Strain distribution in OSB and GWB in wood frame shear walls Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ns064b19j

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  • The overall goal of this study was to gain an insight into the load sharing aspect between oriented strand board (OSB) and gypsum wall board (GWB) in shear wall assembly during racking load. More specifically the objectives of the study were to: (1) evaluate qualitatively the load sharing between OSB and GWB in a wood frame shear wall assembly, (2) analyze the failure progression of GWB and OSB, (3) study the strain profile around fastener on GWB and OSB sides of shear wall, and (4) study the effect of GWB on shear wall behavior. Monotonic tests were conducted on 2440 x 2440 mm walls with 38 x 89 mm Douglas-fir studs 610 mm on center. Two 1220x2440x11.1 mm OSB panels were installed and fastened vertically to the frame with Stanley Sheather plus ring shank nails 102 mm and 305 mm on center along panel edges and intermediate studs, respectively. Two 12.7 mm GWB panels were installed oriented vertically on the face opposite the OSB using standard dry wall screws on some walls. Anchorage to the walls was provided by two 12.7 mm A307 anchor bolts installed 305 mm inward on the sill plate from each end of the wall. In addition to these anchor bolts, walls included hold-downs installed at the end studs of the wall and were attached to the foundation with 15.9 mm Grade 5 anchor bolts making the walls fully anchored. The loading was monotonic and based on ASTM E564-00. Sixteen walls were tested in total, out of which 11 (Type A) were sheathed on both sides with OSB and GWB, while 5 walls were tested without GWB (Type B). Optical measurement equipment based on the principle of Digital Image Correlation (DIC) was used for data acquisition and analysis. DIC is a full-field, non-contact technique for measurement of displacements and strains. The set up consist of a pair of cameras arranged at an angle to take stereoscopic images of the specimen. The system returns full field 3D displacement and strain data measured over the visible specimen surfaces. The tests revealed that load is shared by both OSB and GWB initially in a shear wall assembly. GWB fails locally prior to OSB and load shifts to OSB as GWB starts to fail. Beyond this point, load continues to increase and walls finally fail in OSB. The tests also revealed that load path in wall type A and B is different. Failure in wall type A starts at the uplift corner in GWB and then moves to the uplift corner in OSB. Finally the walls fail at middle of top plate for GWB and OSB both. In wall type B the failure is initiated at the uplift corner in OSB followed by middle region at sill level and ends up at middle section of wall where two panels meet. The uplift corner fasteners are of prime importance in both types of wall and panels. Comparing the strain profiles created using DIC, strains only near fasteners are observed and no detectable strain is observed in the field of the panel. There is a steady built up of strain in wall type B from start to failure and there is no abrupt change in strain during entire loading indicating a ductile failure. Wall type B shows more ductile behavior than wall type A because of the lack of ability of GWB to deform at higher load in wall type A where as OSB in wall type B continues to deform at higher load. Also OSB panel in wall type B experiences higher strains than the OSB panel for wall type A for a given load. In wall type A, there is higher strain around the fasteners in GWB than in OSB in the initial part of loading. GWB is stiffer than OSB, it attracts load and in turn deformation is higher than OSB. But being brittle, GWB fails at around 60% of the ultimate wall capacity and load shifts to OSB. This is indicated by large change in strain in OSB. OSB continues to attract load but the strain in OSB increases at a faster rate till failure indicating a much less ductile behavior than that of wall type B. Contribution of GWB towards strength of the wall is marginal (0.8%) while an increase of 50% was observed in overall stiffness of the walls. Since GWB is stiffer than OSB, it contributes more to the overall stiffness of the wall. Ductility factor of the system increases by 20% and the ductility of the system increases by 13% while energy dissipated by the wall decreases when GWB is included in the shear wall assembly. GWB being brittle reduces the ability to deform before failing and hence a decrease in peak, failure and yield displacements is observed in magnitude of 18%, 13% and 27%, respectively Overall, these tests suggest that initially during loading of a wall the load is shared between OSB and GWB. However, the proportion of load sharing is not known. As GWB fails first the load shifts to the OSB panel which resists it till the failure of the wall. This aspect of load sharing between structural sheathing and gypsum wall board is not incorporated in current design practices. It is recommended that more tests especially with cyclic and dynamic loading be conducted to better understand and quantify the aspect of load sharing.
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