A New Simple Method for Post-tension Strands Friction Coefficient Measurement

Abstract

This article describes a new simple method of measuring the friction coefficients and their values from construction sites in Thailand. The friction coefficients between prestressing strands and their conduits are related to both wobble and curvature. The method requires both measured jacking force and elongation of each strand so the average tensile force along the entire length can be determined from non-linear stress-strain relationship. Then, the total length and total curvature of each strand were calculated from shop drawing to simulate the friction calculation in the design process. The determined friction coefficients will include the effect due to misplace of the conduit layouts from shop drawings. From the experimental measurement of 624 bonded system strands with spiral galvanized metal sheet conduits in post-tensioned slabs structures from two construction sites with two different post-tensioned sub-contractors, the wobble coefficient is 0.0035 per foot and the curvature coefficient is 0.536 per radian for sub-contractor 1 and the wobble coefficient is 0.0040 per foot and the curvature coefficient is 0.578 per radian for sub-contractor 2. The friction coefficients significantly differ than those recommended by ACI318-11 and BS8110-1997 standards.