David Z. Yankelevsky

High strength concrete response to hard projectile impact

Abstract High strength concrete (HSC) becomes more common in practice and may have advantageous implementations. According to existing penetration formulae HSC is expected to enhance the performance of structural elements that are designed to resist projectile impacts. However, scabbing at the rear face is expected to be more severe in elements that are made of HSC, because of the relatively high material brittleness. Therefore, it is important to enhance the ductility of HSC elements, and one possible direction is to use fibers or wire mesh reinforcement. In order to study the influence of the concrete strength and of the reinforcement type on the elements response, penetration tests were conducted on regular strength concrete (RSC) and on HSC plates, with the following types of reinforcement: 5 mm steel mesh, steel fibers, small diameter steel wire mesh, and woven steel fence mesh of various diameters. The plates were subjected to an impact of a cylindrical hard steel projectile, weighing 120 g, with a conical nose and a 1.5 aspect ration. The projectiles were accelerated by a laboratory gas gun to velocities that ranged between 85 and 230 m/sec, which were measured by an electro-optical device. By comparing the response of these plates to an impacting projectile, the effects of concrete strength and of the reinforcement were studied. Major trends of the elements behavior were studied, their responses were compared and are described herein.

LOCAL RESPONSE OF CONCRETE SLABS TO LOW VELOCITY MISSILE IMPACT

A new model to predict the penetration and perforation of concrete slabs impacted by low velocity missiles, is presented. The two-stage model incorporates a first stage penetration algorithm into an infinite medium and a second stage of punching shear. The transition between stages is determined and the penetration time history as well as concrete plug shear resistance and shape are calculated. The present model predictions are compared with several common formulae as well as with test data and good agreement is obtained.

Exact stiffness matrix for beams on elastic foundation

Abstract An exact stiffness matrix of a beam element on elastic foundation is formulated. A single element is required to exactly represent a continuous part of a beam on a Winkler foundation. Thus only a few elements are sufficient for a typical problem solution. The stiffness matrix is assembled in a computer program and some numerical examples are presented.

Effects of Woven Fabric Geometry on the Bonding Performance of Cementitious Composites: Mechanical Performance

Abstract The effect of the geometry of woven fabrics on the bond between monofilament polyethylene yarns and cement matrix was studied in the present work. The fabrics were all plain weave, with varied fills density: 5, 7, or 10 fills per cm; the warps’ density was kept constant at 22 warps per cm. The interfacial bond was evaluated by pullout tests. To characterize the influence of the fabric’s geometry on bond performance, the influence of different parameters of the fabric’s geometry that may affect bond were separated: (1) pullout of a single crimped yarn untied from the fabric to characterize the influence of the shape of the individual crimped yarn; (2) pullout of a single yarn from free fabric (not embedded in the cement matrix); and (3) pullout of a yarn from a fabric embedded in the cement matrix. Straight yarns were also tested for comparison. It was found that the woven fabric provided a considerably better bond to the cementitious matrix than the bond of a single straight yarn. The crimped geometry of the yarn in the fabric was found to have a significant influence on increasing the bond between the woven fabric and the cementitious matrix.

Reliability evaluation in nonlinear analysis of reinforced concrete structures

Modern building codes provide a basis for development of advanced nonlinear models for analysis and design of reinforced concrete (RC) structures. Application of nonlinear models permits direct evaluation of reliability of the whole structure at the stage of a structural analysis. In this paper a probabilistic method for reliability evaluation of plane frame structures with respect to ultimate limit states is proposed. The method is based on a combination of the nonlinear finite element structural model and the first-order reliability method (FORM). Implementation of the FORM for nonlinear analysis of RC structures is considered. Uncertainties associated with the structural model are taken into account and their influence on structural reliability is examined via sensitivity analysis.

ENHANCED BONDING OF LOW MODULUS POLYMER FIBERS-CEMENT MATRIX BY MEANS OF CRIMPED GEOMETRY

Abstract Low modulus polymeric yarns (fibers) can be used as primary reinforcement in thin sheet cement products if their bonding to the matrix can be made sufficiently high. Straight yarns usually have a low bond due to the hydrophilic nature of the yarn and its low modulus which does not allow for sufficient clamping stresses to develop and enable effective frictional bond. In the present paper the potential of modifying the shape of the yarn to achieve a crimped geometry was studied to enhance its bond resistance. Such geometry can be achieved in the production of individual yarns and it is the geometry which exists in woven fabrics. The crimped yarns investigated here were obtained by untying yarns from woven fabrics. The fabrics were produced especially for this work to achieve controlled geometry of the yarn, which was characterized in terms of the wave length and amplitude of the crimped shape. The bonding performance was characterized by pull-out tests of the crimped yarns. The crimped shape enhanced the bonding considerably, and the pull out resistance was found to be a linear function of the product of the wave amplitude of each crimp and the number of waves (i.e., yarn length/wave length) along the yarn. The dominant bonding mechanism was mechanical anchoring.

Analysis of a beam column on elastic foundation

Abstract The analysis of beams on elastic Winkler foundation is very common in engineering. In many applications, transverse as well as axial forces exist. An exact analytical solution of a finite element beam column resting on a Winkler foundation is performed from which the exact stiffness terms are determined. The stiffness matrix is incorporated into a common beam program. Nodes are required only at points of discontinuity in stiffness, loading, or supports. Comparisons are made with case results appearing in literature.

On Blast Pressure Analysis Due to a Partially Confined Explosion: I. Experimental Studies

ABSTRACT The paper presents a second part of a study aiming at understanding some characteristics of an interior explosion within a room with limited venting. Internal explosions may occur as a result of an ammunition storage explosion, or a charge explosion within a room in a terrorist action or a warhead explosion following its penetration into a closed space. This paper follows an experimental study and validation of the problem and aims at an extended numerical investigation of several aspects of the problem. The analysis has been performed for a room with rigid walls having limited venting by using AUTODYN 12 commercial program implementing the Eulerian multi-material approach, and the results were compared with the experimental measurements. An effective simplified model with lumped parameters based on the Bernoulli equation has been developed for the quasi-stationary phase of the detonation products outflow from the room through the venting openings.

Analysis of beams on nonlinear winkler foundation

Abstract The analysis of beams on Winkler foundations is very common in engineering. In many practical problems nonlinear foundation behavior should be considered. Based on an exact solution for a finite beam on a linear elastic foundation an iterative procedure for a piecewise linear foundation is formulated. The approach is implemented in a continuous beam computer program and demonstrated in two examples.

Punching shear in concrete slabs

Punching shear in concrete slabs is a serious problem in certain structural systems, such as flat slabs. Current analysis is based either on empirical and simplified analytical techniques or on theoretical models, mainly based on the theory of plasticity. This paper presents a new model, based on rigid post-fractured behavior, utilizing the post fracture properties of concrete at the rough crack interfaces that are developed. The model predicts the force—displacement resistance during punching, the stress distributions along the cracked interfaces, as well as the shape of the punched concrete plug. Comparison of the model predictions with various test data shows good correspondence. ( 1998 Elsevier Science Ltd. All rights reserved.