifts and acceleration for the two threats was also presented. The study concluded that base isolation improved the performance of containment and internal structure underground shock. Goel and Matsagar [44] discussed various blast mitigation techniques that need to be adopted to design new structures and safeguard heritage and important structures without intervention to parent structure. The study emphasized on the importance of assessment of behavior of structure against blast load and general guidelines for analysis and design of blast resistant structures were also presented. The study examined the effect of increase in standoff distance on structure along with the shape and the placement of building under blast load. It was recommended to use lightweight energy absorbing materials and geometry of structure should be as simple as possible. In addition to the above-mentioned blast mitigation strategies, thorough description of construction and behavior of sacrificial walls was also portrayed under blast load. Burrell et al. [45] investigated the performance of reinforced concrete and steel fiber reinforced concrete (SFRC) column under blast load. Total eight columns of size 152 × 152 mm were constructed and transverse reinforcement was designed using Canadian Standards Association (CSA) 2004. Out of eight columns two were constructed with plain self-consolidating concrete (SSC) and six SFRC columns were constructed with SCC and steel fibers. The total height of column was 2468 mm and longitudinal reinforcement comprised of 4–11.3 mm diameter bars (Bar area = 100mm²). The hoops for seismic columns were placed at 38 mm spacing whereas for non-seismic columns they were placed at 75 mm spacing. The concrete type and percentage of fiber content (0 to 1.5%) was also varied. The blast induced shock waves were stimulated using the state-of-the art shock-tube testing facility at the University of Ottawa with the clear height of column between the steel support was 1980 mm. All the columns were subjected to four increased blast loads until failure. A detailed investigation on the effect of seismic detailing on RC columns, effect of steel fiber on blast behavior of columns and the failure mode of the columns under blast was also concluded. The present researchers are also implementing energy dissipating techniques to mitigate the blast loads and the results validate the effectiveness of control devices in improving the structural responses when subjected to blast loads. Zhang and Phillips [46] demonstrated the importance of base isolation technique for blast loaded structures. A five-storey base isolated building was modelled with nonlinear bumpers which is a passive energy dissipation device consisting of nonlinear spring with cubic stiffness installed at base level subjected to blast and earthquake excitations. The study was based on the assumption that the model remains linear elastic during all external dynamic excitation and all the major members remain functional and nonstructural components may get damaged. In addition, the model is a lumped parameter model with one degree of freedom on each storey. The seismic excitations were performed using 4^th order Runge–Kutta scheme with a fixed time step of 1/10000 s and blast load was performed using 4th-5th order Dormand-Prince with a variable time step. The simulations were performed using MATLAB’s SIMULINK environment. Northridge and Tohoku earthquake ground motions were selected for seismic excitation and blast load was calculated for charge weights of 500 kg and 100 kg of Trinitrotoluene (TNT) under medium to long standoff distance. The study concluded that the use of supplemental passive devices improved the behavior of structure under blast without affecting the seismic design. Mohebbi and Dadkhah [47] studied the performance of a hybrid control system designed to protect a ten-storey shear frame structure against earthquakes when subjected to external blast load on different locations. Different types of control systems were installed to mitigate the response of the structure namely, low and high-damping base isolation systems and hybrid base isolation system with constant and variable voltage for MR damper. The blast load was calculated considering both positive and negative phase blast effects as detailed in UFC 3-340-02. It was observed that semi-active hybrid base isolation system was more effective than the high-damping base isolation system in reducing the structural responses of the building. Kangda and Bakre [48] evaluated the performance of base isolated structures in mitigating the responses of fixed base structures subjected to air blast loading. The study concluded that the lead rubber isolators are effective in mitigating the blast effects and proper selection of isolation parameters is must in obtaining maximum reduction in structural design parameters. The study also reviewed the performance of the six-degree system subjected to negative phase blast effects. Thus, the present researchers must investigate the role of energy dissipaters in the form of isolators to reduce the effects of blast loading as extensively reviewed in the field of earthquake loading. Some interesting references of base isolation in the field of seismic research are summarized as follows. Jangid [49] highlighted the significance of bidirectional excitations in the design of sliding structures. The study was extended further by Rao and Jangid [50] to propose an effective sliding system. The performance of shear type buildings and bridges equipped with different sliding isolated devices was presented by Jangid [51]. Kulkarni and Jangid [52] carried a parametric investigation on the structural flexibility equipped with base isolation subjected to earthquake excitation. Soni et al. [53] investigated the effectiveness of double variable frequency pendulum isolated subjected to earthquake excitations. The threat assessment steps to be followed for blast resistant design of structures is illustrated by Fig. 4 based on the methodology presented by Remennikov [54].

