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Long-term behavior of prestressed LVL members / Matthew Davies in Journal of structural engineering, Vol. 137 N° 12 (Décembre 2011) 

Public ISBD [article]  in Journal of structural engineering > Vol. 137 N° 12 (Décembre 2011) . - pp. 1553-1561 Titre : Long-term behavior of prestressed LVL members : I: Experimental tests Type de document : texte imprimé Auteurs : Matthew Davies, Auteur ; Massimo Fragiacomo, Auteur Année de publication : 2012 Article en page(s) : pp. 1553-1561 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Creep  Frames  Prestressing  Stress  relaxation  Timber  construction  Time  dependence  Laminated  wood Résumé : 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. DEWEY : 624.17 ISSN : 0733-9445 En ligne : http://ascelibrary.org/sto/resource/1/jsendh/v137/i12/p1553_s1?isAuthorized=no [article] Long-term behavior of prestressed LVL members : I: Experimental tests [texte imprimé] / Matthew Davies, Auteur ; Massimo Fragiacomo, Auteur . - 2012 . - pp. 1553-1561. Génie Civil Langues : Anglais (eng) in Journal of structural engineering > Vol. 137 N° 12 (Décembre 2011) . - pp. 1553-1561 Mots-clés : Creep  Frames  Prestressing  Stress  relaxation  Timber  construction  Time  dependence  Laminated  wood Résumé : 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. DEWEY : 624.17 ISSN : 0733-9445 En ligne : http://ascelibrary.org/sto/resource/1/jsendh/v137/i12/p1553_s1?isAuthorized=no

Long-term behavior of prestressed LVL members / Massimo Fragiacomo in Journal of structural engineering, Vol. 137 N° 12 (Décembre 2011) 

Public ISBD [article]  in Journal of structural engineering > Vol. 137 N° 12 (Décembre 2011) . - pp. 1562-1572 Titre : Long-term behavior of prestressed LVL members : II: Analytical approach Type de document : texte imprimé Auteurs : Massimo Fragiacomo, Auteur ; Matthew Davies, Auteur Année de publication : 2012 Article en page(s) : pp. 1562-1572 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Creep  Frames  Losses  Prestressing  Stress  relaxation  Timber  construction  Time  dependence  Laminated  wood Résumé : 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. DEWEY : 624.17 ISSN : 0733-9445 En ligne : http://ascelibrary.org/sto/resource/1/jsendh/v137/i12/p1562_s1?isAuthorized=no [article] Long-term behavior of prestressed LVL members : II: Analytical approach [texte imprimé] / Massimo Fragiacomo, Auteur ; Matthew Davies, Auteur . - 2012 . - pp. 1562-1572. Génie Civil Langues : Anglais (eng) in Journal of structural engineering > Vol. 137 N° 12 (Décembre 2011) . - pp. 1562-1572 Mots-clés : Creep  Frames  Losses  Prestressing  Stress  relaxation  Timber  construction  Time  dependence  Laminated  wood Résumé : 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. DEWEY : 624.17 ISSN : 0733-9445 En ligne : http://ascelibrary.org/sto/resource/1/jsendh/v137/i12/p1562_s1?isAuthorized=no