Time-dependent Behavior of Prestressed Wood Howe Bridges

William Howe’s 1840 patent truss was intended to be posttensioned by tightening nuts on iron rods. Since the wood diagonals simply bear on node blocks, the two diagonals in a panel are both functional only if precompressed. To properly rehabilitate Howe bridges and to guide new designs, it is important to understand the prestress forces that are induced initially and how they change with time. Results of linear elastic analyses for the actions of tensioning vertical rods and temperature changes are reported. Tensioning vertical rods at one location induces forces primarily in the two adjacent panels and causes very small upward vertical displacements. An increase in temperature decreases prestress forces; therefore, prestressing should not be done at low temperatures. A linear viscoelastic analysis is formulated using a standard-linear-solid constitutive model. The formulation is used to predict the time-dependent behavior of the Pine Bluff Bridge in Indiana. Time histories of forces and displacements are computed. Displacements increase and prestress forces decrease over time because of the viscous behavior of wood. Prestress losses decrease as the relative axial stiffness of the metal rods decreases.