## 4 Load Test Results and Discussion
This is mainly because the flexural stiffness of a specimen mostly depends on the rotational stiffness of the joint [27], which increased after the joint bolts of the specimens were retigithened after the fire tests. (2) The flexural stiffnesses of the strengthened specimens S60C and S120C were similar but much higher than those of the untrengthened and untreated specimens. Consequently, the mid-span deflections of the strengthened specimens were significantly smaller under equal loads. (3) The load deflection curves of S60C and S120C basically coincide. The curves of specimens S60 and S120 are also similar. Thus, the duration of fire exposure had little effect on specimens' deformation behavior (at least up to 120 min). Joint opening is an important indicator of deformation in a shield tunnel and it is often investigated to assess a tunnel's safety [28]. Figure 16 presents the joint opening increment versus vertical loading data for all of the specimens. It shows that:  
(1) The joint opening increments of the untrengthened specimens S60 and S120 were smaller than that of the untreated specimen S0 under equal loads. The previously-mentioned explanation is that the rotational stiffness of the fire-damaged joints became higher than that of the untreated specimen when the joint bolts were re-tigithened after the fire test. (2) The joint opening increments of strengthened specimens S60C and S120C were generally similar. At the end of the loading process, an increment of only 0.5 mm was observed, which was significantly smaller than those of the untrengthened and untreated specimens. Figure 17 presents the strain profiles in the strengthening parts of Specimens S60C and S120C under different load levels. The details of the strain gauges are presented in Figure 3. It can be observed that:  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & **Ultimate Load** & \multirow{2}{*}{**Change Ratio**} & **Initial Stiffness** & \multirow{2}{*}{**Change Ratio**} \\  & **(kN)** & & **(kN/mm)** & & \\ \hline S0 & 1236 & — & 40.1 & — \\ S60 & 936 & \(-\)24.3\% & 56.6 & 41.1\% \\ S120 & 936 & \(-\)24.3\% & 41.7 & 4.0\% \\ S60C & \textgreater{}1455 & \textgreater{}17.8\% (\textgreater{}55.4\%\textasci(1) All of the positions where strain gauges were attached were in tension. The deformations were approximately symmetrical about the mid-span. Further, the maximum and minimum strains were at the mid-span and the ends of the strengthening parts, respectively. (2) The strains recorded by gauges 11-15 were much higher than those of gauges 1-5 throughout the entire loading process. Thus, the strengthening played a significant role in determining the tension and bending resistance. The variation in the local shear force between the strengthener and the segment can reflect the performance of the local connection between the strengthener part and the segment. The five sections (I-V) defined in the longitudinal direction are shown in Figure 3. Assuming that the plane-section assumption still applies, the longitudinal strains at different positions in each section can be determined quantitatively based on the measured strains obtained from strain gauges 1-15. Thus, the axial force on each section can be obtained via an integration of the observed stress distribution. The elastic modulus of the steel was assumed to be 200 GPa. Any hardening of the steel after yielding and any contribution of the core concrete in tension were neglected. The local shear force between the strengthener and the segment was determined via the difference in axial force between neighboring sections, as presented in Figure 18. The following observations were made:  
(1) Regarding Specimen S60C, when the applied load was less than 17.5% of F\({}_{\text{v, max}}\) the local shear force in regions II-III and III-IV increased much faster than in regions I-II and IV-V. There was thus significant strengthening near the joint during the initial phase of loading.