## 4 Finite Element Analysis Model  
### Material Model  
The finite element software ABAQUS was used to establish a model for composite GFRP-confined concrete square columns for simulation analysis. As the model involves multiple components of concrete, GFRP tubes, and external GFRP, the convergence of the model is poor. Therefore, the first-order element with good convergence was used in the selection of various models. The eight-node hexahedron complete integral element (C3D8R) with reduced integral was used in concrete and GFRP tubes, and the best results can be obtained at the lowest cost for three-dimensional problems. External GFRP adopts four-node conventional shell element (S4R), which has stable performance and wide application range [23; 24]. The plastic damage model of concrete is a continuous damage model based on plastic, which takes into account the tensile and compressive properties of materials and has good convergence. It can simulate the ultimate compressive stress and plastic deformation of concrete under low confining pressure. Therefore, the plastic damage model was adopted in this paper [25; 26], and the CDP model parameters are shown in Table 6.  
Relations of concrete in uniaxial stress state is determined by the following formulas:  
\[\sigma_{t}=(1-d_{t})E_{0}\Big{(}\varepsilon_{t}-\varepsilon_{t}^{pl}\Big{)} \tag{1}\]  
\[\sigma_{c}=(1-d_{c})E_{0}\Big{(}\varepsilon_{c}-\varepsilon_{c}^{pl}\Big{)} \tag{2}\]  
\[d_{t}=1-\sqrt{\sigma_{t}/E_{0}\varepsilon_{t}} \tag{3}\]  
\[d_{c}=1-\sqrt{\sigma_{c}/E_{0}\varepsilon_{c}} \tag{4}\]  
\[\varepsilon_{t}^{pl}=\varepsilon_{t}^{ck}-\frac{d_{t}\sigma_{t}}{(1-d_{t})E_{ 0}} \tag{5}\]  
\[\varepsilon_{t}^{ck}=\varepsilon_{t}-\varepsilon_{0t}^{pl} \tag{6}\]  
\[\varepsilon_{0t}^{cl}=\sigma_{t}/E_{0} \tag{7}\]  
\[\varepsilon_{c}^{pl}=\varepsilon_{c}^{in}-\frac{d_{c}\sigma_{c}}{(1-d_{c})E_{ 0}} \tag{8}\]  
\[\varepsilon_{c}^{in}=\varepsilon_{c}-\varepsilon_{0c}^{cl} \tag{9}\]  
\[\sigma_{t}=(1-d_{t})E_{0}\Big{(}\varepsilon_{t}-\varepsilon_{t}^{pl}\Big{)} \tag{1}\]0  
Note: \(\sigma_{t}\) and \(\sigma_{c}\) are tensile stress and compressive stress, respectively; \(\varepsilon_{t}\) and \(\varepsilon_{c}\) are tensile strain and compressive strain, respectively; \(\varepsilon_{t}^{pl}\) and \(\varepsilon_{c}^{pl}\) are tensile plastic strain of concrete and compressive plastic strain, respectively. \(d_{t}\) and \(d_{c}\) are uniaxial tensile damage evolution coefficient of concrete and uniaxial compression damage evolution coefficient of concrete, respectively; \(\varepsilon_{t}^{ck}\) is tensile cracking strain; \(\varepsilon_{0t}^{cl}\) is undamaged tensile elastic strain; \(\varepsilon_{c}^{int}\) is compressive inelasticity strain; and \(\varepsilon_{0c}^{pl}\) is undamaged compressive elastic strain. We referred to the Code for Design of Concrete Structures (GB50010-2010) [27; 28] for the compressive constitutive of concrete, and adopted the material property test data for the concrete compressive strength, GFRP tube elastic modulus, and GFRP elastic modulus. \begin{table}
\begin{tabular}{c c c c c} \hline \(\boldsymbol{\
abla}\) (\({}^{\circ}\)) & \(\boldsymbol{\epsilon}\) & \(\boldsymbol{\sigma_{\text{bb}}}\)/\(\boldsymbol{\sigma_{\text{c0}}}\) & \(\boldsymbol{K_{\text{c}}}\) & \(\boldsymbol{\mu}\) \\ \hline
30 & 0.1 & 1.16 & 0.6667 & 0.0005 \\ \hline \end{tabular} Note: \(\boldsymbol{\
abla}\)—dilatancy angle, \(\boldsymbol{\epsilon}\)—flow potential eccentricity, \(\sigma_{\text{bb}}\)/\(\sigma_{\text{c0}}\)—the ratio of initial equibiaxial compressive yield stress to initial uniaxial compressive yield stress, \(K_{\text{c}}\)—the ratio of the second stress invariant on the tensile meridian, and \(\boldsymbol{\mu}\)—viscosity parameter.  
\end{table} 
Table 6: Parameters of the concrete plastic damage model. Both GFR tubes and wrapped GFRP adopt ideal linear elastic model. Combined with the literature [29; 30], we assigned properties to the above materials.