Resist Blast Loads Research Articles

Assessment of Retrofitting Techniques for a Four-Story Masonry Building to Resist Blast Loading

Assessment of Retrofitting Techniques for a Four-Story Masonry Building to Resist Blast Loading

A comprehensive review of ultra-high performance concrete (UHPC) behaviour under blast loads

Blasts resulted from industrial/residential accidents and deliberate attacks can impose extremely hazardous loading conditions upon nearby structures. The repercussions of these blasts encompass not only direct effects such as blast overpressure and fragments but also indirect outcomes like progressive collapse of structures. In the pursuit of enhanced protection for concrete structures, substantial endeavors have been directed towards developing innovative construction materials with outstanding properties, among which the ultra-high performance concrete (UHPC) has emerged as a notable representative. Drawing from existing experimental, numerical and theoretical studies, this paper presents an inclusive overview of recent progress in UHPC structural members (slabs, beams and columns) and UHPC-based composite structures (mesh reinforced UHPC, UHPC-filled steel tube and UHPC strengthening of normal reinforced concrete (NRC) structures) in their ability to withstand blast loads. While discussing the exceptional material and structural dynamic performance of UHPC, recommendations are offered for the further research directions in utilising UHPC structures to resist blast loads, which include exploring eco-friendly UHPC, integrating 3D printing technology and employing machine learning analytical methods.

Improving blast protection of double-layered system for infilled masonry wall in frame structure

To dissipate a large amount of blast energy as well as realize a reasonable load transfer to the beam and column instead of the vulnerable wall in the frame structure, this study employs a novel double-layered system to protect the infilled vulnerable masonry wall to resist blast loads. The system comprises a sacrificial cladding directly exposed to the blast to dissipate energy and connectors to facilitate load transfer and mitigation. Based on the quasi-static compressive test on the connector, the field blast test on the sacrificial cladding and frame structure with an infilled masonry wall, their respective numerical models are established and validated with ANSYS/LS-DYNA, which facilitates the model establishment of the frame structure protected by the double-layered system subjected to blast load. The numerical results indicate that the system effectively mitigates the damage to the masonry wall and reduces the load transferred to the frame compared to that without protection. However, large bending deformation of the sacrificial cladding may still result in collision damage to the vulnerable wall which makes it difficult to be applied in engineering practice. To improve the protective performance of the double-layered system and reduce the possibility of the occurrence of a collision between the sacrificial cladding and the wall, several measures, including connector layout optimization, rear plate strengthening with stiffeners, and appropriate connector height determination have been proposed. The numerical results demonstrate that installing connectors at four edges, adding 5 × 5 stiffeners on the rear surface of the sacrificial cladding, and appropriately designing the height of the connector can significantly enhance the double-layered system’s protective performance, which has practical significance in protecting vulnerable masonry wall in frame structure from blast loads.

Model analysis of carbon fiber reinforcement properties for reinforced concrete beams to resist blast loads

Safety is paramount in Oil & Gas plants, and continuous monitoring and improvements ensure that all measures are taken to protect them. The purpose of this paper is to examine how composite materials can be used to improve the structural reinforcement of concrete beams. Concrete structural beams have been improved in the past by using varying Fiber Reinforced Properties (FRP). It has been investigated how Carbon Fiber Reinforced Properties (CFRP) composites perform under blast loads and how they behave, respond, and perform as reinforcement for reinforced concrete beams. The response of RC beams to blasts was analyzed using a software modelling program called ANSYS that can mimic RC beam properties when reinforced with CFRP in concrete structures. The reason CFRP was chosen was because its properties showed great potential and it is well suited for testing and analysis. As well as absorbing a lot of energy, this material is strong, elastomeric, and alkali-resistant. A numerical analysis and model analysis have been performed with the help of the ANSYS software program. In the experimental results, CFRP was found to increase the flexural and shear strength of RC beams. The RC beams reinforced with CFRP has outperformed RC beam (control beam) in factors such as in Deformation, Equivalent Stress, and Shear Stress by a minimum percentage difference of 0.784% and maximum of 7.09% depending on the layers of CFRP and load applied on the beams in each factor.

