STRESS-STRAIN MODEL FOR FRP-STRENGTHENED WOOD COLUMN UNDER AXIAL COMPRESSION
Based on Tsai-Wu criterion, the plastic yielding formulation and the corresponding yielding-surface equation of triaxially compressed wood columns are developed. From the equilibrium condition of external forces and the strain compatibility, an elasto-plastic incremental model has been presented to describe the stress-strain relationship when the FRP-strengthened wood column is uniaxially compressed. The experiments of the uniaxially compressed columns circumferentially wrapped by CFRP or GFRP sheets have been performed for 12 different strengthening cases. It is shown that experiment data and corresponding analytical results are consistent before the strengthened column reaches its ultimate compressive strength. However, the analytical predictions are higher than the experimental results when the stress in the member exceeds its ultimate compressive strength. It is observed from the experiments that the hoop tensile strain is small and far less than the ultimate tensile strain when the strengthened wood column reaches its compressive strength. Moreover, based on the experimental data, the formula to predict the hoop strain of strengthened columns at ultimate compressive strength is formulated and it can be used as the close condition in the interactive procedure of the numerical model. According to the Bazan-Buchanan criterion, the σ1-e1 curve beyond the ultimate compressive strength is simplified as a declinedstraight line, which corresponds to the experimental results well. So the proposed model can be applied to the engineering design.