We design reinforced concrete members and often we are asked to observe the steel reinforcement in field before the concrete is placed. It is our job to make sure the concrete foundations, beams, columns, etc. are built the way they were designed. During our observations we often find that steel beam stirrups, used in reinforced concrete design, are not installed correctly and it isn’t always clear to the installer why they are important.
Historically, beam stirrups had been used sparingly in residential construction. However, in recent years concrete beam sizes have gotten shallower and spans have increased. In our experience, this has been the result of architectural design and building occupant requirements. The increased cost of foundation elements, such as drilled piers, has also been a factor. Increasing concrete beam spans, to reduce the need for additional piers, has resulted in the need for the use of steel stirrups.
Concrete beams vary in depth. The deeper the beam, the more shear capacity. When the depth is not adequate, steel stirrups must be added to increase the shear capacity of the beam. These stirrups are usually one piece of steel that is bent into a rectangular shape. Often small diameter steel is used, such as #3 and #4 rebar. The stirrup typically wraps around the bottom and top bars of the beams.
A designer should specify the size, spacing and location along the length of the beam where the stirrups are required. We like to specify the stirrup dimensions in our sections, so that the stirrup can be manufactured prior to installation. Stirrups will be required at areas of high shear, such as bearing points and below large point loads.
The installer should be careful to fabricate the stirrup from one piece of steel and adequately overlap each end (contact the Structural Engineer or refer to the ACI code for variations). Too often the stirrup is not pre-fabricated and the installer tries make the stirrup in the field, after the horizontal bars are already in place. This is usually obvious, because the stirrup is constructed from two pieces with inadequate lap splice. It is much easier and efficient to install a stirrup at the same time the horizontal reinforcement is being installed. Always contact the Structural Engineer with any questions about size, shape, spacing and installation of stirrups prior to inspection. This will help prevent last-minute changes, while the concrete truck is waiting.
Visit the Reinforced Concrete Design section of our blog for more on concrete design.