Strongback Beam for 7-Wire Unbonded Monostrand Evaluation


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Title: Strongback Beam for 7-Wire Unbonded Monostrand Evaluation

Author(s): C. MacDougall and F. M. Bartlett

Publication: Special Publication

Volume: 211


Appears on pages(s): 47-66

Keywords: friction; post-tensioned concrete; remaining prestress force; unbonded monostrand tendons; wire breakage; wire strains

Date: 2/1/2003

A unique large-scale testing apparatus has been designed and built to facilitate the stressing and testing of 18.3 m lengths of steel prestressing strand. The strands are tested in a draped configuration to simulate the load and boundary conditions typical of an unbonded monostrand tendon in a post-tensioned concrete slab. Individual wires can be cut to simulate the occurrence of wire breaks due to corrosion. The apparatus consists of a steel wide flange beam with a parabolic profile plate that aligns the tendon in a draped configuration while providing access along the entire length for strain gauges. The overall length was selected, using mechanical models presented elsewhere, so that complete strain recovery could be observed in a wire with a break at the tendon mid-length. Other geometric variables were set to mimic the typical drape-to-span ratio of an unbonded monostrand tendon in a post-tensioned slab. Test results for intact 7-wire monostrand tendons correspond well to those predicted using the mathematical model of Machida and Durelli. Standard errors of 220 N and 133 microstrain were determined for the tendon load and wire strains, respectively. Thus, the apparatus can be used to accurately measure the mechanical response of unbonded 7-wire tendons.