Strength and Detailing of Beams with High-Strength Headed and Hooked Stirrups

Thirty-nine large-scale reinforced concrete beams were tested under monotonic three-point bending to investigate the use of stirrups with mechanical anchors (heads) or hooks and Grade 80 (550) reinforcing steel. Grade 60 and 80 (420 and 550) No. 3, No. 4, and No. 6 (0.375, 0.5, and 0.75 in. [10, 13, and 19 mm]) bars were used as stirrups, which were spaced at one-quarter to one-half of the member effective depth. Other variables included beam depth (12 to 48 in. [310 to 1220 mm]), beam width (24 and 42 in. [620 and 1070 mm]), longitudinal reinforcement strain corresponding to the nominal beam shear strength (nominally 0.0011, 0.0017, or 0.018), and concrete compressive strength (4000 and 10,000 psi [28 and 69 MPa]). Headed stirrups that: a) engage (are in contact with) the longitudinal bars; or b) have a side cover of at least six headed bar diameters and at least one longitudinal bar within the side cover, produce equivalent shear strengths as hooked stirrups, and both details allow stirrups to yield. The results affirm that beams designed for the same Vn with either Grade 60 or 80 (420 or 550) stirrups exhibit equivalent shear strengths. A nominal shear strength based on a concrete contribution equal to 2√ fcbwd may be unconservative when ρ1fytm < 85 psi (0.59 MPa) in members with a/d = 3, h ≥ 36 in. (910 mm), ρ < 1.5%, and no skin reinforcement.