Blast resistance of ECC and composite slabs subjected to near-field explosion

Due to the unfavorable blast resistance of reinforced concrete (RC) slabs, particularly with the frequent occurrence of penetration and spalling damage, engineered cementitious composite (ECC) provided a promising alternative for enhanced blast resistance through its direct application as a building material or by reinforcing existing RC slabs due to its excellent tensile ductility and toughness. In the present study, the blast resistance of the RC slab, the ECC slab, and the composite slab (RC slab reinforced by ECC layer) was investigated and compared through experimental and numerical approaches. Firstly, the material properties of the ECC were acquired through the quasi-static compressive test, as well as quasi-static and dynamic tensile tests, which felicitated the parameter calibration in the K&C model for ECC. Then the numerical models of the RC slab, the ECC slab, and the RC/ECC composite slab were established and validated with the blast test results. The test and numerical results showed that the ECC and composite slabs exhibited superior anti-blast performance in terms of failure mode, damage extent, and load mitigation compared to the RC slab. Owing to the remarkable ductility and toughness, the ECC and composite slabs could greatly reduce the possibility and severity of penetration and spalling damage. Furthermore, five significant parameters impacting the blast resistance of ECC and composite slabs were summarized through dimensional analysis including the tensile strength of ECC σt, the total explosive energy Q, the standoff distance R, the slab thickness H, and the slab span L. Then a dimensionless number was proposed to determine the failure mode and predict the residual deflection at the center of the ECC and composite slabs. The results could serve as a reference for the design of ECC and composite slabs to resist blast loading in the field of structural protection.

Numerical Study on the Behaviour of Hybrid FRPs Reinforced RC Slabs Subjected to Blast Loads

The safety of civilian and military infrastructure is a concern due to an increase in explosive risks, which has led to a demand for high-strength civil infrastructure with improved energy absorption capacity. In this study, a Finite Element (FE) numerical model was developed to determine the effect of hybrid Fibre Reinforced Polymer (FRP) as a strengthening material on full-scale Reinforced Concrete (RC) slabs. The reinforcing materials under consideration were Carbon (CFRP) and Glass (GFRP) fibres, which were subjected to blast loads to determine the structural response. A laminated composite fabric material model was utilized to model the failure of composite, which facilitates the consideration of strain rate effects. The damaged area of the laminate is determined in the FE model, and it is in good agreement with the corresponding experimental results in the literature. Models containing different stacking sequences were built to demonstrate their efficiency in resisting blast loads. In general, the damaged area was reduced when a hybrid reinforcement with CFRP as the top layer was used.

Blast behaviour of precast segmental vs monolithic concrete beams prestressed with unbonded tendons: A numerical investigation

This study aims to numerically investigate the response of precast/prefabricated segmental concrete girders/beams (PSCBs) under detonations of high explosives. A new and effective method of applying prestressing force in tendons in LS-DYNA is explored and used. The working mechanisms of PSCBs vs conventional monolithic beams in resisting blast loads are comprehensively examined and discussed. The numerical results show that blast-induced damage to the beams is primarily caused by three different mechanisms, namely the propagations of blast-induced stress waves and plastic hinges, and the beam free-vibration deflection (global response). There are similar stress wave propagation and the resultant damage in the monolithic and segmental beams at the early stage when the joints are closed. The different damage patterns in these two beams are mainly due to the propagation of plastic hinges and the beam's free-vibration deflection. Damage is more severe and well distributed in the monolithic beam whereas damage to PSCB mainly concentrates in the segment next to the explosive and around the joints. The energy absorption mechanism provided by the relative movement between the segments is the main reason for the reduced damage in the PSCB. Although having larger displacement, the PSCB shows better blast resistance compared to the monolithic beam. Either increasing the prestressing reinforcement ratio or effective prestressing stress reduces the peak and residual displacement of the beam but increases concrete damage. Increasing the number of segments leads to a larger beam displacement response but reduces concrete damage. Energy dissipation (ED) bars are found to effectively unite the segments and hence result in the segmental beam having a similar blast behaviour as the monolithic counterpart.