Fig. 4

Methodology for blast resistant design of structures

3 Studies on Underground Blast Loading

The man-made inventions in form of explosives, dynamites and development of nuclear energy were intended for the welfare of human society. Lately, these inventions have threatened the very existence of human life due to its applications in the warfare activities. Conducting underground blasting tests and storing life threatening chemicals in storage facilities pose a threat to the human life and property. Regardless of the disadvantages associated with the blasting activities some groups of researchers are working hard to adopt these inventions for the growth of the future generations. In field of mining engineering, controlled blasting is performed to fragment large rock masses into smaller ones. Recently, these activities have resulted in ground vibrations and damaged the nearby structures. Thus, the need to understand the underground blast induced vibration needs attention. In this section, research focused on the mine blast phenomenon has been broadly investigated under following two parts. The first section details the empirical and experimental tests conducted to understand the underground blast vibrations are presented. The final section reviews the role of structural engineers in developing techniques to protect structures against underground excitations are discussed.

3.1 Analytical and experimental Mine Blast Tests

The magnitude of blast induced ground vibrations is primarily influenced by the intensity of the explosion, the distance of the structure from the blasting source, the soil and rock characteristics and the type of structure. The damages caused to structures under this erratic phenomenon may range from heavy, such as partial collapse, cracking and local fatigue to minor damages, such as that of nonstructural components only. The significance of protecting structures from the damaging effects of underground blast induced ground motion is progressively gaining importance. Constraints on space, cost, and safety issues have limited the number of experimental investigations on underground blasting. Site specific empirical models have been proposed by various researchers to predict the severity of blast induced vibrations in terms of peak particle velocity (PPV).

IS: 6922 [2] discussed important terminologies and parameters required to assess the underground blasting phenomenon. The guideline proposed the empirical formula to calculate peak particle velocity (PPV) given by Eq. 3 and restricted the PPV values to protect the structures from damage for different soil conditions. It is directed in the code that Q is the charge per delay in kg, R is the distance from the blast point in m and the value of K is taken as 880 for soft rock, soils and weathered rock and 1400 for hard rock. In addition, the code provides safe distance of the structure from the blast point and empirical equation to design structures for seismic effects of blasts in terms of design acceleration. Finally, the code summarized necessary reasons to monitor the ground vibrations useful for excavation in buildup areas.

$${\text{PPV}} = {\text{K}}\;\left( {\frac{{{\text{Q}}^{2/3} }}{{\text{R}}}} \right)^{1.25} \quad \left( {{\text{mm}}/{\text{s}}} \right)$$

(3)

Siskind et al. [55] discussed the damage potential and disturbance caused to the neighboring human population due to the blast-produced ground vibration from surface mining. The study also determined safe levels of ground vibrations in terms of peak particle velocity and suggested appropriate ground vibration measurement techniques. It was found out that from the experimental investigation of structure responses that an amplification factor of nearly 1.5 and 4 were obtained for structures as a whole (racking) and mid walls, at their respective resonance frequencies. Moreover, for blast vibrations above 40 Hz, all amplification factors for frame residential structures were less than unity. The ground vibrations also aggravated the human annoyance due to wall rattling, secondary noises, and the presence of air blast. The study recommended that the peak particle velocity levels of 0.5 to 2 in/sec safe for residential type structures however based on psychological responses the peak particle velocity limits ranged from 0.5 to 0.75 in/sec. Rigas and Sebos [56] proposed an analytical model for the prediction of compressional and shear waves produced by an explosion in stratified soil and highlighted the PPV equations as documented by [57, 58]. The study considered the effects of reflections on particle velocities based on the cube root scaling model of R/W^1/3. The study proposed enhancement factors equal to 2 and 4 for compressional and shear waves respectively for safety analysis of underground and above ground structures subjected to surface explosion. The study also proposed finite element analysis method in case of large-scale problems to determine structure response. Carvalho and Battista [59] analyzed a reinforced concrete frame structure using experimental and theoretically generated underground blast induced vibrations. An actual RC thirteen storey residential building subjected to underground blasting during excavation works for the construction of an underground station in the city of Rio de Janeiro was studied. The experimental values of the blast-induced vibrations in the structures and their architectural components were obtained by micro-accelerometers installed along the height of the building. The analytical model of the building was developed using finite element method subjected to blast load in terms velocity vector (ẋ) was calculated using Eq. 4 whereas the peak particle velocity (PPV) is calculated using Eq. 5. It was observed that the building developed damages in non-structural elements and accelerated settlement of the shallow footing foundations. The study recommended use of steel piles for the foundations to mitigate the damages incurred to structures.

