ROCK ANCHORS - STATE OF THE ART. PART 1: DESIGN

This article (section 2 of part 1 of a 3 part review) is concerned with the bond between the cement grout and the steel tendon and with the tendon itself. It is primarily a review of extensive published work and numerous references are given. The mechanisms of the bond are first discussed; they involve adhesion, friction and mechanical interlock. Notes are given on transmission lengths of wires, strands and bars. Comprehensive tables summarize grout/wire, grout/strand and grout/bar bond values that have been employed or recommended in practice. The author comments that they show greater uniformity for strand/grout values than for the others, that the actual safety factor is usually in excess of two and that the average bond developed by bars is significantly higher than with wires or strands. He discusses the distribution of the bond along the length of the tendon and refers to the effect of rust. He then discusses the rival merits of bars, strands and wires. British standards and code of practice are discussed and tables summarize details of British prestressing wire and strand. Comprehensive tables summarize allowable stresses and safety factors which have been recommended, and also design stresses and safety factors that have been employed. Reference is made to tendon spacers. The author points out that few load/extension tests have been carried out with long tendons and there is also need for study of the behaviour of tendon curvature and the splaying of multi-component tendons near the top anchor head. In view of the inconsistencies between theory and practice, he concludes that there should be more attention paid to uplift capacity and to the possibility of variation in bond stress. /TRRL/

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