Blast retrofit of one-way reinforced concrete members using externally bonded FRP and FRP anchorage

This paper presents the results of nine as-built and carbon fiber reinforced polymer (CFRP) retrofitted reinforced concrete panels subjected to simulated blast loading using a pneumatically operated shock tube. The objective of the study was to characterize the blast response of CFRP retrofitted reinforced concrete panels, with and without supplemental mechanical anchorage applied to the CFRP. The results indicate that retrofitting can significantly increase the strength and stiffness of reinforced concrete flexure members and greatly enhance the displacement time-history response over non-retrofitted members. Debonding of the externally bonded CFRP was the failure mode for all retrofitted members. FRP anchors, designed to prevent or delay debonding failures through mechanical end-anchorage, were found to substantially enhance the performance of panels experiencing critical diagonal crack debonding. However, the FRP anchors were found to have no substantial effect on retrofit performance for the case plate-end interfacial debonding failures. In addition, the displacement time-histories for as-built and FRP retrofitted panel obtained through detail single degree of freedom analysis were found correlate well with those obtained experimentally. Finally, a discussion on the practical considerations of using externally bonded FRP retrofits to resist blast loads and recommendations for protective design are presented.

Blast fragility analysis of RC columns considering chloride-induced corrosion of steel reinforcement

Chloride-induced corrosion of steel reinforcement is one of the key factors that influences the life cycle performance and load-carrying capacities of concrete structures. For concrete structures that might be subjected to explosion hazards during the service life, the fragility analysis with the consideration of chloride-induced rebar corrosion provides quantitative information on lifecycle performance of structures against blast loading. This study performs the fragility analysis of reinforced concrete (RC) columns considering chloride-induced corrosion of steel reinforcement subjected to blast loading. General corrosion and pitting corrosion are considered in this study. Monte Carlo simulations are carried out in the study to calculate the failure probabilities. The mean and standard deviation of the degraded diameter and yield stress of the reinforcement up to 90 years after the corrosion initiation is established, which is incorporated in the existing empirical formulae for constructing the P-I curves of RC columns to predict its deteriorated blast loading-resistance capacity with time. In the analysis the blasting scaled distance is used as the demand intensity measure, random fluctuations of the peak reflected pressure, positive blast loading duration, concrete compressive strength, thickness of the concrete cover, diameter and yield stress of steel reinforcement are considered in the simulations. The blast fragility curves of typical RC columns are generated with respect to different damage levels corresponding to the number of years after corrosion initiation. It is found that the damage probability of RC columns increases significantly with steel reinforcement corrosion deterioration. Parametric analyses have shown that the column size is a more dominant factor, in comparison with the reinforcement ratio, of the RC columns on the blast resistance capacity corresponding to the minor damage levels. It is also found that the longitudinal reinforcement ratio is a less dominant factor than transverse reinforcement ratio on blast fragility curves of RC columns to resist blast loading. The blast resistance capacity of RC columns deteriorates significantly with corrosion damage.

Comparative study on the blast load response of woven and lattice core metallic sandwich panels

Sandwich structures are potential alternatives to monolithic plates for the purpose of absorbing energy and managing the impulse associated with blast loading. The study of sandwich structures subjected to blast loading is a rapidly expanding area of activity. Sandwich structures are extensively used in blast dissipation applications on account of their excellent physical and mechanical properties. These sacrificial cladding structures absorb energy through progressive plastic deformation of the front-facing plate and the core layer; thus minimizing the transfer of the peak force to the non-sacrificial structure. The core absorbs most of the energy and hence has a significant influence on the blast behavior of sandwich structures. The core layer can have several topologies, which in turn have various configurations. The present study numerically evaluated and compared the effectiveness of using Triangular Woven and Pyramidal Lattice core configurations in sandwich panels for resisting blast loads. The parametric study consisted of the effect of change in outer layer thicknesses of sandwich panels and the application of successive blast loads in the performance of the sandwich panels. The study was conducted for seven outer layer thickness combinations modeled in ANSYS Workbench Explicit Dynamics. All the models were subjected to a blast load with the same scaled distance. The comparison of the simulated results was used to determine the more suitable core configuration for dissipating blast loads. Pyramidal lattice panels were found to be 5% and 48% more efficient than a triangular woven panel of comparable relative core density under normal blast load and successive blast load respectively.

