# Study on Seismic Performance of Steel Fiber Reinforced Concrete Pier Under Bending-Torsion Coupling
## 2 Overview of the Experiment
### Experimental Phenomenon  
For the no-torsion pier SFRC-1, several horizontal fine cracks were generated on the front of the pier base during the initial stage of loading. As the loading displacement increased, the horizontal cracks were continuously developed and extended. When the longitudinal steel bar firstly reached yield, short horizontal cracks were observed in the middle of the pier. Moreover, the first through crack, with a width of 2-3 mm, was formed 10 cm above the pier base. As the loading displacement continued to increase, the horizontal  
Figure 4: Loading system. Figure 3: Experimental loading site. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Diameter/mm** & **Sample Number** & \
\begin{tabular}{c} **Yield** \\ **Strength/MPa** \\ \end{tabular} &
\begin{tabular}{c} **Ultimate** \\ **Strength/MPa** \\ \end{tabular} \\ \hline \multirow{3}{*}{16} & Sample 1 & 435 & 625 \\  & Sample 2 & 441 & 619 \\  & Sample 3 & 437 & 626 \\ \hline \multirow{3}{*}{8} & Sample 1 & 430 & 614 \\  & Sample 2 & 422 & 620 \\ \cline{1-1}  & Sample 3 & 424 & 615 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Strength of steel bars. cracks in the middle of the pier extended laterally, and short vertical cracks developing upwards appeared in the middle and lower parts of the pier sides. The second through crack, with a width of 1 mm, was formed 17 cm above the pier base, accompanied by the sound of steel fibers being pulled out. When the horizontal bearing capacity was reduced to 85% of the peak load, two through cracks were formed within 25 cm of the pier base, and the height of the crack reached 75 cm. At this moment, the concrete at the bottom of the pier was crushed, and partial concrete near the main cracks was peeled off. It could be seen that the main cracks on the specimen were horizontal cracks, showing obvious bending failure characteristics. Due to the presence of SFRC, the failure mode of the specimen was different from that of an ordinary concrete pier. The addition of steel fibers improved the tensile strength of the concrete and the flexural bearing capacity of the bottom of the pier, which was subjected to the maximum load. The pier body was kept relatively intact until failure, with no significant areas of concrete spalling and no loss of vertical bearing capacity. The final failure mode of SFRC-1 is shown in Figure 5a. Compared with SFRC-1, SFRC-2 and SFRC-3 suffered different degrees of bending-torsion damage due to the different torsion-bending ratios. As the experiment progressed, diagonal cracks firstly appeared in the middle and upper parts of SFRC-3, which had a higher torsion-bending ratio. Under cyclic bending-torsion action, the diagonal cracks in the middle and upper parts of the two piers gradually increased, forming an 'X'-shaped intersecting crack network. As the loading displacement increased, the diagonal cracks continued to extend upwards and downwards, forming a spiral that traversed through the entire cross-section. The cracks on SFRC-3 were wider and longer than those on SFRC-2. In the later stage of loading, the steel bars at the crack section of SFRC-3 yielded, and the cracks in the pier developed rapidly, with a length of 95 cm. The concrete at 20-50 cm from the top of the pier bulged and fell off extensively, resulting in the exposure of the spiral stirrups and longitudinal bars. For SFRC-2, the crack length reached 90 cm. Some intersecting diagonal cracks with a width of 7 mm and a height of about 25 cm were formed at 20 cm from the top of the pier, and the concrete protective layer bulged but did not fall off. According to the experimental phenomena, it can be seen that the presence of torque causes the main cracks on the specimen to be diagonal cracks, and shear failure becomes the main failure mode. The greater the torque, the more serious the damage of the specimen.