## 3 Results and Discussion
### The Effects of Pre-Tension Force Relaxation  
The GFRP plates experience a creep deformation under sustained loads [27]. When a GFRP plate is subjected to the preloading force of high-strength bolts during the service period, creep deformation is induced, and the pre-tension force of the high-strength bolts should relax. The slip load decreases as the pre-tension force is relaxed [17]. Table 10 summarizes the variation in the results of the G-G slip-critical connection, and the G-S24# slip-critical connection was due to the creep deformation of the GFRP plates. We found that the slip load decreased as the preloading time increased in both connections. The residual rate of the shear resistance of the G-S24# connection was much higher than that of the G-G connection. After one day of preloading time, the slip load of the G-S24# slip-critical connection decreased by 6%. After 30 days, it decreased by only 9%. The test results showed that compared to the G-G connection, the G-S24# connection still had good slip resistance, even after a long period of preloading time.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \begin{tabular}{c} **Connection** \\ **Methods** \\ \end{tabular} & \begin{tabular}{c} **Specimen** \\ **Time (day)** \\ \end{tabular} & \begin{tabular}{c} **Preloading** \\ **(kN)** \\ \end{tabular} & \begin{tabular}{c} **Slip Load F** \\ **(kN)** \\ \end{tabular} & \
\begin{tabular}{c} **Average Value** \\ **(\%)** \\ \end{tabular} &
\begin{tabular}{c} **Residual Rate** \\ **(\%)** \\ \end{tabular} \\ \hline \multirow{4}{*}{G-G} & A–0-1 & \multirow{4}{*}{0} & 2.95 & \multirow{4}{*}{2.78} & \multirow{4}{*}{/} \\  & A–0-2 & & 2.64 & & \\ \cline{1-1} \cline{2-5}  & A–1-1 & & 2.40 & & \\ \cline{1-1} \cline{2-5}  & A–7-1 & & 2.36 & & \\ \cline{1-1} \cline{2-5}  & A–7-2 & & 2.28 & & \\ \cline{1-1} \cline{2-5}  & A–30-1 & & 2.17 & & \\ \cline{1-1} \cline{2-5}  & A–30-2 & & 2.23 & & \\ \hline \multirow{4}{*}{G-S24\#} & B-0-1 & \multirow{4}{*}{0} & 6.32 & \multirow{4}{*}{6.29} & \multirow{4}{*}{/} \\  & B–0-2 & & 6.26 & & \\ \cline{1-1} \cline{2-5}  & B-1-1 & & 5.82 & & \\ \cline{1-1} \cline{2-5}  & B–1-2 & & 5.94 & & \\ \cline{1-1} \cline{2-5}  & B–7-1 & & 5.79 & & \\ \cline{1-1} \cline{2-5}  & B–7-2 & & 5.73 & & \\ \cline{1-1} \cline{2-5}  & B–30-1 & & 5.72 & & \\ \cline{1-1} \cline{2-5}  & B–30-2 & & 5.68 & & \\ \hline \end{tabular}
\end{table}
Table 10: Slip load variation under the pre-tension of high-strength bolts.  
Figure 10: The load–displacement curves of connections with different prevailing torques.  
[PERSON] et al. [15] performed bolt pre-tension relaxation tests for up to half a year to investigate the slip load reduction of GFRP slip-critical connections with steel cover plates. The configuration of the G-S24# connection in our paper is similar to that of [PERSON]'s results [15]. The residual rate of slip load from [PERSON]'s tests [15] and our own tests are presented in Figure 11. From this figure, the G-G connection would lose more than 20% of shear resistance within a month. The residual rate curve's decreasing rate of slip load of the G-S24# connection was lower than that of [PERSON]'s tests within 30 days, and the downward trend seemed to be stable. The slip load in [PERSON]'s tests [15] dropped 16% after half a year. Therefore, we speculate that the slip load of the G-S24# connection would be reduced by no more than 16% after standing for half a year, retaining the vast majority of its original bearing capacity.