Long-Term Behavior of Posttensioned Wood Howe Trusses

摘要

Two full-scale Howe trusses were instrumented and posttensioned to study their time-dependent behavior in outdoor conditions. Force, relative humidity, and temperature data were acquired for more than 2 years. In addition, a single 8 x 8 (nominal size in inches) Douglas fir member was also instrumented, posttensioned and data were acquired for more than 5 years. The data indicated that a permanent prestress state can be achieved in Howe trusses posttensioned using current technologies. Wood viscosity caused some prestress losses, which occurred primarily in the first few months for the Douglas fir members. Linear viscoelastic analyses reliably predicted observed behavior. Because wood and steel have different coefficients of thermal expansion, there were daily and seasonal prestress force fluctuations, which can be predicted using linear elastic structural models. The data showed that extended periods of very high relative humidity did not significantly change member forces. Nevertheless, Fickian diffusion analyses were performed to predict wood moisture content within a cross-section under atmospheric equilibrium moisture content boundary conditions. (C) 2020 American Society of Civil Engineers.