Hezi Y. Grisaro

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• Doctor of Philosophy
• Professor (Assistant) at Technion – Israel Institute of Technology

This paper presents a study of the velocity distribution of fragments caused by explosion of a cased charge. Available formulas usually predict a uniform fragments velocity at the middle part of the cylinder and only few sources recommend evaluation of a non-uniform velocity distribution. According to few, yet available, experimental results, the l...

The dynamic response of blast-loaded structural elements includes multiple modes of vibrations. An equivalent Single Degree of Freedom (SDOF) system is a popular fast-running engineering tool for design to capture the global behavior of the structural element, assuming a single and dominant Mode of Vibration (MOV). Typical models include a nonunifo...

Extreme loads can arise from accidents such as vehicle collisions or airplane crashes, as well as deliberate acts of terrorism or military attacks involving blasts and fragmentation. Blast overpressure can also occur accidentally, for example, from explosions of hazardous materials such as gas. Distinguishing between accidental and deliberate loads...

This research explores probabilistic models for steel wide-flange columns under static axial and transverse far-field blast loading acting on their strong axes. Using ANSYS LS-DYNA, the responses of five sections (W150X24, W200X71, HEB100, HEB320, and HEB500) were evaluated through 50 finite element (FE) simulations. These simulations incorporated...

The application of sacrificial claddings, such as aluminum foams, offers a practical approach for mitigating blast induced damage to structures. These lightweight, porous materials are prized for their energy-absorbing properties, which reduce peak overpressure from blast loads. Previous research has predominantly examined uniform-density foams, of...

Progressive collapse is a critical concept in structural engineering, referring to the failure of a building or buildings following the loss of a key structural component. This phenomenon can lead to a chain reaction where subsequent failures occur, resulting in the collapse of an entire building or a significant part of it. In this study, we will...

In recent decades, the response of structures to blast loads has garnered increasing attention. More specifically, using Reinforced Concrete (RC) for the structural elements is very common in protective structures and therefore, the prediction of their dynamic response is of interest. To assess the resistance of RC structural elements to blast load...

Due to the deficiency of current design guidelines for blast loadings on steel structures, this research develops probabilistic models for steel wide-flange columns under axial and far-field blast loading on both their weak and strong axes. A total of 160 finite element (FE) simulations were conducted using ANSYS LS-DYNA, with columns subjected to...

A parametric numerical analysis was conducted to investigate the response of steel wide-flange sections (or I section) to combined axial and lateral far-field detonations upon their weak axis. Two steel sections, W150X24 and W200X71, were selected for this investigation. Utilizing ANSYS LS-DYNA as the Finite Element (FE) tool with a plastic kinemat...

Among the methods for mitigating blast-induced damage to protected structures (PSs) is the ‎installation of sacrificial claddings on structures under threat. Various materials can be used for ‎these claddings, including Aluminum Foams (AFs). AFs, known for their energy-absorbing ‎properties, effectively reduce peak overpressure from blast loads. Ex...

This study employed a parametric numerical analysis to evaluate the residual axial capacity of wide-flange steel sections under combined static axial load and transverse far-field blast loading acting on its strong axis. These simulations entailed the variation of the Axial Load Ratios (ALRs) at levels ranging from 0-80% (of the capacity of the und...

The response of structures to blast loads commonly includes damage that cannot be avoided. The trivial method to enhance the structural resistance under blast loads is by stiffening its cross-section, which may lead to an uneconomical design regarding material weight and cost. An alternative method is to use sacrificial layers placed on the surface...

Extreme loads can arise from accidents such as vehicle collisions or airplane crashes, as well as deliberate acts of terrorism or military attacks involving blasts and fragmentation. Blast overpressure can also occur accidentally, for example, from explosions of hazardous materials such as gas. Distinguishing between accidental and deliberate loads...

AF are cellular materials that exhibit energy absorption capabilities, making them desirable to use as sacrificial protective cladding applied on structural members. Thereby, the structural resistance to blast loads is increased. During the foam compaction, inner and local buckling of the material walls occur. The described phenomenon reduces the p...

Aluminum foams are porous and recyclable lightweight materials with outstanding mechanical features, such as their energy absorption capabilities, their most noteworthy property. In view of these qualities, this class of material is a great candidate to use as sacrificial layers applied on structures under threat for blast load protection. Few work...

One of the methods to assess a response spectrum of structural elements to various blast loads is by developing Pressure-Impulse (P-I) diagrams. The P-I diagram is a limit curve that represents the combination of pressure and impulse for which the damage level of the structure is achieved, which is represented by a given maximum dynamic displacemen...

