Conventional definition of ductility index which are based on yielding of the reinforcement, are inappropriate for the evaluation of structural ductility of concrete beams reinforced or prestressed with FRP tendons. In this paper, a new definition of ductility index is expressed as the ratio of the total energy to the elastic energy at the failure state of a beam. It is applicable to beams with steel as well as brittle FRP reinforcements, thus providing a common basis for comparison. From the experimental results of prestressed concrete beams, the beams with a carbon fiber reinforcement plastic (CFRP) tendon have higher flexural cracking load, flexural yielding load, and flexural fracture load. While the displacement at the fracture stage was lower compared to steel tendons, excessive steel reinforcement lead to a lower ductility index. Prestressed concrete beam with a CFRP tendon have sufficient ductility when ruptured by crushing of concrete or used with unbonded tendon. Accordingly, ductility of structure could be secured by overcoming reinforcement failure as inducing compress-controlled failure based on overreinforcement designs in the beam which is used in CFRP tendons. Therefore, the practical design method for prestressed concrete beams with CFRP tendons turn out to be overreinforcement, and use of unbonded tendon.
Since the cable anchorage zone in a prestressed concrete cable-stayed bridge is subjected to a large amount of concentrated tendon force, it shows very complicated stress distributions which can cause serious local cracks. Accordingly, it is necessary to investigate the parameters affecting the stress distribution, such as the cable inclination, the position of the anchor plate, the modeling method, and three-dimensional effects. The tensile stress distribution in the anchorage zone is compared to the actual design condition by varing the stiffness of spring elements in the local modeling, and an appropriate position for the anchor plate is determined. The results provide elementary data for the stress state in the anchorage zones and encourage more efficient designs.Key words: finite element analysis, bursting stress, spalling stress, cable anchorage zone, cable-stayed bridge.
Abstract. This paper conducted several flexural tests of RC (reinforced concrete) columns retrofitted by SMA (shape memory alloy) wires. The RC columns with lap spliced reinforcements at the base show less ductile behaviour than those with continuous reinforcements. SMA wire jackets can increase the ductility of the RC columns with lap spliced reinforcements and thus this study used the two kinds of SMA wires to retrofit such RC columns; the first one is NiTi SMA wire that remains in martensite at room temperature and the other is NiTiNb SMA wire that is austenite at room temperature. Both the SMA wires show shape memory effect and provide active confinement for concrete. The SMA wires were prestrained to 7% strain and released. At the state, some residual strain remained and can be recovered by heating. The prestrained SMA wires were wrapped around the plastic hinge region of the RC columns and then the temperature of the wires raised by heating. Then, the SMA wires induced an active confinement on the RC columns. Each wire was measured for recovery and residual stress to obtain the information of active confinement. This active confinement and the resistance of the SMA wires increased the ductile behaviour of the RC columns in lateral direction. The parameters in this study are the kinds of SMA and the pitch of the wires. All types of the specimens retrofitted the SMA wires showed good ductile behaviour without degrading the flexural strength.
This study provides an idea of adjustable bearing using wedges, which adjusts elevation of a superstructure during construction and/or in service. Adjustable bearings can release developed stress in a superstructure causing from settlement and so on. This study prepares a wedged adjustable bearing and assess its performance from a few static loading tests. A tolerance of the bearing to adjust elevation is estimated to be 0.42 mm, which is in good performance. The deformation of 2.0 mm occurs due to loading of 1470 kN and the deformation is removed with releasing the loading. Thus, any residual deformation is not remained. Finally, finite element analyses are conducted to estimate stress in the sole plate of the adjustable bearing, which is distributed uniformally.
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