Full-Scale Blast Tests on a Conventionally Designed Three-Story Steel Braced Frame with Composite Floor Slabs

This paper summarizes the findings of two full-scale blasts tests on a steel braced frame structure with composite floor slabs, which are representative of a typical office building. The aim of this research study was to experimentally characterize the behavior of conventionally designed steel braced frames to blast loads when enclosed with conventional and blast-resistant façade. The two tests involved a three-story, steel braced frame with concentrical steel braces, which are designed to resist typical gravity and wind loads without design provisions for blast or earthquake loads. During the first blast test, the structure was enclosed with a typical, non-blast-resistant, curtainwall façade, and the steel frame sustained minimal damage. For the second blast test, the structure was enclosed with a blast-resistant façade, which resulted in higher damage levels with some brace connections rupturing, but the building did not collapse. Observations from the test program indicate the appreciable reserved capacity of steel brace frame structures to resist blast loads.

Full static resistance of castellated steel beams with hexagonal web openings for blast response predictions

To avoid structural collapses by explosions and terrorist attacks, castellated steel beams (CSBs) could become an alternative to parent steel beams (PSBs) in structures due to high energy absorption capabilities, which offered through a combination of high strength and ductile response. The static resistance function, obtained by either experiments or FE modeling, is considered an important item of data in calculating the dynamic response of a system using Single Degree of Freedom models. This research estimated and promoted the static resistance of CSBs to enhance their ductility and dissipate blast energy through large deformation kinematics. Quasi-static experiments on CSBs were performed to investigate the effect of end connections and different strengthening techniques (closed holes and vertical stiffeners) on the static resistance of CSBs. Load-deflection and strain data were recorded for each specimen up to failure. Moreover, a Finite Element (FE) model was developed and verified against tests. Good matching through all phases of resistance was obtained between the FE and test results. An extensive parametric study was carried out to predict the full static resistance of CSBs under the effect of cross-section geometries. Different ranges of spans were investigated to determine the minimum effective span-to-depth ratio. By providing vertical stiffeners as well as bolted head plate connections, CSBs obtained a larger capability of deformation through the tension membrane stage to resist blast loads under simultaneous high tension and shear forces.

Blast Resilience Enhancement of Cable-Supported Façade Utilizing Super-Elastic Shape Memory Alloy

Due to an increased level of threat, the design of buildings to resist blast loads has gained importance. The most vulnerable component of the building is typically the cladding system, generally being lightweight and directly facing the hazard with large, exposed surface areas. A large number of cladding materials are available, with architectural intent constantly driving the development of new products and systems. Glazed facades are aesthetically appealing and are popular among architects. In this paper, the use of super-elastic shape memory alloy (SMA) to improve the blast resilience of a cable-supported system is explored. The critical component of these systems is the high strength steel material cable. In the present study it is rendered hybrid (Steel-SMA), by attaching SMA cable(s) to the steel cable at one or both ends, and the dynamic performance under blast is studied. It is shown that the introduction of SMA in the cable has the potential to improve the resilience of the facade system.

