Jonathan Ochshorn's Structural Elements for Architects and Builders, Third Edition
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Chapter 5: Reinforced concrete

Reinforced concrete is plain concrete in which steel bars have been strategically placed so that they compensate for the low tensile strength of the concrete. This allows concrete — historically (i.e., before the late nineteenth century) a material that was, like masonry, stressed primarily in compression — to become quite a bit more versatile. This versatility comes about not because reinforcing concrete with steel makes the concrete suitable for use in pure tension members, but rather because it opens up the possibility of using concrete where either bending, or other combinations of compression and tension are expected: not only in all sorts of slabs, beams, girders, and columns, but also in more exotic shell structures such as hyperbolic paraboloids.

For the record, it should be noted that steel bars added to concrete bending and compression elements may also be placed where compression, and not only tension, is expected. In columns, for example, buckling can cause bending in either of two directions about a column's weak axis, so the location of added tension and compression stresses is unknown. Therefore, reinforcing bars must be placed to resist tension on both sides of the column, even though one side will not be subjected to tension stress. In beams, compression steel is often provided for two reasons. First, it is convenient to provide at least a nominal amount of steel in all four corners of a rectangular concrete cross section, and not only in the tension zone. Doing so provides a framework (or cage) that enables the whole ensemble of necessary reinforcement to be tied together and lifted in one piece into formwork before the concrete is cast. Second, compression steel is often desired because it reduces deflection of beams, especially those with long spans and relatively narrow depths.