Anchorage Strength of Conventional and High-Strength Hooked Bars in Concrete

Abstract

Key factors affecting the anchorage strength of hooked bars are investigated and design guidelines for the development length of hooked bars that apply to both conventional and highstrength steel and concrete are presented. In this study, 337 beam column joint specimens were tested. Parameters included number of hooks (2, 3, or 4), concrete compressive strength (4,300 to 16,510 psi [30 to 114 MPa]), bar stress at failure (22,800 to 141,600 psi [157 to 976 MPa]), bar diameter (No. 5, 8, and 11 [No. 19, 25, and 36]), concrete side cover (1.5 to 4 in. [38 to 102 mm]), quantity of confining reinforcement in the joint region, hooked bar spacing (3 to 11 bar diameters measured center-to-center), hook bend angle (90° or 180°), placement of the hook (inside or outside the column core, and inside or outside of the column compressive region), and embedment length. Using a subset of 214 simulated exterior beam-column joints, expressions are developed to characterize the anchorage capacity of hooked bars as a function of embedment length, concrete compressive strength, bar diameter, and amount and orientation of confining reinforcement. The results of this study show that front failure plays an important role in the behavior of hooked bars, which contrasts with the findings of previous studies. The provisions in the 2014 ACI Building Code become less conservative as the concrete compressive strength and bar diameter increase. The contribution of concrete compressive strength to the anchorage capacity of hooked bars can be represented by the concrete compressive strength to the 0.29 power, in contrast to the 0.5 power currently used in the ACI 318-14 Code. Confining reinforcement, expressed as the area of confining reinforcement per confined hooked bar, provides in an incremental rather than percentage increase in the anchorage capacity of hooked bars. Confining reinforcement parallel to the straight portion of the hooked bars contributes to the anchorage capacity of both 90° and 180° hooked bars. The contribution of confining reinforcement oriented perpendicular to the straight portion of the hooked bar differs from that of confining reinforcement parallel to the straight portion of the hooked bar and may be similar to the contribution of confining reinforcement to the development and splice strength of straight bars. Hooked bars with 90° and 180° bend angles produce similar anchorage capacities and can be used interchangeably. Increasing concrete side cover from 2.5 to 3.5 in. (64 to 89 mm) does not increase the anchorage capacity of hooked bars. These observations are incorporated into a new design equation that allows for the conservative design of hooked bars at concrete strengths up to 16,000 psi and steel stresses up to 120 ksi, well above current Code limits.