THE EFFECT OF MOISTURE CYCLING ON CREEP OF SMALL GLUED LAMINATED BEAMS

Creep information for sawn lumber exposed to constant and cyclic humid ity environments previously published by the authors is supplemented b y this study using glued laminated material of about the same size, te sted on the same apparatus, and using the same general procedure. For both sawn and glued laminated material, the relative creep was measure d at about 55% of their average ultimate strength. Relative creep was measured for specimens matched with respect to modulus of elasticity ( MOE) and exposed to a constant relative humidity (65 F and 65% RH) for about 1,000 hours, and a cyclic relative humidity (65 F and 90% RH fo r 82 hours followed by 65 F and 40% RH for 82 hours). This 164-hour cy cle was repeated six times for about 1,000 hours of cyclic exposure. T his is a more severe change in equilibrium moisture content of the env ironment than is likely to occur in building structures. The result sh owed relative creep of the glued laminated material increased by 40% t o 72% as a result of cycling. One very low MOE pair showed an increase of 167%. The increase for solid sawn lumber previously reported was 2 00% to 400% due to cycling. Results also showed that the ratio of rela tive creep for cycled to constant humidity exposure did not change app reciably as time under load was increased. These differences are attri buted to permeability differences between glued laminated wood and saw n lumber. The gluelines are believed to retard water vapor movement an d reduce the moisture change rate and extremes of wood moisture conten t in any given cycle.