$${\dot{\text{x}}}\left( {\text{t}} \right) = - {\text{v}}\;{\text{e}}^{{\frac{{ - {\text{t}}}}{{{\text{t}}_{{\text{d}}} }}}}$$

(4)

$${\text{v}} = 48.2\;\left( {2.52^{{ - {\text{n}}}} } \right)\;{\text{s}}\;\left( {\frac{{\text{R}}}{{{\text{Q}}^{1/3} }}} \right)^{{ - {\text{n}}}}$$

(5)

The peak particle velocity is represented by v (m/s) whereas t_d is the arrival time given by R/c. R (m) is the distance between the charge center and structure undergoing vibration due to the explosion and c is wave propagation velocity (m/s) in soil obtained as the square root ratio of E and γ_d where E is Young’s Modulus in N/m² and γ_d is the average mass density in kg/m³. Wu et al. [60] conducted blast tests to examine the difference in blast wave characteristics on soil surface, at soil-rock interface and also inside the rock mass for granite rock deposit at Sweden. The study proposed empirical formulations given by Eqs. 6, 7 and 8 to calculate peak particle acceleration (PPA), peak particle velocity (PPV) and principal frequency (PF) respectively on soil surface under different loading densities. Empirical equations for at soil-rock interface and also inside the rock mass were also derived and compared. It was observed that the PPA and PPV on the soil surface were higher than those at the soil-rock interface for the same scaled range. The empirical formulae derived for predicting PPV were also compared with Dowding’s empirical formulae and the comparison was found fairly reasonable.

$${\text{PPA}} = 330.3\;\left( {\frac{{\text{R}}}{{{\text{Q}}^{1/3} }}} \right)^{ - 2.75} \quad \left( {\text{g}} \right)$$

(6)

$${\text{PPV}} = 2.733\;\left( {\frac{{\text{R}}}{{{\text{Q}}^{1/3} }}} \right)^{ - 2.34} \quad \left( {{\text{m}}/{\text{s}}} \right)$$

(7)

$${\text{PF}} = 131.3\;\left( {\frac{{\text{R}}}{{{\text{Q}}^{1/3} }}} \right)^{ - 0.47} \quad \left( {{\text{Hz}}} \right)$$

(8)

