Use of Headed Bars as Shear Reinforcement

Abstract

Thirty-nine beams with a shear span-to-depth ratio of 3 were tested with the goal of determining whether headed deformed bars can be used in reinforced concrete members in place of stirrups as shear reinforcement as well as whether shear reinforcement with yield strengths up to 80 ksi [550 MPa] may be used without problems related to strength or serviceability. Grade 60 and Grade 80 [Grade 420 and Grade 550] No. 3, No. 4, and No. 6 [No. 10, No. 13, and No. 16] headed bars and stirrups were used as transverse reinforcement, and were spaced between onequarter and one-half of the member effective depth. The shear strength of members reinforced with U stirrups and crossties was compared with the strength of matching specimens reinforced with headed bars as shear reinforcement. Stirrups were anchored around longitudinal bars, as required by ACI 318-14. Headed bars were anchored using one of three details: (1) engaged with longitudinal bars, that is, with the bearing face of the head in contact with a longitudinal bar; (2) not engaged with longitudinal reinforcement, with the headed bar outside of the longitudinal reinforcement and close to the side of the member; and (3) not engaged with longitudinal reinforcement, with the headed bar inside of the longitudinal reinforcement and at least 4 in. from the side of the member. Member depths ranged from 12 to 48 in. [310 to 1220 mm] with widths of 24 and 42 in. [620 and 1070 mm]. Test specimens were designed to represent beams, walls, and mat foundations. Modifications to the ACI 318-14 Code are proposed, which will in turn impact the design of nuclear power plants worldwide through changes in ACI 349-13 and ACI 359-13. The results show that members with adequately anchored headed deformed bars have shear strengths that are equivalent to members with stirrups. Adequate anchorage of headed bars is provided by (1) direct contact between the bearing face of the head with longitudinal reinforcement or (2) placing the headed bars inside at least one longitudinal bar and providing side concrete cover to the headed bar of at least five headed bar diameters. Placing headed bars outside of longitudinal reinforcement and close to the side of a member may result in reduced shear strength. Grade 80 [Grade 550] shear reinforcement provides the same strength and similar serviceability as Grade 60 [Grade 420] shear reinforcement. Shear crack widths increase with beam depth.