Tension development length and lap splice design for reinforced concrete members

摘要

The key factors that govern development (anchorage) and splice performance of reinforcing bars in tension are described. The bond design provisions of the AASHTO (American Association of State Highway and Transportation Officials) LRFD (Load and Resistance Factor Design) Bridge Design Specifications, the American Concrete Institute Building Code Requirements for Structural Concrete (ACI 318-05), ACI Committee 408, CEB-FIP Model Code 1990, and Eurocode 2 are summarized and compared with test results. In terms of the key factors, the bond strength of bars not confined by transverse reinforcement is proportional to the concrete compressive strength to the 1/4 power. The contribution of transverse reinforcement to bond strength increases with the area of the transverse reinforcement per bar and the relative rib area and diameter of the bar being developed or spliced and has been found to be proportional to the concrete compressive strength to a power between 3/4 and 1.0. For deep members or for high-slump (conventional and superplasticized) concrete, the effect of bar placement may be greater than accounted for in current design provisions. Of the five design procedures compared in this paper, those developed by ACI Committee 408 provide the best match with test results for both developed and spliced bars. For developed bars, the other four design procedures result in a high percentage of tests for which the test to calculated strength ratios (T/C) are less than 1.0, with the greatest percentage of low T/C values resulting from the AASHTO requirements. The relative safety of the latter four design procedures improves for lap splices. A provision in ACI 318-05 that allows a 20% reduction in development and lap splice length for bars smaller than 19 mm appears to be unsafe and should be removed or modified.