Wu et al. [61] proposed a numerical model to simulate ground motion time histories on surface and in the granite mass. The numerical model proposed in the study was validated with the help of verified recorded data in field blast tests carried out at the granite site under consideration. The attenuation relationship to estimate peak particle acceleration, peak particle velocity and principal frequency were derived considering the location of blast motions to be on ground surface and in the free field at the granite site. Wu and Hao [62] studied the dynamic response of a two span two storey RC frame structure subjected to numerically simulated underground blast-induced ground motions using DRAIN2D-X computer program. The blast load is simulated based on the earlier studies by Wu and Hao [63] considering the effect of soil structure interaction and ground motion spatial variations and neglecting those leads to inaccurate prediction of structural response. The study also evaluated damage characteristics of frame structures such as displacement, acceleration, bending moment and shear force along the column height subjected to ground motions of different frequencies. It was observed that the structural response and damage indices are highly ground motion frequency dependent. Erten et al. [64] studied the ground vibration intensity caused by blasting at a limestone quarry located in the city of Izmir, Turkey. The study also determined a safe charge weight per delay to mitigate the adverse effects of ground vibrations on buildings and structures within 500 m. Based on the blasting practices, the study proposed charts to calculate safe charge weight for different distances between blasting and structures. It was also observed that the structural damage occurred was due to the amplification factor as well as poor foundation condition of the structures around the quarry. Ataei [65] estimated the peak particle velocity (PPV) of blast vibration by conducting and recording blasts data in Karoun 3 Dam and Power Plant project located in southwest of Iran. It was observed that the most important factor in PPV estimation is the weight of charge per delay and distance from blast center. The study also proposed a method to control blasting by conducting sufficient number of experiments and measurements in different locations of the project region. Empirical equations and charts are also plotted to estimate the PPV based on the experimental blasts conducted in the study. Mesec et al. [66] carried out 246 field tests records to measure peak particle velocity. The study proposed empirical relationship given by Eq. 9 that holds in good correlation with established peak particle velocity (PPV) and scaled distance (SD) for the sediment rock deposits comprising mainly limestone and dolomite. The study was carried out at the quarry of Vukov Dol located on the eastern slopes of the Medvednica Mountain near Zagreb, Croatia. The results obtained from the study also predicted the influence of ground vibrations level in the environment in reference to different world standards.

$${\text{PPV}} = { 5}0{8 }\left( {{\text{SD}}} \right)^{{ - {1}.{37}}}$$

(9)

Mohammed and Mohammed [67] investigated blasting parameters to study its effects on the two pipelines in Egypt. The main objective of the research was to determine the vibration level that will not damage the oil pipelines located close to limestone quarry of the National Cement Company, Egypt. The study also aimed to develop a standard based on peak particle velocity relationships to mitigate the effects of the blasting activities on the environment. It was observed that the maximum allowable charge per delay without any environmental effect was 591 kg at safe distance of 650 m (the minimum distance between the quarry blasts and the two pipelines).

Kumar et al. [68] collected a total of 120 published field blast data points from four different researchers in terms of peak particle velocity PPV and scaled distance (SD) for various soil sites. The study combined three different soil properties, namely, unit weight (γ_w), degree of saturation (S), and Young’s modulus (E) to predict the empirical model given by Eq. 10 to estimate the peak particle velocity using Curve Expert 1.37 software.

$${\text{v}} = \left( {\frac{{\text{E}}}{{{\upgamma }_{{\text{w}}} }}} \right)^{0.229} {\text{SD}}^{{ - \left( {1.6985 - 0.175{\text{xS}}} \right)}}$$

(10)

It was concluded that the applicability of the present model for blast related design is very good, as it considered extensive experimental data and compared with a large number of available empirical models. It was found that the present model predicts fairly for fully saturated soils irrespective of soil type and predicts higher values (critical values for design) for partially saturated soils.

Kumar et al. [69] proposed the empirical model given by Eq. 11 for the estimation of blast vibration parameters in terms of peak particle velocity for various types of rocks considering different engineering properties. The equation was derived from a total of 1089 published field blast data proposed by 13 different researchers. The study used the curve fitting technique to fit the equation given by the software CurveExpert 1.37 where γ_d is average mass density, f_c is the uniaxial compressive strength, peak particle velocity given by v and SD is the scaled distance (m/kg1/2), which is defined as the ratio of distance from charge point, R (m), to the square root of charge mass, Q (kg), expressed in TNT.

$${\text{v}} = \frac{{{\text{f}}_{{\text{c}}}^{0.642} {\text{SD}}^{ - 1.463} }}{{{\upgamma }_{{\text{d}}} }}$$

(11)

It was concluded that the proposed empirical model is capable of providing the estimate of predicted PPV that reasonably matches with experimental data and published empirical predictions. Thus, many researchers have thoroughly investigated the soil behavior under blast induced vibrations and developed empirical formulae in form of peak particle velocity for different site conditions. Based on knowledge of intensity of ground vibration generated due to mine blasts and accidental underground explosions the responsibility of structural engineers to design and protect structures against the underground blasting is necessary. In the next section, the recent trends in protection of structures against underground blasting is discussed.