Wrapping and anchoring effects on CFRP strengthened reinforced concrete arches subjected to blast loads

AbstractThis paper studies the effectiveness of bonding carbon fiber reinforced polymer (CFRP) with additional wraps or anchors to strengthen reinforced concrete (RC) arches resisting blast loads, which is of great significance to optimize the CFRP strengthening methods. To improve the anti‐explosion behavior, RC arches are bonded with CFRP sheets in longitudinal and transverse directions, and anti‐explosion behaviors were investigated through experiments. The performance of strengthened arches is discussed in terms of damage degrees in contrast to the un‐strengthened arch. Under explosive loading, the resistance of composite arches to spalling, shearing, and bending is much better than that of control arch. The results show that the anti‐explosion effect of wrapping or anchoring, which can prevent CFRP debonding or peeling‐off, is much better than only bonding CFRP in the longitudinal direction. Besides, simplified analysis models are proposed to assess the blast‐resistant capacity of CFRP strengthened RC arches, which show good agreement with the experiment.

Assessment of Dynamic Response of 3D Ultra High Performance Concrete Frame Structure under High Explosion Using Johnson-Holmquist 2 Model

Most of the 3D frame structures are not initially designed to resist blast loads. Due to the limitation of financial resources as well as the complex technical requirements, study the dynamic behavior of 3D frame structure under blast load, particularly for advanced materials like Ultra High Performance Concrete (UHPC), faces many challenges. Therefore, numerical simulation can be a good alternative. The objective of the research is to investigate the dynamic behavior of a UHPC building subjected to blast loading through a 3-D numerical model with the direct simulation of blast load. This building studied in this paper is a real, 6-story office building in Vietnam, which was originally designed for dead load, live load, wind, and earthquake. The building is subjected to surface blast of 500kg Trinitrotoluen (TNT) equivalent charge weight with close-in distance and a variable height of blast source. In this study, Johnson-Holmquist 2 damage model (JH-2) model is implemented to simulate the UHPC structures subjected to blast loading. Base on the theory of the JH-2 model, a subroutine, integrated with Abaqus software, is designed by the authors to calibrate the input parameters of UHPC material. A total of 8 cases of the explosion scenario are considered, in which the design of the original column is revised by using UHPC composite structure in external columns. The results indicate that there is a tremendous increase in response when the blast occurs at the mid-height of the building and the severe damage is observed in the external column in front of the explosion. The obtained result in terms of blast wave pressure during time history as well as peak deflections and damage of building under blast loading is assessed. The blast loading resistance effect of the revised design compared to the original design is also demonstrated as a beneficial alternative in blasting condition.

Blast mitigation using sandwich panels with saffil foam

Abstract The demand for the protection of important government and public buildings has increased in recent days to resist blast loads due to frequent terrorist activities occurring around the world. Sandwich panels are preferred structural elements that can be used to protect the parent structure by mitigating the blast pressure. Numerical analysis of sandwich panel consisting of steel plates of equal thicknesses separated by a saffil foam core is carried out to evaluate its response under blast load. Parametric analysis of sandwich panels with varying foam thicknesses and standoff distance is performed using LS-DYNA®. The steel face and back-sheets in sandwich panels are modelled using piecewise linear plasticity material model and saffil foam is modelled using crushable foam material model available in LS-DYNA®. Moreover, back sheet of sandwich panels is stiffened using six different stiffener configurations. Finally, behaviour of conventional sandwich foam panel is compared with the stiffened sandwich foam panels for the considered stiffener configurations. Response of these different configurations of sandwich panels is evaluated based on peak central point displacement at the back sheet of the panel and it is observed that provision of stiffeners considerably improves the blast mitigation capacity of the sandwich panel.

Peak overpressure and impulse due to diffraction over a cylinder and/or multi-reflection of a shock wave in structural design- Part I

