Anchorage of High-Strength Reinforcing Bars with Standard Hooks

Abstract

Hooked bars are used to anchor reinforcing steel where member dimensions prevent straight bars from developing their full yield strength. Prior to the current study, the quantity of data has been limited with regards to the capacity of hooked bars–particularly when high-strength steel or concrete is used. As a result, current design provisions in ACI 318-14 limit yield strength and concrete compressive strength to 80,000 psi and 10,000 psi, respectively, for the purpose of determining the development length of hooked bars. The purpose of this study was to determine the critical factors that affect the anchorage strength of hooked bars in concrete and to develop new design guidelines for development length allowing for the use of high-strength reinforcing steel and concrete. In this study, a total of 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), bar diameter (No. 5, No. 8, and No. 11), concrete side cover (1.5 to 4 in.), amount of transverse reinforcement in the joint region, hooked bar spacing (3db to 11db 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. The results of this study show that current ACI 318-14 code provisions are unconservative for larger hooked bars and higher compressive strength concrete. The effect of concrete compressive strength on the anchorage capacity of hooked bars is less than represented by the 0.5 power currently used in ACI provisions; the 0.25 power provides a more realistic estimate of capacity. The addition of confining transverse reinforcement in the hook region increases the anchorage capacity of hooked bars–the value of the increase depends on the quantity of confining reinforcement per hooked bar. Hooked bars with 90° and 180° bend angles exhibit similar capacities, and no increase in capacity was observed when increasing side cover from 2.5 to 3.5 in. Anchoring a hooked bar outside the column core or outside the compressive region of a column provides less capacity than anchoring the hooks at the far side of a beam-column joint or in a wall with a high side cover. Hooked bars also exhibit a reduction in capacity if the center-to-center spacing is less than seven bar diameters. These observations are used to develop a new design equation that allows for the conservative design of hooked bars.