Long-Term Behavior of Prestressed LVL Members. II: Analytical Approach

Abstract

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.