Time-Dependent Behavior of Posttensioned Wood Howe Bridges
Publication: Journal of Structural Engineering
Volume 132, Issue 3
Abstract
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.
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Acknowledgments
The Federal Highway Administration supported the Historic American Engineering Record studies of the Pine Bluff Bridge. The wood species used for the prestressing experiments were identified by Ronald W. Anthony, of Fort Collins, Colo. The actual timbers used for the experiments were lent to Case Western Reserve University by Carl Steepleton of Alliance, Ohio. Neil Harnar, Department Engineer, instrumented and monitored the long-term prestressing tests.
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Information & Authors
Information
Published In
Journal of Structural Engineering
Volume 132 • Issue 3 • March 2006
Pages: 418 - 429
Copyright
© 2006 ASCE.
History
Received: Jun 28, 2004
Accepted: Jul 19, 2005
Published online: Mar 1, 2006
Published in print: Mar 2006
Notes
Note. Associate Editor: J. Daniel Dolan
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