Johan Jönsson

Load carrying Capacity of Curved Glulam Beams Reinforced with Self- tapping Screws.

The aim of this research was to determine the load carrying capacity of curved glulam beams reinforced perpendicular to grain and subjected to climate induced internal stresses. The self-tapping screws used as reinforcement prevent the glulam to split along the grain causing failure perpendicular to grain. Different tests were performed; specimens seasoned in dry and moist climate, specimens exposed to single climate change and reinforced specimens seasoned in a dry and moist climate. Specimens without reinforcement where tested to failure and then reinforced and tested again. From the tests it can be concluded that reinforcement improves the capacity of the beam in some cases up to 50% compared to unreinforced beams. The effect of moisture gradients on the capacity is significant and the worst case is when the beams are in a moistening phase. ZusammenfassungIn dieser Untersuchung sollte die Tragfähigkeit gebogener, senkrecht zur Faserrichtung verstärkter Brettschichtholzträger (BS-Träger) bestimmt werden, die klimatisch bedingten inneren Spannungen ausgesetzt sind. Dabei verhindern die selbstbohrenden Schrauben ein Aufreissen der BS-Träger in Faserlängsrichtung und ein dadurch verursachtes Querzugversagen. Verschiedene Versuche wurden durchgeführt: Versuche mit Prüfkörpern in trockenem und feuchtem Wechselklima, Versuche mit Prüfkörpern, die während der Lagerung nur einem einzelnen Klimawechsel ausgesetzt wurden sowie Versuche mit verstärkten Prüfkörpern in trockenem und feuchtem Wechselklima. Prüfkörper ohne Verstärkung wurden bis zum Versagen belastet, anschliessend verstärkt und ein weiteres Mal geprüft. Die Ergebnisse zeigen, dass die Verstärkung der BS-Träger deren Tragfähigkeit um bis zu 50% steigert. Der Einfluss der Holzfeuchte-Gradienten auf die Tragfähigkeit ist signifikant, wobei der ungünstigste Fall die Befeuchtungsphase ist.

Internal stresses in the cross-grain direction in glulam induced by climate variations

Abstract Results are presented from an experimental investigation to determine internal stress states perpendicular to grain in glulam induced by moisture variations. The stresses are determined by measuring the released strains before and after cutting. Stress distributions are determined for specimens seasoned in constant humidity, specimens exposed to an artificial single climate change, specimens exposed to cyclic climate change and for specimens exposed to natural climate outdoors under shelter. Results for seasoned specimens show that internal stresses exist in glulam without the presence of moisture gradients. Stresses in specimens with an induced moisture gradient become larger when moistening from a specific climate A to another climate B than when drying from B to A. Tests in an outdoor sheltered climate show large variations in strains and stresses. The tensile stress level in the inner part of the glulam cross section exceeds the characteristic strength of 0.5 MPa during a period of approximately 80 days.

A Contact Free Measurement Method to Determine Internal Stress States in Glulam

Abstract A method to determine the internal stress state across the grain of glued laminated wood (glulam), as well as the experimental equipment for measuring released deformation are described in this paper. The internal stress state is calculated from measured released deformations, which are measured by a contact free method based on a digital camera technique. Non-destructive measurement of the modulus of elasticity in the cross-grain direction is presented and the correlation between dynamic and static modulus of elasticity is shown. Results for a single specimen with induced moisture gradient show that the largest stress found in the tests was about 0.57 MPa, which is higher than the characteristic value of tensile strength perpendicular to the grain. This stress is based on released strain over the total length of the slices cut from the specimens. FEM-simulations evaluating the stress distribution within the specimen to account for end effects indicate that the maximum stress is about 30% higher.

The effect of moisture gradients on tensile strength perpendicular to grain in glulam

Experimental results are presented concerning tension perpendicular to grain in glulam sections where internal stresses induced by climate changes are present. The results reveal that moisture induced internal stresses may affect the tension capacity both in a positive and a negative way compared to reference specimens, free from moisture gradients. The tension capacity is reduced during moistening from RH 40% up to RH 80% whereas it is increased during drying from RH 80% to 40%. The tension capacity during wetting is only 50% of that observed for drying specimens. This behaviour can be explained by the combination between initial moisture induced stresses and stresses from external loading. The same tendency was observed for specimens exposed to cyclic climate changes, where it was also found that time and numbers of cycles do not affect the tension capacity in any specific direction.ZusammenfassungEs werden experimentelle Ergebnisse präsentiert, betreffend der Zugfestigkeit senkrecht zur Faser in Glulamabschnitten, in denen durch Klimaänderungen induzierte interne Spannungen vorhanden sind. Die Ergebnisse zeigen, dass durch Feuchte induzierte interne Spannungen die Zugkapazität sowohl positiv als auch negativ beeinflussen können im Vergleich zu Kontrollproben, die frei von Feuchtegradienten sind. Die Zugkapazität wird während des Feuchtevorgangs von 40% bis zu 80% relative Feuchte reduziert, wobei sie sich während der Trocknung von RH 80% auf 40% erhöht. Die Zugkapazität während des Befeuchtens beträgt nur 50% derjenigen, die bei der Probentrocknung beobachtet wird. Dieses Verhalten kann durch die Kombination zwischen initial feuchte-induzierten Spannungen und Spannungen von externer Belastung erklärt werden. Die gleiche Tendenz wurde bei Proben beobachtet, die Wechselklima ausgesetzt waren, wobei man auch herausfand, dass Zeit und Anzahl der Wechsel die Zugkapazität nicht in irgendeiner bestimmten Richtung beeinflussten.

Internal stresses in glulam due to moisture gradients in the grain direction

Abstract An experimental study on the internal stress state perpendicular to grain in glued laminated timber induced by moisture variations along the grain is presented. The stress is evaluated by using measured modulus of elasticity and strain. Different types of tests are performed, in which specimens are exposed to artificial climate change when the specimens are in either a wetting or a drying state, and specimens are exposed to a cyclic climate change. Test results from the specimens that were in a drying state show that the tensile stresses near the end of the specimens at some point exceeds the strength of the wood and cracks. In the opposite test, when the specimens are in a moistening phase, the maximum tensile stresses are lower and in the order of 0.2 MPa during the tested period. The cyclic test, which continued during 20 weeks, showed that there were no accumulated effects regarding strain and stress values. In this test, the average maximum tensile and compression stresses were of the order 0.20 and 0.10 MPa, respectively.