This paper presents an experimental research, investigating the blast wave generated by a line charge, detonating cord (detcord), that is detonated in the air, on the ground, and buried in shallow depth. The experimental setup involved detonating a line charge in an open field, and measuring the blast wave properties at various radial distances fro...

Aluminum foams (AFs) have been known for their ability to absorb energy. Therefore, these kinds of foams could be used as sacrificial protective cladding on structures to enlarge their resistance to blast loads. The AF manages to mitigate the blast effects by absorbing a large amount of energy due to its cellular structure. During the densification...

As primary structural members, steel I-shaped (wide-flange) sections, used for columns and beams in buildings, may be subjected to near-field detonations. A near-field, or close-in detonation consists of high overpressure and impulse magnitudes, and nonuniform pressure distribution along the member, with peak intensity around the charge location. T...

Concrete, as the most widely used construction material, is associated with a high environmental impact. Within the present study, structural optimization is the method of choice to counter this issue. The entire process, from optimization, to design, experiments and numerical simulation is outlined. Embedded in the framework of a design competitio...

Aluminum Foams (AFs) are known for their ability to absorb energy due to their porous structure. Due to this unique feature, they are able to mitigate structural damage and reduce peak displacements under dynamic and blast loads. Energy absorption occurs when a plastic wave propagates through the AF width and lower pressure than the applied blast w...

In this paper, a mechanical model is developed to simulate the behavior of 1D plain reinforced concrete (RC) and fiber reinforced concrete (FRC) members subjected to uniaxial tension, considering the development of discrete cracks. The force bridged over the crack is a function of the crack width, fracture energy, and fiber volume. The model is for...

A pressure-impulse (P-I) diagram provides a quick tool to assess the resistance of a structural system to blast loads, based on a nonlinear single-degree-of-freedom (SDOF) analysis. Thus, instead of performing many SDOF analyses, the P-I diagram is the limit curve representing the combination of pressure and impulse for which the designed damage of...

This paper presents an experimental research to study the behavior of fiber-reinforced concrete slabs with varying fiber volume and type subjected to contact and close-in detonations. A total of 25 full-scale blast tests were carried out. The results are analyzed, discussed and compared to fast-running tools. For the close-in detonations, the damag...

The current paper presents an improved model for the dynamic response of RC wall to a combined load of blast and fragmentation, as a result of a cased charge detonation. The model considers the damage inflicted by the fragments and their additional momentum, as well as the more realistic blast load. The main aim of the study is to compare the more...

The tension stiffening behavior of reinforced concrete (RC) and fiber reinforced concrete (FRC) members is studied under uniaxial loading conditions. The members are comprised of a concrete section with or without fibers, and reinforcing rebars. When a tensile force is applied at the rebar ends, tensile stresses are transferred to the concrete thro...

Explosives installed on a commercial unmanned aerial vehicle (UAV) may be carried and detonated close to a structural element. Although the explosive mass may not be large, the standoff distance is expected to be very small, and therefore a potentially high level of damage can be developed. A popular method to improve the performance of reinforced...

When a wide-flange section is exposed to close-in detonation, local and global deformation modes are developed due to a complex, nonuniform pressure environment. The local deformation includes local distortion and rotation of the flanges, as well as deformation of the web, or even breaching near the web/flange junction. Experimental studies are pre...

The analysis of structural members subjected to close-in detonations involves a complicated dynamic scenario. Since the charge is very close to the structural member, the reflected pressure distribution on the member surface is highly non-uniform. In addition, the level of damage to the structural member may be high because of the large magnitude o...

Detonation of a high explosive close to a structural component results in a blast load that is highly localized and nonuninform in nature. Prediction of structural response and damage due to such loads requires a detailed understanding of both the magnitude and distribution of the load, which in turn are a function of the properties and dimensions...

Combined loading of blast and fragments involves various loading mechanism on a structural element. It includes the momentum inflicted by the blast load and the penetration of the fragments, as well as the structural damage due to the fragment penetrations. A model to account for the fragmentation loading in a realistic, yet simplified approach was...

In the case of a confined explosion, the resulting pressure signals are very complex. The peak gas pressure developed in such event is an important parameter characterizing the pressure signal. Ability to predict the gas pressure with high fidelity is mandatory for any attempt to properly predict the dynamic structural response of the envelope elem...

This paper presents methodology and calculation models that aim to enable a more realistic design of reinforced concrete (RC) protective barriers. The proposed methodology considers the global effect of fragmentation impact, which is commonly neglected in current design procedures. The analysis considers a non-uniform, two-zone damage model over th...

