Long-Term Behavior of Posttensioned Timber Connections

Abstract

Tests regarding the long-term behavior of posttensioned timber connections have been performed at the ETH in Zurich. The aim of these tests is to estimate the loss in tendon force that can be expected during the lifetime of a structure. Several posttensioned beam and beam-column timber specimens exposed to controlled and uncontrolled environmental conditions and loaded only with the axial prestressing force have been monitored for more than one year. The force in the tendon was measured together with the strains in the specimens and temperature and relative humidity in the testing environment. Creep tests performed on small glulam timber blocks loaded in compression have been started as well. The estimated creep coefficients parallel and perpendicular to the grain are used to calculate the losses with an analytical model for the larger posttensioned specimens. A prestressing loss in the range of 5–10%, depending on the geometrical properties and environmental conditions, has been found at the end of the one-year testing. Such a percentage is likely to increase up to 30% at the end of a 50-year service. Careful consideration has been given to identify the different variables affecting the losses of prestressing force in the long-term, including the environmental conditions. The tendon force losses were compared to the results obtained by a simple analytical model. A parametric study based on the model was performed in order to check whether the suggested ash reinforcement in the column is beneficial regarding tendon force losses in the long-term.