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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. II: Analytical Approach

J. Struct. Eng. 137, 1562 (2011); http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000410 (11 pages)

Massimo Fragiacomo1 and Matthew Davies2

1Associate 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
2Design Engineer, Dunning Thornton Consultants Ltd, PO Box 27-153, Wellington, New Zealand. E-mail: matt.davies@dunningthornton.co.nz

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(Submitted 17 November 2009; accepted 24 March 2011; posted ahead of print 26 March 2011)

This paper investigates the long-term behavior of beams and frames made from laminated veneer lumber (LVL) prestressed with unbonded tendons. A formula for the evaluation of the prestress losses during the service life was derived from the integral equations of wood and tendons using the age-adjusted effective modulus method. All phenomena affecting the time-dependent behavior of the system, such as creep, mechano-sorption, thermal and moisture strains of the wood, and relaxation and thermal strains of the steel tendon, were considered. The formula was validated on experimental results showing the possibility to adequately predict the losses, although with some approximation. The formula was simplified further by removing some parameters that have little or almost no effect, and rewritten in a nondimensional format, leading to an easy-to-use closed-form solution suitable for implementation in codes of practice. The most important parameters governing the prestress losses were the proportion of the timber member’s length loaded perpendicular to the grain, and the ratio between the axial stiffnesses of the tendon and timber frame. An increase in these parameters significantly raises the amount of prestress loss. Possible methods for the mitigation of losses in a frame are finally presented and briefly discussed.

© 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 showed during the whole project.

Article Outline

  1. Introduction
  2. Analytical Model
    1. Elastic Solution
    2. Time-Dependent Solution
      1. Time-Dependent Behavior of Timber
      2. Time-Dependent Behavior of Prestressing Steel
      3. Congruence Equation
      4. Age-Adjusted Effective Modulus Method
      5. Derivation of Final Solution
  3. Model Validation and Parametric Study
    1. Calibration of the Toratti Rheological Model for LVL
    2. Experimental-Analytical Comparisons
    3. Influence of the Proportion of LVL Loaded Perpendicular to the Grain
  4. Discussion
    1. Accuracy of the Analytical Solution
    2. Simplification of the Analytical Solution
    3. Determination of the Governing Nondimensional Parameters
    4. Mitigation of Prestress Losses
  5. Conclusions

EDITORIALLY RELATED

  1. Long-Term Behavior of Prestressed LVL Members. I: Experimental Tests
    Matthew Davies et al.
    J. Struct. Eng. 137, 1553 (2011)JSENDH000137000012001553000001

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ARTICLE DATA

Digital Object Identifier
http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000410

PUBLICATION DATA

ISSN

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

Publisher

American Society of Civil Engineers

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