Many times, with limited methods for analyzing elements, structural engineers have used CJ in structural masonry walls to simplify the wall design. Therefore, it makes sense for the structural engineer to locate the joints so that they will have as little impact as possible on wall capacity, within prescribed spacing recommendations. Since the purpose of a control joint (CJ) is to provide a bond break that will permit longitudinal movement and relieve horizontal tensile stresses, horizontal reinforcing is generally not continuous through a control joint. Hybrid design of masonry wall and surrounding frame. A higher f’ m will also reduce reinforcing lap lengths and, depending on the fastener, increase fastener capacity when connecting to the masonry, again reducing material costs.ĭesigning with the actual strength of the block that will be used on a project can be very beneficial and cost effective, especially for projects with a lot of openings or other details that create a complex masonry system. This can mean less reinforcing steel and smaller members for lintel and jamb designs, wall design for axial loads and out-of-plane lateral loads, and in-plane loads on shear walls. The modulus of elasticity of masonry is directly dependent on f’ m, and for design, flexure, and shear checks that are performed against some percentage of f’ m – a higher f’ m means a higher allowable strength.
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The value of f’ m affects numerous parts of masonry design. Visit for information on block strengths in several states. Knowing the actual strength of the material, and designing with it in mind, can lead to cost savings in both labor and materials.
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Block manufacturers have increased the strength of CMU to reduce potential damage to units in production, storage, and site transportation. Many manufacturers, however, regularly produce stronger blocks resulting in stronger structural members. Sometimes, engineers will mistakenly specify a minimum f’m because the design will work at that value and they are not aware that higher design strengths are available or, because they believe that masonry of lower strength will be easier to obtain. When designing structural elements with concrete masonry units (CMU), the compressive strength ( f’ m) of the assemblage of the block, grout, and mortar is crucial to the strength of the element. Make every effort to model the actual masonry element strength. When using FEM, what design strength should be utilized? Since FEM associates load to elements based on their strength/stiffness, accuracy is crucial.
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The first place to start in developing a sophisticated FEM is to make every effort to define the material strength of all structural items correctly. a) CJ next to opening b) CJ away from opening edge.