Anchorage Strength of Standard Hooked Bars in Simulated Exterior Beam-Column Joints

Abstract

The current ACI hooked bar design provisions are based on test results of 38 simulated beam-column joints containing two hooked bars. The provisions address the effects of hooked bar surface condition, concrete cover, amount of confining reinforcement confining the hooks, and type of concrete (normalweight or lightweight). This study uses results of 338 simulated beamcolumn joint specimen tests at the University of Kansas, including two, three, or four No. 5, 8, or 11 (No. 16, 25, or 36) hooked bars with 90° or 180° hooks, along with 61 tests by others to investigate the effects of hooked bar spacing, anchoring the hooked bars outside the column core or halfway through the column depth, concrete tail cover to 90° hooks, and the effect of tail kickout at failure on hooked bar anchorage strength. In the tests performed at the University of Kansas, the center-to-center spacing between hooked bars ranged from 3 to 12 bar diameters, hooked bars were placed inside or outside column core, and hooked bars were extended to the far side of the column core or extended halfway through the column depth. Hooked bars had nominal embedment lengths ranging from 2.5 to 25.2 in. (64 to 640 mm), nominal concrete side cover ranging from 1.5 to 4 in. (38 to 100 mm) in simulated beam-column joints and 11.3 to 24.6 in. (287 to 625 mm) in walls, and nominal concrete tail cover to the hook ranging from 2 to 18 in. (50 to 460 mm). Concrete compressive strength ranged from 4,300 to 16,510 psi (30 to 114 MPa) in simulated beam-column joints and 2,400 to 5,450 psi (17 to 38 MPa) in walls, and bar stresses at anchorage failure ranged from 27,100 to 141,000 psi (187 to 972 MPa) in simulated beam-column joints and 14,200 to 60,000 psi (98 to 420 MPa) in walls. The results show that the center-to-center spacing between hooked bars plays a role in anchorage strength up to a spacing of seven bar diameters. The closer the bars, the lower the anchorage strength per bar, in contrast with the total anchorage strength, which remains constant or increases moderately as the number of hooked bars in a joint increases. The presence of confining reinforcement mitigates the effect of close spacing but does not eliminate it. Hooked bars placed outside the column core or anchored halfway through the column depth exhibit low anchorage strength when compared to hooked bars placed inside the column core or extended to the far side of the column. The reduction in anchorage strength ranges from 4 to 34%, producing an average anchorage strength equal to about 84% of the average strength of hooked bars placed inside the column core or extended to the far side of the column. For hooked bars with a 90° hook, concrete cover to the tail as low as 0.75 in. (29 mm) or tail kickout at failure do not affect the anchorage strength. The likelihood of tail kickout increases with increasing bar size and for hooks with tail cover less than 2 in. (50 mm) and no confining reinforcement. The results from the current analyses were used to modify a previously derived descriptive expression for hooked bar anchorage strength and a design expression for hooked bar development length. These modifications expand the applicability of the descriptive and design expressions to include the effects of hooked bar spacing, placing the hooked bar outside column core, and not extending the bar to the back of the column. Design provisions for ACI 318 are proposed.