Seismic Performance Evaluation of Reinforced Concrete Pile-Cap With a Pile, Exterior Column and Foundation Beam

Abstract eng:
Pile-cap is an important structural joint member. Its function is to transfer the stresses occurring on the columns through a group of piles to the ground, occurring on the complex stresses that occur under earthquake loading. It is difficult to identify if the pile-caps were damaged by the earthquake. It requires a complete excavation of the pile-caps. It’s hard to observe the seismic behavior under earthquake loading. It is very important to clarify pile-cap shear failure mechanism of reinforced concrete (RC) structures. However, shear failure mechanism of a pile, exterior column-beam pile-cap in RC structure is not resolved yet under bi-lateral loading. Currently, there is no research and few valid experiments, regarding the study of pile-caps. As a result, the stress mechanism has not been defined. Therefore, the pile-cap design has been left to the architect’s discretion. Though performance-based design is applied to buildings as they become taller, in case of pile-caps, the lateral and vertical reinforcements is not considered and the pile-cap foundation is currently designed using methods prescribed by structural regulations. Most of the research in these studies has focused on the effects of vertical loading on structural performance and bar configurations in pile-caps. The performance was examined in cases of tension-only or compression-only loading, but the ultimate strength and deformation were not specified. In the previous report, we performed lateral load reversal tests of subassemblages with one pile, column, foundation beam and pile-cap. This report is a series study of grasping the seismic capacity of interior pile-caps, these specimens were carried out to investigate the pile-caps shear performance. The specimens which were the exterior subassemblages of a precast pile, foundation beam and column, were half scale to actual frames. In this experimental study, five specimens were fabricated which can be divided into five types considering the arangement properties. The depth and width of the column and beam section were same, respectively. The length from the center of the column to the pin-roller support of a beam end was 1600mm. The height from the center of the beam to the loading point on the top of the column or to the bottom support was 1415mm, respectively. The shear span ratio was 4.16 in the column, 2.54 in the beam and 3.40 in the pile, respectively. Reversed lateral horizontal loads at the top of a column and constant axial load were applied to all specimens. Three configuration of D10 bars were used in the pile-cap. They were utilized as main reinforcement bars which were called a tie and lateral bar, respectively. Concrete compressive strength was 24MPa. All specimens were designed to fail in pile-cap. The diagonal shear crack strength and ultimate shear strength can be estimated by the prediction method for usual RC beam-column joints to apply the vertical member section to the average between the pile, the column and the pile-cap section or that between column and pile-cap. But in this research, the small diameter pile was applied, which was smaller than the column section in case of the low buildings. Therefore, the quantification of the effective pile-cap section is not resolved in case of large diameter piles applied to high buildings. The shear strength of pile-caps was enhanced by confining effect due to the shear reinforcement amount and the transformation of the surrounding structural members were restrained by them. The difference in the positive and negative maximum strength was occurred by the strut mechanism in loading direction.

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