3.2 Vibration Control Techniques

The concept of base isolation in form of sand layer to protect the structures against blast motions using Autodyn 3D software was first applied by Wu et al. [70]. It was concluded that the proposed technique was effective to reduce the structural response and damage incurred to frame structure subjected to blast induced ground motions in both high and relatively low frequency ranges, even though the isolation effects tend to reduce with decrease of the ground motion principal frequency. Tian and Li [71] analyzed a multi-storey building without or with a sliding base-isolation device subjected to ground shock induced by an in-tunnel explosion. The analysis considered the effect of an adjacent tunnel in between the building and the explosion tunnel using ABAQUS software. The obtained results were also compared with that neglecting the tunnel barrier in soil medium i.e., direct blast wave generated vibrations in soil medium. The blast load is calculated from the technical manual TM 5-1300 and validated by the proposed coupling model. It was observed that the base isolated technique is effective in reducing the structural responses of the building subjected to in tunnel explosions. Mondal et al. [72] studied the effectiveness of New Zealand (N-Z) base isolated system in improving the performance of fixed base systems subjected to underground blast induced vibrations. The study evaluated the performance of both single degree of freedom system (SDOF) and multi degree of freedom system (MDOF) equipped with base isolated system. It was observed that the selected technique is effective in reducing the top storey displacement and absolute acceleration in range of 82–96% and 90–94% respectively. The study also investigated the effect of isolation parameter on the applied response mitigation strategy. Mondal et al. [73] numerically investigated the effectiveness of fluid viscous dampers (FVD) in mitigating the damaging effect of underground blast induced vibrations. The study is conducted on a SDOF system equipped with FVD and cylindrical pot damper. A parametric study is carried out by varying the damping of the damper and the time period of the SDOF system. It was observed that the maximum reduction in peak storey displacement is attained at high damping ratio and higher time period of structure. Conversely, the peak reduction in absolute acceleration is achieved at 30% damping ratio. The maximum reduction is obtained at higher period of SDOF system. Majumder and Ghosh [74] studied the performance of shape memory alloys (SMA) in mitigating the structural responses of a three storied steel frame structure subjected to underground blast induced vibrations. The blast induced vibration was applied in terms of acceleration input time history. The study observed that the applied SMA devices showed better potential in comparison to the conventional steel bracings. Mondal et al. [75] reviewed the performance of different isolation systems such as Laminated Rubber Bearing (LRB), N-Z type base isolator, Pure Friction (P-F) system, Friction Pendulum (FP) system, Resilient-Friction Base Isolator (R-FBI) and Electricite de France (EDF) system. The design parameters of the isolators and their optimum ranges are evaluated from the criterion of peak acceleration reduction of the superstructure while minimizing bearing displacement and its residual deformation. The comparison of results showed that LRB isolators result in maximum reduction in structural responses in comparison with N-Z isolators at the expense of maximum bearing displacements for both high and low blast effects. Amongst the sliding isolators the P-F isolators yielded maximum reduction in structural responses with FP system harvesting least bearing displacements at high blast loading and EDF system showing better response control technique under low blast loading. Mondal et al. [76] compared the performance of shape memory alloy rubber bearings (SMARB) with New Zealand (N-Z) bearings when subjected to blast induced vibrations. The study concluded that the SMARB minimized the structural responses of a five-storey shear type building in comparison with N-Z systems at the advent of higher peak bearing displacement responses. The performance of building was also studied by varying the isolation period and isolation strength of both N-Z and SMARB isolators. Kangda and Bakre [77] proposed the effectiveness of connecting the closely spaced buildings with fluid viscous dampers in reducing the structural responses when compared in unconnected state. Different placement techniques were incorporated in demonstrating the effectiveness of fluid viscous dampers. The connected buildings were subjected to same underground blast induced vibrations and the loading was calculated by changing the blast charge weight whereas the distance between the charge and structure was kept constant. In 2020, Kangda and Bakre [78] also investigated the performance of regular and irregular steel frame buildings under blast induced vibrations. Base isolation systems and fluid viscous dampers were installed to mitigate the performance of fixed base buildings when subjected to ground induced vibrations. The performance of various control devices as demonstrated by various researchers equipped to mitigate the blast effects are summarized in Table 3. The performance indices evaluated depicts the effectiveness of passive control techniques in mitigating the underground blast effects as compared to air blast. In the present study, an attempt has been made to highlight the performance of civil engineering structures subjected to above ground and underground blast loading. The study encapsulates the load estimation and design guidelines to be followed to construct blast resistant structures. The performance of structures installed with seismically effective control devices against blast induced vibrations has also been presented. The influence of high-grade materials in mitigating the blast resultant damages has also been investigated. The study also reports a few case studies investigating the importance of architectural planning in improving the blast mitigating capabilities of structures. The performance of composite materials has proved to be an excellent development in improving the performance of blast resistance capacity of concrete structures [96]. The replacement of conventional glass facade with curtain wailing brackets has improved the blast protection levels of this highly vulnerable component against blast [97]. The role of plants namely thuja hedges acting in mitigating the blast intensities and damage capacities has been recently investigated by Gebbeken et al. [98]. The present study concludes that the development blast resistant materials, energy absorbing devices, barrier systems and innovation security devices are future trends to safeguard civil engineering structures against the fatal and uncertain blast phenomenon. The review recommends the effectiveness of seismically strengthened buildings subjected to underground blast induced vibrations and a micro level investigation critical for the protection of structures subjected to above ground blast damages.

