Experimental Study on Axial Compressive Behavior of the BFRP-Confined Timber Columns with and Without Knots

Timber has gained popularity in the construction industry in recent years due to its low carbon footprint, favorable seismic performance, and esthetic appeal. However, due to the size limit and inevitable natural defects such as knots in the lumber, the axial capacity of timber columns might be insufficient. Therefore, wrapping the timber column with basalt fiber-reinforced polymers (BFRPs), which is an environmentally sustainable material, to improve the load-carrying capacity has been a promising technology. While existing research mostly focuses on defect-free specimens, this study investigates the effects of knots on the structural performance of timber columns wrapped by BFRP. Axial compressive tests were carried out on timber columns, i.e., Douglas fir (knot-free) and camphor pine (with knots), wrapped by BFRP. The results showed that the load-carrying capacity, stiffness, and ductility can be significantly enhanced by the BFRP wrapping. The failure mode of the Douglas fir specimens transitioned from timber crushing failure to shear failure, while the camphor pine specimens failed around the knot area, and the failure mode changed from overall bending to BFRP rupture when the three layers of BFRP were employed. Furthermore, compared to knot-free columns, those specimens containing knots exhibited greater variability in load capacity and recorded a higher percentage increase in strength after reinforcement by BFRP. Based on the test results, three prediction models of the compressive strength of the BFRP-wrapped Douglas fir and camphor pine columns are presented.