The design and planning of structural components, such as columns, to resist blast loads is a complex task. Often standard free-field blast loads, specified in design codes or other literature, are used for the analysis of the object. These loads are then further increased or decreased depending on the topology of the surrounding geometry (Mach-Stem), the shape of the impacted object itself, and blast wave reflection. However, most of these research works focus purely on the assessment of the structural components itself, ignoring a complex fluid mechanics phenomenon such as diffraction, which is of particular interest when circular columns are standing next to each other or placed in front of a façade. The article addresses three main topics. First, the article answers the question whether the area behind the column is protected, meaning constituting a shadowed area. We present findings that the close area behind a column is subjected to higher pressure and impulse values as there would be without the column. Hence, the incident pressure sees significant pressure buildup due to diffraction. This pressure buildup is quantified using pressure increase factors and presented together with the accompanying impulse. Second, this pressure buildup is of relevance for realistic design of a façade behind the column, which is not covered in current design codes at all. We discuss relevant parameters in the design process. Third, directly coupled with the assessment of the pressure buildup behind the column due to diffraction is the assessment of pressure and impulse in the area behind the column due to multi-wave reflection at a façade, leading to a significant pressure and impulse scale-up, which might be relevant for design of a column and/or a façade. This article identifies gaps in understanding diffraction and subsequent multi-reflection of blast wave within a structural design framework and provides insights on how to establish safe design accounting for these effects.

Effect of strength matching on failure characteristics of polyurea coated thin metal plates under localized air blast loading: Experiment and numerical analysis

Abstract A favorable strength matching can be very beneficial for improving the performance of polyurea coated thin metal plates to resist blast loads. This study made an attempt to elucidate the effects of material and interface strengths, including steel and polyurea strengths, and interface normal (NFLS) and shear strengths (SFLS), on the failure characteristics of polyurea coated steel (PCS) plates under localized air blast loading. First, air blast experiments were conducted to identify the deformation and failure of PCS plates with different steel strengths. Subsequently, elaborative numerical simulations were performed to analyze the dynamic response and energy absorption characteristics of PCS plates. Finally, the effects of material and interface strengths on the failure characteristics of PCS plates were numerically evaluated. The results show that the kinetic energy of polyurea fragments significantly contributed to the total energy absorption of polyurea backing layer (PBL) that failed due to spalling. The steel strength had a significant effect, whereas the polyurea and interface strengths had marginal effect on the deformation of front steel layer (FSL). Increase in polyurea strength led to a dramatic decrease in the energy absorption proportion (EAP) of kinetic energy of polyurea fragments, but this EAP slightly varied by changing the steel strength or interface strength for PBLs that failed by spalling. The dominant energy absorption mechanisms of PBLs changed with the variation in steel and polyurea strengths, but did not vary with interface strength, including NFLS and SFLS.

The blast resistant performance of concrete-filled steel-tube segmental columns

Prefabricated segmental concrete column has seen an increased rate of adoption in the construction industry. Much effort has been paid to investigate the performance of segmental column under seismic and impact loadings. The study of its blast resistant performance is very limited. In the meanwhile, field blast tests and numerical simulations have demonstrated that concrete-filled steel-tube (CFST) columns exhibit outstanding blast resistant capacity. This study aims to investigate the blast performance of segmental column with CFST segments. A detailed 3D model of segmental CFST columns are generated using LS-DYNA. The model is validated with field blast testing data on CFST column and laboratory impact test on segmental column. The blast resistant performance of segmental CFST column is compared with conventional monolithic column and monolithic column with prestress. Parametric study is then conducted to investigate the influences of infill concrete strength, steel skin thickness, pre-stress ratio, and number of segments on the blast resistance of the segmental column. It is found that increasing the steel skin thickness could most effectively improve the blast resistance of a segmental CFST column, while increasing the concrete strength, prestress force, and the number of segments, also improve the capacity of segmental CFST column to resist blast loading.

Blast resistance of steel plate shear walls designed for seismic loading

Since a blast loading or explosion can create nonlinear wave action and impact pressure on a structure, it necessary to construct a structure to resist blast loading as like other loads. In this study the nonlinear behaviour of a blast loading is simulated by calculating the pressure diagram with respect to time under the guidance of IS 4991 – 1968, code for “Criteria for Blast Resistant Design of Structures for Explosions above Ground”. The study carried out for different charge weight (100kg TNT, 200kg TNT and 400kg TNT) and standoff distances of 20metre. Nonlinear behaviour of a Blast loading to steel structures with shear plates of thickness 6 mm, 8 mm and 10 mm are modelled in ETABS and the analysis is carried out to obtain base shear, story displacement, story deformation pattern, column forces, etc.