Abstract
A post-tensioned precast structure is expected to reduce a structural damage by concentrating a deformation in a pressure connection between beam and column. Furthermore, the post-tensioned precast structure is also advantageous from the precast construction viewpoint, especially without grouting into a sheath. Previous test results showed that a shear strength of a beam-column joint decreased with increasing a ratio of opening to the joint volume which depends on the size and the number of sheaths to go prestressing bars through.
To investigate the effect of the opening due to sheaths on the shear behaviors of the beam-column joints, 3-D finite element analyses were conducted on post-tensioned precast frames which are previously tested specimens. The experiments were performed on two types of specimens, plane cross-shaped joints (hereinafter referred to as the plane specimen) and space joints that additional beams were orthogonally installed (hereinafter referred to as the space specimen). These specimens were cut from a whole structure, and the common factors are the shear span of column and beam (1.415 m and 1.6 m, respectively), the cross-sectional dimensions of column and beam (350 mm × 350 mm and 250 mm × 400 mm, respectively), the bar arrangements (12-D25(SD490) + 2-D10(SD345) @90 for column, 4-D13(SD345) + 2-D10(SD345) @100 for beam, and 2-D10(KSS785) for joint), and the axial load ratio (0.13). The specimens were designed to reach shear failure at the beam-column joints before the beams fail, in accordance with design guidelines of AIJ. For this reason, the concrete strength of beams doubled (σB=70-85 N/mm2). The prestressing bars (SBPD for bonded specimen by grouting and SBPR for unbonded specimen by opening) having a diameter of 36 mm were inserted into the sheaths and the prestressing were introduced up to the ratio of 0.13 - 0.17 to an axial concrete capacity of the beam. The test parameters are diameters and arrangement of sheaths and the ratio of opening to the joint volume are 0 (by grouting) to 12 %. All of the specimens resulted in shear failure in the beam-column joint.
The pressing and detaching behaviors in the connection between beam and column were modeled with an interface element having an appropriate properties. The concrete was modeled with a solid element and a Drucker–Prager criterion having 15 and 20 degrees of internal friction angle depending on the concrete strength was adopted for the compressive failure, and the linear softening behavior was assumed after maximum strength. Multidirectional crack models having a threshold value of 60 degrees were used for concretes, and the shear stiffness was reduced with increasing a crack width. All reinforcing bars were modeled with embedded elements and assumed as a bi-linear behavior. The prestressing bars were modeled with truss elements, and furthermore, for the PC bars bonded by grouting, interface elements were also used to represent a bond-slip relationship.
The relations of the shear force and drift angle, the cracking patterns, and the behaviors of reinforcement which were obtained from the analysis were roughly consistent with the test results. The compressive damage and confining effect in the beam-column joints were numerically estimated based on the 3-D stress state. Analytical results that the shear strength of the beam-column joint decreased considerably with increasing opening ratio for the plane specimen without beams installed orthogonally. If, however, the orthogonal beams were installed, the shear strength of the beam-column joint increased by 20 % and the decrease in shear strength due to opening was softened when compared with the plane specimen because a high confining pressure was generated around the beam-column joint of the space specimen and the effect of the concrete strength was reduced.
