# Retrofitting of Steel Structures with CFRP: Literature Review and Research Needs
## 4 Numerical Analysis
The results indicated that the adhesive modulus, the CFRP thickness, and the CFRP modulus significantly affect the debonding strength. [PERSON] [59] studied the prediction of the debonding failure in RHS steel tubes strengthened with CFRP using a bond-slip model under an end-bearing load. C3D8, S4R, and COH3D8 elements were used to model the bearing plate, CFRP plate, and adhesive layer. The tie constraint was applied to adhesive surfaces connected to the CFRP plate and tub web. The results indicated that the debonding process in the FE model is much more gradual which can lead to higher stiffness in the load-displacement curve prior to reaching the ultimate load. A numerical investigation on the interfacial behavior of the bond between CFRP laminate and steel beam was presented by [PERSON] et al. [126]. In this study, all parts were modeled using C3D20R solid elements and the common nodes at the interfaces were merged. It is found that interfacial shear stress as well as peel stress are not uniformly distributed over the width of the adhesive layer. Furthermore, the distribution of the peeling stresses along the width of the bond-line is not the same for different strengthened beams. This variation is attributed to the higher axial stiffness of the CFRP laminate as a major parameter. [PERSON] et al. [16] investigated the flexural behavior of the steel beams repaired with CFRP sheets with various initial crack configurations by simulating a three-dimensional model in ANSYS. SOLID45 and LINK8 elements were utilized to model steel beams and CFRP sheets, respectively. These elements can be connected using interface elements as they have the same degree of freedom. The COMIN39 interface element was used to predefine crack propagation at the midspan of the beam. Results showed that the influence of the initial damage level on the failure mode of the repaired beam is negligible, whereas the damage level can have an effect on the web fracture rate of the beams. Moreover, the level of the initial damage determines the initiation of CFRP debonding. [PERSON] [127] studied the rehabilitation of notched steel beams using CFRP plates. In this regard, a mixed-mode cohesive law was used to model notched retrofitted steel beams. The findings show that increasing the CFRP elastic modulus and thickness can enhance the bearing capacity while reducing the ductility and leading to premature debonding failure. [PERSON] et al. [128] explored the effectiveness of using externally bonded CFRP using ductile adhesive. A trapezoidal mixed-mode CZM was used to simulate the debonding behavior of the CFRP-strengthened steel beam. The findings revealed that thicker or shorter CFRP laminates resulted in higher interfacial stresses, leading to earlier debonding, whereas longer CFRP laminates delayed debonding by changing the stress transfer path. Table 7 provides a summary of the numerical studies presented in the literature. The realistic representation of stress distribution and failure modes in the rehabilitated elements with CFRP demands high computational costs. However, simplifications of the assumptions may not capture all real-world behaviors. Therefore, finding a middle ground that takes these factors into account is really important.