Masonry Structures, Overview
Masonry is the oldest building material that still finds wide use in today’s building industries. The most important characteristic of masonry construction is its simplicity. Laying pieces of stone, bricks, or blocks on top of each other, either with or without cohesion via mortar, is a simple, though adequate, technique that has been successfully used ever since remote ages. Naturally, innumerable variations of masonry materials, techniques, and applications occurred during the course of time. The influence factors were mainly the local culture and wealth, the knowledge of materials and tools, the availability of material, and architectural reasons.
The primitive savage endeavors of mankind to secure protection against the elements and from attack included seeking shelter in rock caves, learning how to build tents of bark, skins, turfs, or brushwood and huts of wattle and daub. Some of such types crystallized into houses of stone, clay, or timber. The evolution of mankind is thus linked to the history of building materials.
The first assault to the use of structural masonry happened at the middle of the nineteenth century, when cast-iron beams and columns started to be produced. By the end of the century, skyscraper constructions methods had eliminated the necessity of massive ground-level piers of masonry. Nevertheless, the collapse of masonry as a structural material started in the beginning of the twentieth century, with the introduction of German, French, and British regulations for design of reinforced concrete structures. Concrete was used in constructions of walls as early as the fourth-century BC around Rome. But, only in 1854, a system for reinforced concrete was patented in Britain by W.B. Wilkinson. By the beginning of the twentieth century, it was clear that reinforced concrete was a durable, strong, moldable, and inexpensive material, and masonry was practically forgotten as a structural material in several developed countries.
In Europe, the building solutions using unreinforced structural masonry represent about 15 % to more than 50 % of the new housing construction, taking as reference countries with low seismicity (e.g., Germany, Netherlands, or Norway) but also countries with high seismicity (e.g., Italy). A usual solution is the adoption of masonry units with large thickness in the building envelope to fulfill thermal requirements. It is stressed that an integrated and complete building technology is needed, including units with different shapes and solutions for floors. Reinforced masonry was developed in different countries as a response to the lower performance of unreinforced masonry buildings under large horizontal loading, but no unified solution was found. Diverse solutions with different levels of success coexist, together with recent innovative solutions.
It is common practice to combine prefabricated slabs with load-resisting walls so that formwork, scaffolding, and execution times can be significantly reduced. In the USA, in the last 30–40 years, reinforced masonry became an attractive and efficient solution from a perspective of cost-benefit analysis for buildings in regions of low to high seismicity, e.g., hotels, residential buildings, office buildings, schools, commercial buildings, or warehouses. The American standard solution includes reinforced concrete horizontal bond beams, two-cell blocks filled with grout, and vertical reinforcement. Besides other reinforced masonry approaches, confined masonry is popular in developing countries, as the changes with respect to unreinforced masonry construction are small. In this system vertical and horizontal reinforced concrete elements of small section are included in the masonry. These elements aim at providing an increase of shear and flexural strength, together with a larger energy dissipation capacity and larger ductility with respect to horizontal actions.
The last decades have witnessed an enormous development in the characterization of masonry materials and in numerical methods for structural analysis. Today, with the help of a computer, it is possible to analyze structures with a high level of accuracy. The material science (directly dependent on the observation and analysis of the experimental behavior) has suffered a slower evolution, but important advances in constitutive models and structural component models have occurred in various fields. For a long time, it was believed that the decline of masonry as a structural material was not only due to economic reasons but also to underdeveloped masonry codes and lack of insight in the behavior of this type of structures. This is not presently the case, as modern codes are available, e.g., Eurocode 6 or EN 1996-1-1:2005 (CEN 2005), but also modern engineering books, e.g., Drysdale and and Hamid (2008), Hendry (1998), Pfefferman (1999), or Sahlin (1971), which provide general overviews of design. More specific books, with a focus on earthquake performance are also available, e.g., Tassios (1988) or Tomaževič (1999). It is stressed that the design of unreinforced masonry structures under seismic loading has not yet received general consensus.
Nevertheless, the mechanical behavior of the different types of masonry has generally a common feature: a very low tensile strength. This property is so important that it has determined the shape of ancient constructions. The difficulties in performing advanced testing of ancient structures are quite large due to the innumerable variations of masonry, the variability of the masonry itself in a specific structure, and the impossibility of reproducing it all in a specimen. Therefore, most of the advanced experimental research carried out in the last decade has concentrated in brick/block masonry and its relevance for design. The masonry components are detailed in another essay.
The fact that ancient and modern masonry have so much variability in materials and technology make the task of structural analysis of these structures particularly complex. From a very simplified perspective, it is possible to distinguish masonry as reinforced and unreinforced. The presence of (distributed) reinforcement provides masonry with tensile strength and renders masonry closer to reinforced concrete. In such a case, the orthotropic behavior of masonry and the nonlinear constitutive behavior become less relevant, and the techniques normally used for the design and analysis of reinforced concrete structures can possibly be used. Conversely, in the case of unreinforced masonry structures, the very low tensile strength of the material renders the use of nonlinear constitutive behavior more obvious. This is particularly true in the assessment of existing structures and in seismic analysis.
Masonry is usually described as a material that exhibits distinct directional properties due to the mortar joints, which act as planes of weakness. This description is associated mostly with the material, whereas a different description can be given at structural level.
