Knowing your starters from your stirrups
Nov 11, 2019
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Are you being asked to purchase reinforcing components for your customer, but struggling to know exactly what to order or to specify?
The reinforcing within concrete structures is made up of various components, each serving a different purpose and requiring specification.
In this article we will provide a short overview of the key reinforcing components, their purpose, their variants and some considerations when ordering.
In this article we will look at our stocked items:
Non-stocked items, such as U-bars, Z-bars or items with non-standard dimensions can be ordered on request.
Reinforcing is placed in concrete to create a cost-effective composite building material which utilises:
- The high-compressive strength and low-cost of concrete
- The high tensile strength of steel.
Concrete, on its own, has low tensile strength and is therefore susceptible to crack and breaking when it is placed under tension or stretched, like in an earthquake.
A general point to note is, reinforcing components are normally specified by the overall outside dimension (i.e. including the bar thickness) to ensure correct concrete cover is maintained. As an example, a R10 200mm stirrup will measure 200mm from outside edge to outside edge, and have an internal dimension of 180mm.
Deformed bar is the primary longitudinal component used in reinforcing and has ribbed like surface projections. The ribs improve the bonding, which occurs between the steel and concrete, when the concrete surrounding the bar hardens. These two materials then act as though they are mechanically one.
United Steel stocks deformed bar in Grade 300E & 500E, a variety of diameters and in 6, 9 & 12m lengths as standard. The different grades of deformed bar are usually denoted by the letters D and H, followed by the diameter. For example, D10 is 10mm Grade 300E deformed bar, and H12 is 12mm Grade 500E deformed bar.
Some other things to keep in mind are:
- Deformed bar is sometimes called rebar, which should not be confused with ReidBar. Reidbar is threaded bar used for bracing.
- Plain round bar, is as the name implies plain, and lacks the ribs that give improved concrete bonding. Plain bar is often denoted R10, R12 etc, and is used to make reinforcing components such as links and stirrups.
- Care should be taken if substituting Grade 300E bar for the higher tensile strength Grade 500E bar, as a higher tensile strength is not a benefit in all applications. For example, it is more difficult to bend
As the name implies, starter bars are bars cast into one concrete structure to provide a (over)lapped connection to reinforcing bar in another structure. They provide continuity of reinforcement across a join. For example, starters are placed in a foundation to allow connection to a concrete (block) wall, or to the main floor slab.
Starter bars are made from deformed bar, and come in a variety of diameters & in both Grade 300E & 500E.
Starter bars also come in a variety of shapes, where the end is bent or hooked to
- Provide additional anchorage
- Provide an additional safety factor
- Contain multiple primary bars and reduce the impact of shear stress, similar to stirrups.
United Steel has two stocked starter shapes as shown below. The first is a simple (180 Deg) hook starter and the second is a bent starter with hook (L-Hook starter). Other shapes and sizes are available on request.
To specify a starter the diameter of the bar needs to be defined, along with the shape, upright length, and length of any end bend. For example, a STR12-1200-150 is an L-Hook starter which has a 12mm diameter starter, with 1200mm upright, 150mm bend, and hook at the end of the bend.
Stirrups are rectangular loops made from plain round bar, which have the purpose to confine or restrain the primary reinforcing bars and prevent shear failure. Shear failure is where the structure fails parallel to the direction of force.
Stirrups have a hook at each end of the loop, which provide anchorage and tie the reinforcement in position when hooked around one of the primary longitudinal reinforcing bars. They are called stirrups due to their historic similarity in shape to horse riding stirrups.
To specify a stirrup the diameter of the plain bar is specified, along with the two dimensions that define the rectangle. United Steel stocks stirrups in standard a range of sizes.
Links are lengths of plain round bar, with a hook at each end. Similar to stirrups, they serve the following purposes in reinforced concrete:
- Provide shear reinforcement
- Tie together or restrain the primary longitudinal reinforcing bars, including in some cases those enclosed within a stirrup
- Prevent the primary bars from blowing out under large load.
- Hold the reinforcement while concreting
To specify a Link state the bar diameter and the distance between the hooks.
United Steel stocks links in standard a range of sizes.
Corner bars provide continuity of reinforcement in the corners of a structure, such as a foundation slab, and provide torsional reinforcement. Torsion is the twisting that occurs in a structure when it is loaded – the corners of a foundation slab rise and twist when a load is applied.
Corner bars are made from deformed bar and bent into an L shape, with the legs being of equal or unequal length – see images below. The lengths are determined by the amount of lapped connection required by the primary reinforcing bars in the structure.
To specify a Corner Bar the diameter of the bar is specified along with the length of the legs. United Steel stocks corner bars in standard a range of sizes.