Marcio Antonio Ramalho

A numerical model for the description of the nonlinear behaviour of multi-leaf masonry walls

A nonlinear finite element model was developed to simulate the nonlinear response of three-leaf masonry specimens, which were subjected to laboratory tests with the aim of investigating the mechanical behaviour of multiple-leaf stone masonry walls up to failure. The specimens consisted of two external leaves made of stone bricks and mortar joints, and an internal leaf in mortar and stone aggregate. Different loading conditions, typologies of the collar joints, and stone types were taken into account. The constitutive law implemented in the model is characterized by a damage tensor, which allows the damage-induced anisotropy accompanying the cracking process to be described. To follow the post-peak behaviour of the specimens with sufficient accuracy it was necessary to make the damage model non-local, to avoid mesh-dependency effects related to the strain-softening behaviour of the material. Comparisons between the predicted and measured failure loads are quite satisfactory in most of the studied cases.

Proposal of a test specimen to evaluate the shear strength of vertical interfaces of running bond masonry walls

There is no normalized test to assess the shear strength of vertical interfaces of interconnected masonry walls. The approach used to evaluate this strength is normally indirect and often unreliable. The aim of this study is to propose a new test specimen to eliminate this deficiency. The main features of the proposed specimen are failure caused by shear stress on the vertical interface and a small number of units (blocks). The paper presents a numerical analysis based on the finite element method, with the purpose of showing the theoretical performance of the designed specimen, in terms of its geometry, boundary conditions, and loading scheme, and describes an experimental program using the specimen built with full- and third-scale clay blocks. The main conclusions are that the proposed specimen is easy to build and is appropriate to evaluate the shear strength of vertical interfaces of masonry walls.

Elements of Structural Masonry Reinforced with Sisal Fibers

Abstract: There is great interest in the use of natural fibers as reinforcement to obtain new construction materials due to its low cost, high availability and reduced energy consumption for its production. This paper evaluates the incorporation of sisal fibers of 20 mm and 40 mm in length and volume fraction of 0.5% and 1% for concrete masonry structural blocks, and determines the use of these units to build prisms and mini-walls. Laboratory tests were carried out to characterize the physical of blocks and mortar, in addition to the axial compression tests of the units, prisms, and mini-walls. The sisal had low apparent density and high water absorption, which is a common feature of such material due to the high incidence of permeable pores. The physical properties of the blocks with and without addition complied with the standard requirements established to validate their use. The obtained results showed that the fiber-reinforced mini-walls obtained values very close to or even higher than those obtained for the mini-walls without fibers, demonstrating better performance than the blocks and prisms.

A Non-local Anisotropic Damage Model ForBrittle Materials

The mechanical behaviour of brittle materials, such as masonry and concrete, is characterized by strain-softening after a peak stress value. This phenomenon is the macroscopic manifestation of microcracking, which leads to formation of macrocracks and, possibly, to failure of the material element. This behaviour can be described through the damage mechanics using smeared crack models. These models permit the evolution of the cracking process to be reproduced, but if any region exists in the body where the strain field localizes, damage localizes as well. In other words, the results of finite element analyses made using these models are strongly mesh dependent, and an unrealistic mechanical behaviour is described. This problem can be avoided, or at least reduced, assuming that damage is spread within a region (or ‘process zone’) whose size is supposed to be a material property. This kind of models are generally known as ‘nonlocal damage models’. In this work, a non-local damage model is proposed for brittle materials, such as masonry and concrete, starting from a previous proposal of the authors [1]. The model is characterized by an anisotropic damage tensor, D, which evolves differently under tensile and compressive strains. The damage process driving variable is supposed to be a ‘damage force’, Y, obtained deriving the material Helmoltz free energy with respect to the damage variable. As the maximum eigenvalue of Y attains a critical value (y0T or y0C, according to the sign of the associated strain), the first damage direction is activated. An additional damage direction can activate in the plane orthogonal to the first one if the maximum direct component of Y in that plane attains the damage threshold. Eventually, the third principal damage direction necessarily forms a righthanded triad with the other two. It is important to note that the principal directions of damage remain fixed throughout any load history, so that the model can be included in the class of the non-rotating smeared crack models. The model overcomes some deficiencies of the previous local version, as the model parameters are related to an internal characteristic length of the material to make the specific fracture energy mesh-independent in finite element analyses. The model was calibrated using experimental results on brittle specimens subjected to uniand biaxial stresses. The capability of the model were assessed reproducing experimental shear tests on masonry panels subjected to horizontal monotonic and cyclic loads. In building the panels, two different height-to-width ratios were used, corresponding either to a ‘slender’ panel (1 m in width and 2 m in height) or to a ‘squat’ panel (of the same width as the slender one, but 1.35 m in height). The collapse of the squat panel is matched by growth of cracks in the central zone induced by shear stresses, whereas the failure mechanism of the slender panel is matched by a rocking behaviour. Accordingly, the plot of the horizontal force vs. the horizontal displacement for the squat panel shows a degradation in stiffness and strength in conjunction with a significant dissipation. On the contrary, the same plot for the slender panel is characterized by a smaller dissipation, due to a lower reduction in stiffness, and a lack of softening behaviour [2]. Future evolutions of the proposed model concern the possibility of encompassing also creep-induced damage, by employing different evolution laws for damage originated by increasing stresses and by sustained stresses. Also, the possibility of capturing crackclosure effects, which is a basic requirement to effectively reproduce the material behaviour under cyclic loads, could be taken into account by using different damage variables, separately accounting for damage induced by tensile and by compressive strains.

Experimental And Numerical Study OfMulti-leaf Masonry Walls

A research programme was carried out with the aim of investigating the mechanical behaviour of multiple-leaf stone masonry walls. A number of experimental tests were performed on three-leaf specimens, consisting of two external leaves made of stone bricks and mortar joints, and an internal leaf in mortar and stone aggregate. The specimens differed from each other in terms of interface geometry, stone nature, and loading conditions. A numerical model was developed to predict the nonlinear response of the specimens. The model is characterized by a damage tensor, which allows one to describe the damage-induced anisotropy accompanying the cracking process. Comparisons between the predicted and measured failure loads are quite satisfactory in most of the studied cases. The numerical procedure still needs to be improved to accurately describe the post-peak behaviour, by avoiding mesh-dependency effects related to the strain-softening behaviour of the material.