Applied Mechanics and Materials Vol. 82

Abstract: The spread of terror and war threats aimed against built up urban areas has extended considerably and attracts the attention and concern of many countries. The severity and potential danger of these extreme events was demonstrated most severely in several past events. Many countries, including Israel, are facing different threats of terror and war. Therefore, Israel is continuously engaged in theoretical and experimental research activities, related to response of civilian buildings to various war threats. Some of the experience gained in recent years, in relation to the structural response of RC residential buildings subjected to blast loads, can be very helpful in the development of improved design requirements of such buildings, aimed mainly at localization and reduction of the damage and prevention of the progressive collapse of buildings. Earthquake loading is another extreme loading that may cause large scale destruction to many buildings that had not been properly designed to withstand the strong ground shaking effects. Many countries in seismic areas have developed throughout the years their standards and codes for seismic design, and recently built buildings in these areas are designed according to these codes. Nevertheless, many older buildings, that had been designed and constructed prior to the development of the modern seismic codes, are still more vulnerable. Although the earthquake response and blast protection are entirely different disciplines, there exist many places where both types of extreme loads are potential threats to existing buildings. Therefore a multi hazard approach should be adopted to design the new buildings and to retrofit the existing buildings, to ensure that they will withstand these threats. This paper aims at over-viewing some aspects of these extreme load threats and address the resistance, performance, protection and strengthening of the structures that are subjected to these loads. It aims at generalizing the professional view and providing a better solution to an envelope of different possible extreme events.

Abstract: The main objective of the international research project (COST C26 Action, Chairman F.M. Mazzolani) dealing with “Urban Habitat Constructions under Catastrophic Events” (2006 – 2010) was to increase the knowledge on the behaviour of constructions located in urban habitat and subjected to both natural and/or man-made catastrophic events, such as earthquakes, fire, wind storms, heavy snow loading, gas explosions, accidental impact from vehicles out of control and occasionally due to bomb blasts during terrorist attacks. In this view, it has been planned to define suitable tools for predicting the ultimate response of such constructions under extreme conditions, occurring when both loading and structural resistance are combined in such a way to reduce the safety level below acceptable values. In addition, the preparation of ad-hoc guidelines for damage prevention as well as for repairing of constructions hit by the above situations is planned. Twenty-three European Countries are participating in this project (Austria, Belgium, Cyprus, Czech Republic, Finland, France, Germany, Greece, Hungary, Italy, Lithuania, Macedonia, Malta, Netherlands, Poland, Portugal, Romania, Slovenia, Spain, Sweden, Switzerland, Turkey, United Kingdom). The final Conference was held in Naples on 16 to 19 September 2010 with the participation of additional twenty-three oversee Countries, where the out-put of the project has been presented. A synthetic overview of the main achieved results is given in this paper.

Abstract: Since 9/11, there has been an increased interest in developing a better understanding of the properties of concrete structures under impact and blast loading. Although concrete, as a material, demonstrates extreme brittleness under dynamically applied loads, fortunately, fiber reinforcement significantly enhances such resistance. Yet, the dynamic properties of both concrete and fiber reinforced concrete (FRC) remain poorly understood. This paper provides a historical perspective of our efforts aimed at understanding the impact resistance of fiber reinforced concrete, highlights some of the issues and challenges encountered and identifies the emerging areas where further research is necessary.

Abstract: The behavior of concrete structures is strongly influenced by the loading rate. Compared to quasi-static loading concrete loaded by impact loading acts in a different way. First, there is a strain-rate influence on strength, stiffness, and ductility, and, second, there are inertia forces activated. Both influences are clearly demonstrated in experiments. For concrete structures, which exhibit damage and fracture phenomena, the failure mode and cracking pattern depend on loading rate. Moreover, theoretical and experimental investigations indicate that after the crack reaches critical speed of propagation there is crack branching. The present paper focuses on 3D finite-element study of the crack propagation of the concrete compact tension specimen. The rate sensitive microplane model is used as a constitutive law for concrete. The strain-rate influence is captured by the activation energy theory. Inertia forces are implicitly accounted for through dynamic finite element analysis. The results of the study show that the fracture of the specimen strongly depends on the loading rate. For relatively low loading rates there is a single crack due to the mode-I fracture. However, with the increase of loading rate crack branching is observed. Up to certain threshold (critical) loading rate the maximal crack velocity increases with increase of loading rate, however, for higher loading rates maximal velocity of the crack propagation becomes independent of the loading rate. The critical crack velocity at the onset of crack branching is found to be approximately 500 to 600 m/s.

Abstract: Concrete is a material used all over the world for civil engineering but the mechanisms governing its dynamic behaviour are still not well understood. In this work, spalling tests and edge-on impact experiments have been used to determine the influence of the free-water contained in pores and micro-cracks on the dynamic strength and on the fragmentation process. Moreover, spalling tests have been also used to identify the main mechanisms leading to the difference of behaviour observed between wet and dry concrete.

Abstract: Data on the dynamic fracture energy of concrete are scarce and also not consistent due to different test methods, data analyses and definitions. In [1] the authors summarized and evaluated the test methods. Suggestions for the standardization of dynamic tensile testing were given. In the current paper the discussion is continued. First, definitions for the fracture energy and the relevant parameters are given. Next, theoretical considerations are given for the different rate dependency regimes of the dynamic tensile strength. Fracture and damage mechanics form the basis for the theoretical modeling. Based on the same principles, it is shown that the enhancement of the fracture energy occurs at higher loading rates than for the tensile strength. Phenomenological models to quantify the dynamic fracture energy are still lacking. To quantify the dynamic fracture energy, uniaxial test conditions are required. The Hopkinson bar technique meets this requirement. The paper presents and evaluates available data and relates these to the theoretical considerations.

Abstract: This paper presents a laboratory experimental study on the effect of high strain rate on compressive behavior of plain and fiber-reinforce high-strength concrete (FRHSC) with similar strength of 80-90 MPa. Steel fibers, polyethylene fibers, and a combination of these were used in the FRHSC. A split Hopkinson pressure bar equipment was used to determine the concrete behavior at strain rates from about 30 to 300 s^-1. The ratio of the strength at high strain rates to that at static loading condition, namely dynamic increase factor (DIF), of the concretes was determined and compared with that recommended by CEB-FIP code. Fracture patterns of the specimens at high strain rates are described and discussed as well. Results indicate that the CEB-FIP equation is applicable to the plain high strength concrete, but overestimates the DIF of the FRHSC at strain rates beyond a transition strain rate of 30 s^-1. Based on the experimental results, a modified equation on DIF is proposed for the FRHSC.

Abstract: The purpose of this work is to assess the dynamic mechanical behaviour of a commercial glass similar to that of the laminated glass structures used for protection and security applications in buildings. In particular, the study has been focussed on the influence of the strain-rate on the compressive (standard compression test) and tensile (splitting tensile test) strength of this glass. Tests at different strain-rates have been performed in the range between 10^-3 to 10³ s^-1 using standard test equipment for quasi-static tests and a SHPB equipped with a high-speed camera for the dynamic ones. Test data for compression tend to show that there is no substantial sensitivity to the strain-rate concerning ultimate strength and Young modulus. An appreciable increase in the ultimate tensile strength is revealed at higher strain-rate.

Abstract: In this paper, several material models are analyzed in order to represent and compare the behaviour of laminated glass subjected to blast loads. LS-DYNA and EUROPLEXUS are used for numerical simulation. These codes have different capabilities to describe the mechanical problem, especially the failure behaviour. The results of the simulations are compared to laboratory experimental results in order to validate the accuracy of the material models.