Anchorage and lap capacity of square twisted reinforcement for assessment of existing structures

摘要

Although square twisted section reinforcement is no longer used in new construction, there remain many older structures still in service that were built with such bars. The increasing need for assessment of existing construction means there remains a continuing need for information on performance over older types of reinforcement. Research into plain surface bars essentially ceased when ribbed bars became the norm, and plain bars have consequently been bypassed in developments in understanding of bond since 1960. The basis through which margins of safety are introduced for design has also changed as formal methods for analysis of reliability have developed. An analysis of test data on bond of square twisted bars, also termed Type 1 deformed or Ransome type bars is presented, and a new semi-empirical formulation for anchorage capacity of straight square twisted bars in R.C. structures consistent with the reliability-based approach included in the forthcoming second generation of Eurocodes is derived. The expression parallels that for ribbed bars in the fib Model Code 2010. A comparison with provisions of some historical international design codes suggests that in some cases the margin of safety against failure of straight plain bar anchorages may have been lower than would be required by current norms. Implications for practice are briefly discussed.