The effect of shear blocks in OSB double layer gridshells

Bending active gridshells result from made by deforming, usually on site, an initially flat grid composed of straight members into a 3 dimensional curved shape. A timber bending active gridshell sufficiently stiff for even a long span may be assembled from individual flexible layers. Each grid layer is independently deformed into a 3-D curved shape. The layers are then locked together using shear blocks to form a stiff and strong composite structural unit. Bending active timber gridshells require long defect-free lengths (to avoid fracture during forming) and small member cross-sections (for flexibility). The use of solid timber laths usually involves costly processing to cut out defects and re-join the pieces. This study examines an engineered timber product, Irish Orientated Strand Board [OSB], which promises to address these difficulties. This research investigates the degree of composite action generated by shear blocks in a double layer bending active gridshell made from Irish OSB. The study also investigates the material stiffness of OSB. Gridshell behaviour is typically geometrically non-linear and depends on the forming process. Computer analysis can address both issues. It is convenient in such computer models to treat a multi-layer grid as a single layer grid of equivalent stiffness. This equivalent stiffness depends on the effect of the shear blocks. Collins has established a global value for the degree of composite action (and hence an equivalent bending stiffness) for best fit of experimental deflection data with model predictions in a double curved Irish OSB experimental gridshell. However Collins did not investigate directly the degree of composite action in individual members. This research investigates the degree of composite action of individual flat double layer strips of Irish OSB and compares these with the best-fit global value noted by Collin’s. Significant differences from Collins’ value are noted and possible explanations discussed. Suggestions for further research are described.