Basic Mechanical Behavior of Masonry

Masonry is a composite material, comprising units, mortar, grout, and accessory materials. Because of this, its mechanical behavior is complex. Using nonlinear finite-element analysis, addressing the behavior of constituent materials and of the interface relationships between them, it is possible to describe the force-deformation behavior of masonry elements. For design, however, this approach is neither practical nor necessary. For design purposes, masonry is normally idealized as an isotropic material, with nonlinear stress-strain behavior in compression (much like concrete) and linear stress-strain behavior in tension. Compressive capacity is governed by crushing (often characterized by complex local behavior) and tensile capacity, by the bond strength between units and mortar. The crushing strength of masonry can be evaluated by compression tests on masonry prisms. Design of masonry elements is based on a specified compressive strength of masonry, fm², whose role is analogous to that of the specified compressive strength of concrete, fc², in concrete design. The specified compressive strength of masonry is the basis for design and forms part of the contract documents. These contract documents require verification that the masonry comply with the specified compressive strength, either by compression tests of prisms or by conservative relationships involving the compressive strengths of the units  fication (MSJC 2008a,b). One advantage of using the conservative relationships involving the compressive strengths of the units and the type of mortar (the so-called unit strength method from Tables 1 and 2 of the MSJC Specification) is that it is possible to verify compliance with the specified compressive strength fm with no project-specific material testing whatsoever. The compressive strength of the units is verified by the manufacturer as part of quality control and compliance with the unit specification. The mortar can be specified by proportion, and compliance with that specification is verified by verifying proportions (no mortar testing). The grout can be specified by proportion and compliance with that specification is verified by verifying proportions (no grout testing). The minimum probable strength of the masonry is then obtained from Table 1 or 2 of the MSJC Specification (MSJC 2008b). Material tests can be performed for quality control or to verify compliance with a specified strength, but they are not necessary if the designer chooses specification criteria that do not require testing. Masonry elements requiring structural calculation are designed using the specified compressive strength, verified as noted above, and prescribed tensile bond strengths based on extensive experimental investigation.



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