Conventional and High-Strength Hooked Bars—Part 1: Anchorage Tests

Abstract

Keywords: anchorage; beam-column joints; bond and development; highstrength concrete; high-strength steel; hooks; reinforced concrete; reinforcement. INTRODUCTIONProvisions for calculating the development length of hooked bars in U.S. design codes, such as the ACI 318-14, "Building Code Requirements for Structural Concrete"; ACI 349-06, "Code Requirements for Nuclear Safety-Related Concrete Structures"; and the "AASHTO LRFD Bridge Design Specifications" (AASHTO 2012) are based primarily on studies performed in the 1970s by Minor and Jirsa (1975), Marques and Jirsa (1975), and Pinc et al. (1977). These studies included only a small number of specimens containing standard hooks and a limited range of material properties (Grade 60 [420] reinforcing steel with yield strengths of 64 and 68 ksi [441 and 469 MPa] and concrete compressive strengths between 3750 and 5400 psi [26 and 37 MPa]). Neither high-strength steel bars nor high-strength concrete, now commonly available in construction practice, were included in these studies.The main objective of this paper is to present the results of a study of key parameters affecting the anchorage strength of standard hooked bars with a much wider range of material properties. For the purpose of this study, standard hooked bars are defined according to the provisions in Section 25.3 of ACI 318-14. Due to the magnitude of the study, the results are presented in a series of papers. The specific objectives of this paper are to describe the experimental program, provide detailed information about the observed mode of failure of the specimens, and present an evaluation of the experimental results in the context of the development length provisions for hooked bars in ACI 318-14. A second paper will present a statistical analysis of the test results and formulate equations to characterize hooked bar anchorage strength for normal and high-strength materials. Subsequent papers will evaluate specific parameters affecting hooked bar anchorage strength and develop code change proposals. RESEARCH SIGNIFICANCEThe use of high-strength steel and concrete is becoming more common in the construction industry due to benefits such as lower congestion, smaller member dimensions, and increased useable floor area. Current provisions in ACI 318-14 for hooked bar anchorage are based on limited test results that include a single grade of reinforcement and a narrow range of concrete compressive strengths. An experimental program with an expanded range of material properties was necessary to develop a better understanding of the main parameters that affect anchorage strength and to formulate code provisions applicable to the full range of material strengths available in present-day reinforced concrete construction. EXPERIMENTAL PROGRAMA total of 337 beam-column joint specimens-276 with two hooked bars and 61 with three or more hooked barswere tested to investigate the anchorage strength of hooked bars (Searle et al. 2014; Sperry et al. 2015a,b). The parameters of the study were bar size, bar stress a...

Anchorage Strength of Closely-Spaced Hooked Bars

“…This analysis shows that the bar stress predicted by Eq. (2) becomes progressively less conservative as the concrete compressive strength and bar size increase, matching the observations by Sperry et al (2017a).…”

“…Ajaam et al (2017) measured strain along the straight portion of the hooked bars and on hoops serving as the confining reinforcement within the joint region. The results showed that hoops oriented parallel to the embedment length of hooked bars and located within eight to 10 bar diameters of the straight portion of the hooked bar within the joint region exhibited increases in strain at earlier loading stages than those located further away from the hooked bars and yielded prior to hooked bar anchorage failure, confirming the previous findings by Sperry et al (2015bSperry et al ( , 2017b.…”

“…Sperry et al observed that the effect of concrete compressive strength on the anchorage strength of hooked bars is proportional to the compressive strength raised to the 0.29 power, rather than the square root of compressive strength currently used in Code provisions. Sperry et al (2015bSperry et al ( , 2018 also found that for 180-degree hooked bars, confining reinforcement in the form of closed hoops increases anchorage strength regardless of orientation (parallel or perpendicular to the embedment length), while for 90-degree hooked bars, confining reinforcement oriented parallel to the embedment length increases anchorage strength more than confining reinforcement oriented perpendicular to the embedment length. Ajaam et al (2017) measured strain along the straight portion of the hooked bars and on hoops serving as the confining reinforcement within the joint region.…”

“…Sperry et al (2015bSperry et al ( , 2017aSperry et al ( ,b, 2018 evaluated tests of 245 simulated beam-column joint specimens with two hooked bars fabricated using normalweight concrete with compressive strengths ranging from 2570 to 16,500 psi (17.7 to 114 MPa). Bar stresses at failure ranged from 30,800 to 143,900 psi (212 to 992 MPa).…”

“…Bar stresses at failure ranged from 30,800 to 143,900 psi (212 to 992 MPa). Sperry et al (2015bSperry et al ( , 2017a observed that for specimens containing two widely spaced hooked bars, anchorage strengths calculated based on the provisions of ACI 318-14 overestimate anchorage strengths for larger hooked bars and overestimate the effects of concrete compressive strength and confining reinforcement. Sperry et al observed that the effect of concrete compressive strength on the anchorage strength of hooked bars is proportional to the compressive strength raised to the 0.29 power, rather than the square root of compressive strength currently used in Code provisions.…”