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Journal of Structural Engineering / Volume 137 / Issue 12 / TECHNICAL PAPERS

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Long-Term Behavior of Prestressed LVL Members. I: Experimental Tests

J. Struct. Eng. 137, 1553 (2011); http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000405 (9 pages)

Matthew Davies1 and Massimo Fragiacomo2

1Design Engineer, Dunning Thornton Consultants Ltd, PO Box 27-153, Wellington, New Zealand. E-mail: matt.davies@dunningthornton.co.nz
2Associate Professor, Dept. of Architecture, Design and Urban Planning, Faculty of Architecture, Univ. of Sassari, Palazzo del Pou Salit, Piazza Duomo 6, 07041 Alghero, Italy (corresponding author). E-mail: fragiacomo@uniss.it

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(Submitted 17 November 2009; accepted 17 February 2011; posted ahead of print 19 February 2011)

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.

© 2011 American Society of Civil Engineers

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.

Article Outline

  1. Introduction
  2. Relevant Literature Review
  3. Experimental Program
    1. Long-Term Tests in Variable Environment
    2. Long-Term Tests in Controlled Environment
  4. Experimental Results
    1. Relaxation of Prestressing Tendon
    2. Losses in Frames—Controlled Environment
    3. Losses in Frames—Uncontrolled Environment
    4. Flexural Creep—Uncontrolled Environment
  5. Influential Factors to Losses in Prestressing Load
    1. Influence of Specimen Length
    2. Influence of Tendon Relaxation and Climate-Induced Strains
    3. Influence of Pure Creep
    4. Influence of Mechano-Sorption and Climate-Induced Strains
    5. Summary of Influential Factors
  6. Discussion
  7. Conclusions

EDITORIALLY RELATED

  1. Long-Term Behavior of Prestressed LVL Members. II: Analytical Approach
    Massimo Fragiacomo et al.
    J. Struct. Eng. 137, 1562 (2011)JSENDH000137000012001562000001

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ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000405

PUBLICATION DATA

ISSN

0733-9445 (print)  
1943-541X (online)

Publisher

American Society of Civil Engineers

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