Initial stiffness of integrally-formed GFRP T-joint in GFRP transmission tower

It is recognized that the semi-rigidity of the glass fiber reinforced polymer (GFRP) joint should be considered in the structural analysis of GFRP structures, because of the relatively small initial stiffness resulting from the intrinsic property of GFRP material. Regarding the integrally-formed GFRP T-joint in the GFRP transmission tower, the study of its semi-rigidity remains inadequate. This paper aims to develop the relevant method for approximating the initial stiffness of the integrally-formed GFRP T-joint, in which the joint connects a vertical column with circular hollow section (CHS) and a horizontal bracing beam with rectangle section (RS). The finite Element (FE) models are first established, and validated by comparing the FE analysis results with the experimental results extracted from the previous study by the authors. The parametric study is subsequently conducted by using the validated FE model to investigate the sensitivity of the initial stiffness to the joint's main geometrical parameters, i.e., the diameter and the thickness of the column, and the width and the height of the bracing beam. The total number of the specimens numerically modeled for the parametric study is 255. The mechanical model that reveals the main physical source of the initial stiffness is proposed. Based on the parametric analysis results, the approximating formula is then concluded, which is capable of well predicting the initial stiffness of the joint. The formula is first validated by comparing the theoretical results with the FE analysis results, considering the simply-supported T-shaped structures with/without the tapered haunches. Then, the GFRP line-suspension module (GFRP-LSM) utilized in a GFRP transmission tower is employed for further experimental validation. It is found that the structural analysis results, with consideration of the semi-rigidity of the joints, agree well the continuum shell element-based FE analysis results. In terms of the top displacement, the result obtained via the structural analysis is close to the measured data, indicating an error of 9.03%. In contrast, a significant discrepancy would be found when the semi-rigidity of the joints is neglected in the analysis, and the error would increase to 21.86%. The comparison again highlights the importance of considering the semi-rigidity of the GFRP joints in the structural analysis of a GFRP structure.