Table 3 Performance of various control devices subjected to air and underground blast loading

4 Summary and Discussion

The criteria and code provisions for the calculation of blast phenomenon are limited. Hence to predict the phenomenon of blast loading extensive research has been carried out. The protection of civil engineering structures against such a complex loading phenomenon needs extensive research. The finite element models to predict the responses for the structures have been researched but their solutions are quite difficult. Lots of investigations have been carried out considering structural elements such as beams, columns and slabs subjected to blast loadings reporting the individual behavior of these elements both experimentally and analytically. Recently investigations to improve the performance of these structural elements have also been carried out. The present research has been aimed to encapsulate the investigations carried out in the field of blast protection techniques. The blast techniques studied in the past are based on many assumptions and are also relatively time consuming in parametric investigations. The stick system models are mostly used in buildings for time and frequency domain analysis in the field of earthquake engineering. The vibration control techniques in the form of dampers and isolations have been adopted to improve the structural performance in the field of earthquake engineering with some research emphasizing the applicability of seismic design in the field of blast mitigation. Very few studies have investigated the performance of stick models equipped with passive control techniques such as dampers and isolation systems in mitigating the responses of structures subjected to blast loading. The performance of full-scale frame buildings with regular and irregular vertical elevations have also been analyzed in past when subjected to blast induced vibrations to understand the complete structural behavior of buildings. The performance of buildings equipped with isolation systems subjected to underground and surface blast phenomenon has proved its effectiveness as studied in the field of earthquake engineering. The technique of inter connecting the adjacent buildings with and without base isolation systems when subjected to underground blast vibrations have also been reported in improving the structural performance. Some critical blast protection approaches enlisted by various researches and proposed for further investigation are as follows:

• 1. The performance of high strength concrete with fibers and closed transverse reinforcement has proved to be an effective technique in improving the performance of blast prone structures.

• 2. The vibration control techniques namely isolation and damper systems perform exceptionally when subjected to underground blast induced vibrations.

• 3. The building façade, barriers such as sacrificial claddings, and blast wall play a critical role in improving the blast protection levels.

• 4. Carbon fiber reinforced polymers (CFRP) and steel fiber reinforced polymers have proved to be excellent composite materials for improving the performance of blast resistance capacity of concrete structures.

• 5. The recent trends in blast protection technique recommend potential protective plants namely thuja hedges acting as intelligent barriers in mitigating the blast intensities and damage capacities.

• 6. The present study concludes that the development blast resistant materials, energy absorbing devices, barrier systems and innovation security devices are future trends to safeguard civil engineering structures against the fatal and uncertain blast phenomenon.