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Experimental Investigation of Development Length Required for Large-Diameter Headed Reinforcing Bars in Tension

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  • Creators:
  • Corporate Creators:
    University of California, San Diego. Dept. of Structural Engineering
  • Corporate Contributors:
    California. Department of Transportation
  • Subject/TRT Terms:
  • Publication/ Report Number:
  • Resource Type:
    Tech Report
  • Geographical Coverage:
    United States
  • Edition:
    Final Report
  • Corporate Publisher:
    California. Department of Transportation
  • Abstract:
    The study presented in this report was to investigate the anchorage capacities of No. 14 and No. 18 headed bars, including the influence of the embedment length, concrete strength, head size, parallel tie reinforcement, and bar group on the anchorage strength, and evaluate the applicability of the development length requirements for No. 11 or smaller headed bars in ACI 318-19 and the 10th Edition of AASHTO LRFD Bridge Design Specifications to larger diameter bars. Thirteen bar anchorage tests were conducted. Eleven of these had single bars and two had double side-by-side bars. Heads with net bearing areas of 4Ab and 9Ab, were considered. The target compressive strengths of the concrete used were 4,000, 6,000, and 8,000 psi. The specimens had bar embedment lengths equal to 70% or 100% of the minimum according to ACI 318-19. The test results show that the anchorage failure of a headed bar is normally initiated by concrete breakout but eventually governed by bar pullout when the bearing strength of the concrete at the T head has been weakened by the opening of breakout cracks. The bar force at which concrete breakout occurs is approximately proportional to the square of the embedment length, rather than the embedment length to the power of 1.5 as specified in ACI 318 for embedded anchor bolts. After the occurrence of breakout cracks, parallel tie reinforcement can play a significant role in providing the necessary anchorage capacity. The specimens that satisfied the development length formula in ACI 318-19 performed satisfactorily. Two specimens that satisfied the AASHTO formula and had a head size of 9Ab had anchorage failures. This indicates that the AASHTO formula over-estimates the influence of the head size. The unified formula in AASHTO considers only the bond strength and the bearing resistance at the T head, and does not explicitly account for concrete breakout. It can be perceived that if anchorage failure is initiated by concrete breakout, the size of the T head will not have a significant influence on the anchorage resistance, as supported by the data in this study and other prior studies on smaller diameter bars, which have shown that the use of a head size greater than 4Ab will not necessarily improve the anchorage capacity and the benefit is small at best. Even though the ACI formula appears to be safe for the large-diameter bars, it is more conservative for No. 18 bars than No. 14 bars, probably due to the fact that the development length is proportional to the bar diameter to the power of 1.5. A rational method has been proposed in this study to determine the development lengths for headed bars and the required parallel tie reinforcement. It is based on the anchor bolt provisions in ACI 318, which have been improved to account for the influence of the bar embedment length and the contribution of the parallel tie reinforcement in an accurate and rational manner.
  • Format:
    PDF
  • Funding:
    65A1051
  • Collection(s):
  • Main Document Checksum:
    urn:sha-512:eee5ebdbbc43c0bb0022ff5c43d7e033a94286b5022cdf8f735ce7182aa2da8e844661ba368f9f5c9d32859f74be9cbaa14fca2fdefdc31256e6d99be8d7a228
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  • File Type:
    Filetype[PDF - 17.59 MB ]
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