Stirrup reinforcing is a major factor in adding strength to concrete support columns. Though concrete is strong by itself, it's even stronger with steel bars running through its length. To ensure that this composite material is as strong as possible, engineers use stirrups, also made of steel, to keep everything straight and in order when the mixed concrete is poured.
Stirrups are frames made from high-strength steel wire. The stirrups are bent using a machine into a supportive cage that runs the length of the concrete column. The stirrup, though it is not as strong as steel rebar, acts like the metal wire in windows that prevents them from just being smashed. It holds the concrete into a shape while lending support.
Stirrups are often used in conjunction with steel rebar. The rebar -- long, strong lengths of steel -- is put inside the mold so that when the concrete is cast it has steel bones, so to speak. The bar is held in place by the stirrup, which ensures that the rebar is straight and that it's providing the maximum amount of support and strength to the column in which it's placed.
Stirrups are separate from one another. Rebar is placed into the base where the concrete will be poured. The stirrups, which are little more than square bands, are placed around the steel rebar at regular intervals to strap them tightly into a given shape. This forms a skeletal square or rectangular column inside the mold. The concrete is poured over the skeleton, embedding it inside the column.
The stirrups provide resistance, which is more specific than strength. The concrete and the rebar are supporting the load coming down on the column from above. The stirrups act like tendons, keeping things together when there's pressure and flexing. Though not essential, the stirrups are definitely helpful. They're also used in beams, which are laid horizontally rather than vertically as columns are.
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