Development of Large High-Strength Reinforcing Bars with Standard Hooks and Heads

Abstract

Hooked and headed reinforcing bars are viable alternatives for development of reinforcing steel when member geometry does not allow for a straight deformed bar to fully develop its yield strength. Current design provisions in ACI 318-19 Building Code impose limitations on the use of hooked and headed bars larger than No. 11 (that is, No. 14 and No. 18 bars), mainly due to a lack of experimental data. This research continues a comprehensive study of the anchorage and development of high-strength hooked and headed bars to expand the available data to include No. 14 and No. 18 bars. Forty-two large-scale simulated beam-column joint specimens containing No. 11, No. 14 and No. 18 hooked and headed bars are tested. Of the 42 specimens, 12 contain hooked bars and 30 contain headed bars. The effects of bar size, bar spacing, bar location, embedment length, confining transverse reinforcement in the joint region, placement of bars within the cross-section, concrete compressive strength, compression strut angle, and effective beam depth on anchorage strength are investigated. Two loading conditions are used. In loading condition A, the joint shear is 80% of the total applied force to the bars, simulating the forces in an exterior beam-column joint with the beam located at the midheight of the column. The joint shear is reduced to ⁓69% of the total applied force in loading condition B. All hooked bar specimens and 15 headed bar specimens are tested under loading condition A, while the other 15 headed bar specimens are tested using loading condition B. Concrete compressive strengths range from 6,390 to 15,770 psi for hooked bars and from 5,310 to 16,210 psi for headed bars. Bar stress at failure ranges from 87,300 to 130,600 psi for hooked bars and from 54,900 to 148,300 psi for headed bars. Center-to-center bar spacing, s, ranges from 3.5db to 10.6db for hooked bars and from 2.7db to 10.6db for headed bars, where db is the nominal hooked or headed bar diameter. Confining reinforcement, Ath, or parallel ties, Att, in the joint region ranges from 0 to 0.465Ahs and 0 to 0.827Ahs for hooked and headed bars, respectively, where Ath or Att equal the total cross-sectional area of tie legs within 10db from the top of the bars and Ahs is the total area of the bars being developed. Headed bars with net bearing areas between 4.2 and 4.4 times the bar area are used.

The test results are compared with the current provisions for the development length of hooked and headed bars in Chapter 25 of ACI 318-19. Descriptive equations to characterize anchorage strength of hooked and headed bars developed previously for No. 11 and smaller bars are evaluated. New descriptive equations are developed to represent the anchorage strength for bars as large as No. 18. The equations are compared with the test results in the current study and available in the literature. New design provisions for development length are developed for hooked and headed bars and evaluated with respect to test results and ACI 318-19 provisions.

The descriptive equations for hooked and headed bars developed in this study accurately account for concrete compressive strength, confining reinforcement, and bar spacing. The ability of the equations to accurately represent anchorage strength is insensitive to variations in compression strut angle and effective beam depth. While the contribution of confining reinforcement to anchorage strength increases with bar size, the effect of increasing confining reinforcement for headed bars is much lower than for hooked bars and much lower for headed bars than observed in prior studies. Under loading condition A, all hooked bar specimens, even those without confining reinforcement, carried the joint shear and exhibited an anchorage failure, whereas shear-like failures were observed in some headed bar specimens under similar conditions. These observations reveal the distinct role of the tail of the hook in helping to carry the joint shear, and indicate the difference in joint shear under loading conditions A and B is a key factor in the type of failure and anchorage strength of headed bars. Larger headed bars need confining reinforcement on the order of 0.5Ahs to carry the joint shear demand under loading condition A.

The development length provisions in ACI 318-19 are unnecessarily conservative for No. 14 and No. 18 hooked and headed bars. For both hooked and headed bars, providing confining reinforcement below the minimum amounts required by ACI 318-19 contributes to anchorage strength. Similar to No. 11 and smaller hooked and headed bars, the effect on anchorage strength of concrete compressive strength is best represented by the 0.25 power for design. The bar location factor o of 1.25 in ACI 318-19, applied to bars terminating inside column longitudinal reinforcement (column core) with side cover < 2.5 in. or bars with side cover < 6db, can be safely reduced to 1.15 for design. The proposed design equations for hooked and headed bars are applicable to concrete with compressive strengths up to 16,000 psi, steel with yield strengths up to 120,000 psi, and bars as large as No. 18. The proposed modification factors for confining reinforcement (expressed as Ath/Ahs or Att/Ahs) and bar spacing (expressed as s/db), in the form of a single expression or simplified expressions that address the effects of confining reinforcement and bar spacing independently, provide more flexibility for designers to take advantage of a range of values for Ath/Ahs or Att/Ahs and s/db and, ultimately, permit the use of shorter development lengths than the provisions in ACI 318-19 for all bar sizes.