stirrup

A stirrup is a closed loop of reinforcing steel (rebar) that wraps around the main longitudinal rebars in a concrete beam or column. It is bent into a rectangular or square shape and positioned perpendicular to the beam's length, tying the main rebars together and holding them in their proper spacing during concrete placement. The stirrup's job is to resist shear forces, the horizontal stresses that try to make one part of the beam slide past another, especially near supports where shear is highest.

Stirrups are typically made from smaller diameter rebar than the main reinforcement, usually No. 3 or No. 4 bar (10 mm or 13 mm diameter in metric). They are spaced closer together near points of high shear stress and farther apart in regions of lower stress. For example, a beam might have stirrups at 4 inch (100 mm) spacing near the supports and 8 inch (200 mm) spacing toward midspan. The spacing, bar size, and bend configuration are all specified in structural drawings calculated by the engineer.

Closed vs. Open Stirrups

A fully closed stirrup forms a complete rectangle or square and provides maximum shear reinforcement. An open stirrup (also called a U-stirrup) has one side that does not close; it is used in situations where the top or bottom of the member is not under significant stress, or where ease of installation is prioritized. Some codes require closed stirrups in seismic zones or high-stress regions because they provide better confinement of the concrete core.

During concrete placement, stirrups can shift or become misaligned if not properly secured. Concrete vibration, worker traffic, and the weight of fresh concrete can move them. They must be tied to the main rebars with wire to hold them in position until the concrete sets. Improper spacing or missing stirrups can lead to diagonal cracks in the concrete beam, a visible sign of shear failure in the concrete itself.

The term comes from the resemblance to a horse stirrup: a loop that hangs and supports. In older trades and some regions, stirrups are also called shear reinforcement, ties, or hoops, though hoops is more commonly applied to circular reinforcement in columns. Understanding stirrup placement and spacing is fundamental to concrete frame design, and inspection of stirrup location and spacing is a critical quality control task on any reinforced concrete project.