Detailed micro-modeling – units and mortar in the joints are represented by continuum elements, whereas the unit-mortar interface is represented by discontinuum elements.
Simplified micro-modeling – expanded units are represented by continuum elements, whereas the behavior of the mortar joints and unit-mortar interface is lumped in discontinuum elements.
Macro-modeling – units, mortar, and unit-mortar interface are smeared out in a homogeneous continuum.
Accurate modeling of masonry structures requires a thorough experimental description of the material. Obtaining experimental data, which is reliable and useful for numerical models, has been hindered by the lack of communication between analysts and experimentalists. The use of different testing methods, test parameters, and materials preclude comparisons and conclusions between most experimental results. It is also current practice to report and measure only strength values and to disregard deformation characteristics. Only recently, information in the post-peak or softening regime became more widely available.
Softening is a gradual decrease of mechanical resistance under a continuous increase of deformation forced upon a material specimen or structure. It is a salient feature of quasi-brittle materials like clay brick, mortar, ceramics, rock, or concrete, which fail due to a process of progressive internal crack growth. Such mechanical behavior is commonly attributed to the heterogeneity of the material, due to the presence of different phases and material defects, like flaws and voids. Even prior to loading, mortar contains micro-cracks due to the shrinkage during curing and the presence of the aggregate. The clay brick contains inclusions and micro-cracks due to the shrinkage during the burning process. Stone also, usually, contains inclusions and micro-cracks. The initial stresses and cracks as well as variations of internal stiffness and strength cause progressive crack growth when the material is subjected to progressive deformation. Initially, the micro-cracks are stable which means that they grow only when the load is increased. Around peak load an acceleration of crack formation takes place and the formation of macro-cracks starts. The macro-cracks are unstable, which means that the load has to decrease to avoid an uncontrolled growth. In a deformation-controlled test, the macro-crack growth results in softening and localization of cracking in a small zone while the rest of the specimen unloads.
The properties of masonry are strongly dependent upon the properties of its constituents. Compressive strength tests are easy to perform and give a good indication of the general quality of the materials used. It is difficult to relate the tensile strength to the compressive strength due to the different shapes, materials, manufacture processes, and volume of perforations in the units. The bond between the unit and mortar is often the weakest link in masonry assemblages. The nonlinear response of the joints, which is then controlled by the unit-mortar interface, is one of the most relevant features of masonry behavior. Two different phenomena occur in the unit-mortar interface, one associated with tensile failure (mode I) and the other associated with shear failure (mode II).
Different test setups have been used for the characterization of the tensile behavior of the unit-mortar interface. These include (three-point, four-point, bond-wrench) flexural testing, diametral compression (splitting test), and direct tension testing. An important aspect in the determination of the shear response of masonry joints is the ability of the test setup to generate a uniform state of stress in the joints. This objective is difficult because the equilibrium constraints introduce nonuniform normal stresses in the joint.
Finally, the uniaxial behavior of the composite material is anisotropic, i.e., depending on the loading direction with respect to the material axes, namely, the directions parallel and normal to the bed joints. The compressive strength of masonry in the direction normal to the bed joints has been traditionally regarded as the sole relevant structural material property, at least until the recent introduction of numerical methods for masonry structures. A test frequently used to obtain this uniaxial compressive strength is the stacked bond prism, but it is still somewhat unclear what the consequences are of using this type of specimens in the masonry strength. It has been accepted by the masonry community that the difference in elastic properties of the unit and mortar is the precursor of failure in compression, with uniaxial compression of masonry leading to a state of triaxial compression in the mortar and of compression/biaxial tension in the unit. Uniaxial compression tests in the direction parallel to the bed joints have received substantially less attention from the masonry community. However, regular masonry is an anisotropic material and, particularly in the case of low longitudinal compressive strength of the units due to high or unfavorable perforation, the resistance to compressive loads parallel to the bed joints can have a decisive effect on the load-bearing capacity.
The constitutive behavior of masonry under biaxial states of stress cannot be completely described from the constitutive behavior under uniaxial loading conditions. The influence of the biaxial stress state has been investigated up to peak stress to provide a biaxial strength envelope, which cannot be described solely in terms of principal stresses because masonry is an anisotropic material. Therefore, the biaxial strength envelope of masonry must be described either in terms of the full stress vector in a fixed set of material axes or in terms of principal stresses and the rotation angle between the principal stresses and the material axes.
Masonry is a building material with many variations and different levels of success in modern construction worldwide. Masonry is also widely available in the built heritage, as most of the pre-twentieth-century buildings are made of masonry. Masonry consists of units laid with a certain bond, usually including mortar in the joints. Masonry can be typically unreinforced or reinforced and an understanding of this composite material requires the characterization of its components. Because masonry is a composite material, different modeling strategies are available either with the discretization of the bond or not. These approaches are denoted by micro- and macro-modeling, usually combined with advanced nonlinear analysis tools. Adequate seismic design and simple assessment of unreinforced masonry buildings are a challenging task and the applicability of the methods depends on the existence of box behavior for the building. In the absence of box behavior, macro-block analysis is a popular and powerful approach. Finally, it is noted that adequate characterization of the experimental behavior is needed for an effective analysis and design of masonry structures, and this is currently available in the literature.
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