Finding solutions for openings in laminated wood wind turbine towers

Abstract

Construction with laminated veneer lumber (LVL) still is an emerging field within timber engineering. Introducing openings in timber laminates creates challenges caused by the material’s orthotropic properties. Analytical investigations about stress concentrations forming around these openings have been conducted, showing that both layup of the laminate and orientation of fibres greatly impact resulting stresses. Fluctuation of stresses over rotation angle is significantly higher in a unidirectional laminate than in a plywood laminate. Classical laminate theory was applied to investigate possibilities of computing material properties for timber laminates with arbitrary layup. A relation predicting material properties both for LVL and plywood was found. Theoretical values indicate standard values may underestimate material properties significantly due to different layups given the same properties. Two methods were investigated to reduce stresses around openings: adding reinforcing structures and modifying laminate layup. Reinforcing structures showed that redistributing stresses within a timber laminate wall is especially challenging when reinforcement is not integrated into the wall. Stiffeners with varying stiffness placed in the wall yield best results, reducing stress concentrations in both normal in-plane material directions and in-plane shear. Maximum stress and Tsai-Hill failure criteria were applied to evaluate material utilization in the laminate. Modifying layup showed introducing higher strength material around the opening has a more substantial impact on performance than creating layups with different lamina orientations. Reinforcing structures were observed to have a smaller impact on material utilization than modifying the laminate material. Combination of the two methods revealed that reinforcing a laminate with a non-unidirectional layup is not beneficial and counteracts laminate mechanics.