Experimental study on withdrawal resistance for a screw in cross-laminated timber-bamboo composite

Hybrid cross-laminated timber (CLT), being a derivative of generic CLT, offers the advantages of combining different materials to achieve improved performance. This study focused on investigating the withdrawal capacity of self-tapping screws (STS) in a hybrid CLT, i.e., cross-laminated timber-bamboo (CLTB), which is composed of high-density bamboo scrimber and low-density fast-growing Chinese fir lumber. The effects of screw diameters, penetration length and penetration sides on withdrawal performance were evaluated and the experimental results were further compared with the predicted values obtained from existing theoretical formulas. The experimental findings revealed a significant increase of 297% in withdrawal capacity when the outer layer of CLT is replaced with high-density bamboo scrimber. Compared to Chinese fir CLT, the penetration length of STS has a smaller influence on the withdrawal capacity of CLTB, with a minimum increase of 20.9% for CLTB, and a maximum increase of 119% for Chinese fir CLT. In addition, STS diameter significantly affects the CLTB withdrawal capacity by 66.4%, whereas the effect is 9.9% for Chinese fir CLT. When inserted into the narrow side, STS should only penetrate the transverse layer. The withdrawal capacity of bamboo scrimber transverse layer in type A was 260% higher than those of Chinese fir transverse layer in types B and C. Finally, significant differences were found between the predicted and experimental values for all three types of CLT. One of four existing theoretical formulas demonstrates improved accuracy in predicting the withdrawal capacity of CLTB.