## 3 Experiment Results and Discussion  
### Failure Process and Modes  
#### 3.1.1 Unreinforced Specimens  
As shown in Figure 10, all unreinforced specimens UP-1, UP-2, and UP-3 with different length-diameter ratios (4, 6, and 8) displayed flexural failure modes involving the crushing of concrete at the bottom of the pier column and buckling of longitudinal rebars. All unreinforced specimens experienced the generation, development, and expansion of bending and shear cracks, as well as the crushing of concrete at the pier column root. (a) For specimen UP-1, some initial circumferential fine transverse cracks appeared along the pier column bottom when the lateral displacement reached 6 mm. The longitudinal rebars yielded when the displacement reached 8 mm. It was generally considered that the specimen yielded due to the unrecoverable plastic deformation according to the hysteretic curve, and \(\Delta_{y}\) was taken as 8 mm. At the end of the \(\Delta_{y}\) displacement cycle, several transverse cracks began to expand approximately 15 mm from the pier column bottom. With the increase in load displacement, the original cracks continued to extend in the horizontal direction, and several new transverse cracks were generated on the surface of the pier column during the \(2\Delta_{y}\) to \(4\Delta_{y}\) loading cycles. During the \(5\Delta_{y}\) loading cycles, some transverse cracks gradually developed into diagonal cracks and penetrated each other. Once the loading displacement reached \(6\Delta_{y}\), the width of the penetrating cracks increased sharply, and slight spalling of concrete occurred around the cracks accompanied by a slight \"splitting\" sound. With further load displacement, the concrete on both sides of the tension and compression zone at the pier column bottom spalled off rapidly in blocks, and a plastic hinge gradually formed. Once the loading displacement increased to \(8\Delta_{y}\), the concrete at the pier column root was crushed and exhibited significant peeling, leading to the longitudinal rebars and stirrups being exposed and buckling. Meanwhile, the horizontal load dropped to 85% of the peak load, and the test was terminated. (b) For specimen UP-2, when the lateral displacement reached 12 mm, several initial fine cracks began to appear at the pier column bottom; meanwhile, the longitudinal rebars had not reached the yield strength, and \(\Delta_{y}\) was taken as 18 mm. During the \(2\Delta_{y}\) to \(6\Delta_{y}\) loading cycles, the transverse and diagonal cracks at the pier column bottom gradually widened, extended, and penetrated, resulting in concrete crushing and spalling at the column bottom and exposure of steel rebars at the pier column root. The test ended when the loading displacement reached \(7\Delta_{y}\) due to the significant damage to the specimen. (c) For specimen UP-3, the initial cracks appeared when the lateral displacement reached 12 mm, and \(\Delta_{y}\) was taken as 20 mm. During the \(2\Delta_{y}\) to \(9\Delta_{y}\) loading cycles, the development of cracks and damaged condition at the pier column root were similar to that of specimens UP-1 and UP-2. The test ended when the displacement reached \(10\Delta_{y}\).