As mentioned above, masonry and concrete tend to be strong in compression but weak in tension. In olden times, builders took advantage of this fact by the use of arches above door and window openings. Arches are an interesting structural form because they act totally in compression. Therefore, arches above windows have the downward load of the wall and floor/roof loads above transferred around the arch keeping the structure in compression, so tension cracks didn’t occur in the masonry arch. Barrel vault ceilings are really just three dimensional arches, so they also worked as compression members only..A lintel is defined as a horizontal structural member supporting the wall load above a window or door. Lintels can be precast concrete, steel structural shapes, stone, wood or other products. Some common lintel types are shown below. The Construction Supervisor should be aware of the concept of arch action with regard to lintel design. A lintel designed for arch action only needs to carry the actual wall weight in the 45 degree angle triangle directly above the opening. Since typical precast CMU lintels are designed this way, if a change that places a concentrated load in this triangle happens during the course of the project, the Construction Supervisor should understand the ramifications and point it out to the Design Professional.
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