Long-Term Behavior of Prestressed LVL Members. I: Experimental Tests
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Volume 137, Issue 12
Abstract
This paper presents the results of experimental tests undertaken to resolve questions regarding the viability of multistory prestressed timber structures, specifically the reduction in the prestress load over time. Reduced- and large-scale specimens were tested for 1 year in controlled and uncontrolled heated indoor conditions. Specimens included beams and two-bay frames made from radiata pine laminated veneer lumber (LVL) with a box-shaped cross section. The prestress load was applied through the center of the section by using an unbonded tendon. All relevant quantities such as load in the prestress tendon, deflection, timber moisture content, environmental temperature, and relative humidity were monitored throughout the test. Functions were fitted to the one-year experimental curves and then used to indicate more clearly the trends of the prestress losses during the testing. For a beam in which all timber is loaded parallel to the grain, a reduction in prestress of 1.4% was found after 1 year, whereas for a frame in which 11% of its length is loaded perpendicular to the grain, the loss increased to 7% because of the presence of columns. Furthermore, an attempt was made to separate the contributions made to prestress losses by key factors, namely creep and mechano-sorption of the LVL that is parallel and perpendicular to the grain; relaxation of the prestressing strand; and shrinkage/swelling on account of environmental variations. The most important factor is the proportion of timber that is loaded perpendicular to grain compared with the timber loaded parallel to grain.
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Acknowledgments
The technical support and financial contribution provided by Carter Holt Harvey is gratefully acknowledged, with a special thanks to Mr. Hank Bier for the personal interest demonstrated during the whole project. Particular thanks are also owed to John Maley and the staff of the University of Canterbury’s Department of Civil Engineering laboratory.
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Published In
Journal of Structural Engineering
Volume 137 • Issue 12 • December 2011
Pages: 1553 - 1561
Copyright
© 2011 American Society of Civil Engineers.
History
Received: Nov 17, 2009
Accepted: Feb 17, 2011
Published online: Feb 19, 2011
Published in print: Dec 1, 2011
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