The response of I-shaped, or wide-flange, sections subjected to close-in detonations has been evaluated through numerical simulations. A simulation approach for close-in detonation, which includes two stages, is suggested. The first stage includes the modelling of the detonation process through computational fluid dynamics (CFD) and the second stag...

The peak gas pressure developed as a result of a confined explosion is an important parameter characterizing the pressure signal and is crucial for assessment of the structural response of the confined space envelope elements. The gas pressure depends on the amount of the released energy, including the afterburning energy. A new analytical model is...

This paper describes a second stage of a comprehensive experimental study of the fragment damage effect on the mechanical properties of reinforced concrete (RC) elements. In the first stage, RC elements were exposed to fragmentation impact in a field test series. These specimens were then brought to the laboratory together with reference undamaged...

This paper presents analysis of perforated witness plates that were part of an experimental setup of previously published field tests. The witness plates were placed in the field tests to check the fragmentation pattern of the cased charge. A qualitative inspection of the plates shows that the spatial mass distribution of the fragments was not unif...

Combined loading of blast and fragmentation impact is one of the common extreme loadings on structures. In the design and analysis of protective structures, the blast and fragment loadings are commonly decoupled. Yet, previous studies have shown that there is a synergistic effect of these loads, which is manifested by more severe damage and structu...

The combined loading of blast and fragments on reinforced concrete (RC) elements has been studied in field tests of RC T-walls that were subjected to detonated cylindrical cased charges. Characterization of the combined loading has been experimentally studied and analyzed. This analysis includes the pressure time-history of a control bare charge an...

In this paper, the combined loading caused by blast and fragmentation impact has been studied. While there are methods for predicting the reflected blast wave pressure and impulse time history, the multi-fragment impulse has been rarely studied, and in fact, it is frequently neglected. This study deals with several aspects regarding the combined lo...

Design of structures to extreme loads include explosive loading that generates fragments impact and overpressure (blast). These structures are commonly made of reinforced concrete and they are called ‘protective structures’, where structures that are not designed to extreme loading can still be exposed to such loads. It is therefore important to ev...

One type of extreme loading on structures is the combination of air blast and fragments impact. This combined loading is a complex phenomenon, which includes the transmitted momentum of the blast and fragments impact to the structure, as well as local damages due to the fragments penetration. It has been shown that combined loading can cause more s...

The performance of explosives, which expressed by the peak overpressure and impulse, is commonly compared to standard TNT charge. The TNT equivalency factor (EF) represents the required amount of explosive mass to yield the same parameter (overpressure or impulse) as TNT charge at the same distance. Several studies suggest a unique value for the EF...

Fragmentation of cased charges is of interest in the design of protective structures. To assess the global response of a protective structure due to the fragments impact, their spatial mass distribution should be evaluated. Experimental results indicate that this distribution is not uniform, as commonly assumed. In this paper, a simplified model is...

Design of structures to extreme loads include explosive loading that generates fragments impact and overpressure (blast). These structures are commonly made of reinforced concrete and they are called 'protective structures', where structures that are not designed to extreme loading can still be exposed to such loads. It is therefore important to ev...

Peak overpressure and impulse are the most important parameters in the explosive performance estimation. Available models commonly consider trinitrotoluene explosive as the standard charge. In this article, the trinitrotoluene equivalency factor is studied through verified one-dimensional numerical simulations. The equivalency factors for impulse a...

A common type of extreme loading on structures includes combination of blast and fragments impact. Because of the complex nature of this combined load design and verification procedures of protective structures usually decouple the effects of overpressure and of the fragmentation impact. Although it has been shown that the simple addition of the se...

Many explosives are covered with a steel casing. The fragmentation process of the casing dissipates part of the detonation energy and therefore cased charges yield lower overpressures and impulses than the same charges without a casing (bare charges). It is often required to assess the mass of an equivalent bare charge, which will produce similar i...

A model is proposed for the prediction of residual deformations of a composite reinforced concrete barrier with a rear steel liner, due to impact of a non-deforming projectile. The model is based on representation of the response of the rear steel liner to the impact of the projectile and the ejecting concrete fragments by a single degree of freedo...

In assessing vulnerability of a concrete barrier it is important to know the projectile's residual velocity, in case of perforation. The residual velocity is a measure of the resistance level provided by the barrier in case it is breached. It is also an important parameter in studying the response of barriers, whether experimentally or analytically...

An important aspect in the design of protective structures is minimization of the spalling phenomena at the protected faces of its walls. One way to solve this design aspect is to attach a protective sheet, commonly made of steel, to the rear face of a protective barrier. This paper proposes a model to assess the perforation resistance of a composi...