4.3 Bar Bending, Standard Hooks & Mandrel Bending Diameters

4.3 Bar Bending, Standard Hooks & Inside Bend Diameters

A standard hook combines a bend and a straight tail. Inspect both on the correct bar mark, then verify the location and embedment on the approved detail. A short bend diameter or tail can affect anchorage. A correctly shaped hook can still be installed at the wrong elevation or on the wrong side of the longitudinal bar.

Read standard hook dimensions

Let $d_b$ be the nominal diameter of the bar forming the hook. Under ACI 318-19 Table 25.3.1, standard longitudinal hooks use the following inside bend diameters. A 90-degree hook has a $12d_b$ tail. A 180-degree hook has a tail of at least $4d_b$ and at least 2.5 inches.

Table 25.3.2 governs standard hooks for stirrups, ties and hoops. Inside bend diameter is at least $4d_b$ for No. 3–5 and $6d_b$ for No. 6–8. Select the applicable tail separately.

Worked measurements. A No. 10 longitudinal hook requires an inside bend diameter of at least $8(1.270)=10.16$ inches. A No. 6 longitudinal hook requires $6(0.750)=4.50$ inches inside diameter, or 2.25 inches radius. For a No. 3 135-degree stirrup hook, $6(0.375)=2.25$ inches, so the 3-inch minimum tail controls. For a No. 5 135-degree hook, $6(0.625)=3.75$ inches governs.

Hook tail length is a fabrication dimension. Hooked tension development length under Section 25.4.3 is measured from the critical section to the outside end of the hook, with minimums of $8d_b$ and 6 inches in addition to the applicable calculation. Section 18.8.5 has separate rules for special moment frame joints. Check the approved anchorage detail and the applicable code case.

Distinguish a seismic hook and crosstie

ACI 318-19 Chapter 2 and Section 25.3.4 use a bend of at least 135 degrees for a seismic hook, with at least 90 degrees for a circular hoop. The straight extension must be at least $6d_b$ and at least 3 inches. The hook engages the longitudinal reinforcement and the extension projects inside the stirrup or hoop. Check the actual bar being engaged and the tail direction.

Under Section 25.3.5, a crosstie is continuous with a seismic hook at one end and a standard hook bent at least 90 degrees at the other. Both engage peripheral longitudinal bars. The standard end uses the applicable Table 25.3.2 tail. Alternate the 90-degree ends of successive crossties engaging the same bars except where Sections 18.6.4.3 or 25.7.1.6.1 apply. Read the approved detail for the member and identify the actual arrangement before checking alternation.

Field bending and straightening

Under ACI 318-19 Section 26.6.3.2, bend reinforcement cold before placement unless the licensed design professional permits otherwise. Field bending of reinforcement partially embedded in concrete requires a detail in the construction documents or the licensed design professional's permission. Verify the bend location and inside diameter and check any signs of damage to the bar, coating or adjacent concrete. A bent dowel's observed offset, grade, size and location help the designer evaluate its disposition.

Where ACI 301-20 applies and field bending or straightening of partially embedded bars is permitted, Section 3.3.2.8 gives default methods when the documents do not specify one. It permits cold bending No. 3 through No. 5 one time if the bar is above 32°F. Other sizes use the specified preheating procedure. This includes at least five bar diameters of heat application on each side of the bend center, no heating below the concrete surface, no more than 500°F at the concrete interface, and preheat of 1100–1200°F maintained through bending. Measure as specified, and do not artificially cool until the bar is below 600°F. Do not heat ASTM A1035 bars under this procedure. Protect any epoxy coating during permitted heating and repair damaged coating after authorized work. Under Section 3.3.2.8(c), check the minimum inside bend diameter in its table and verify that the beginning of the bend is at least the specified bend diameter from the concrete surface.