Abstract
The correct use of non-adherent prestressing techniques in beam-column connections can significantly increase the seismic performance of precast concrete frames, making them a competitive alternative to the traditional cast-in-place concrete structures. In this sense, a fundamental factor of the nodes' design is represented by the correct calibration of the ratio between the amount of mild steel and that of post-tensioned reinforcements, in order to provide the prescribed flexional and rotational capacity to the beam-column interface, and to guarantee at the same time the required dissipative and re-centering capacity. In this paper, a simple algorithm for the optimal and quick pre-dimensioning of the above mentioned parameters is proposed. It is based on the knowledge of the materials' mechanical parameters and of the target stresses and deformations corresponding to the required performance level, which can be computed either through a Force Based Design approach or by a Displacement Based Design approach. In the case of centered post-tension and in the presence of symmetrical mild steel, the procedure can also be effectively represented in a graphical form, making it is easily possible to determine the post-tensioning stress that should be applied to the cables in order to guarantee that they remain in the elastic field after the seismic event, and to obtain the recentering of the system.
Highlights
Over the last thirty years, the scientific community has been extensively involved in research programs aimed at investigating the possibility of an organic approach to the use of precast components, in order to overcome the traditional idea that their design should “emulate” that of cast-in-place structures
The recent ACI standards [3] contain design provisions for precast concrete structures exposed to high seismic risk
Precast concrete frames are often introduced in buildings with the sole purpose of operating as a bracing element, whereby the effects of gravity loads become negligible if compared to seismic stress, and the adoption of symmetrical mild reinforcements together with a centered post-tension results rational
Summary
Over the last thirty years, the scientific community has been extensively involved in research programs aimed at investigating the possibility of an organic approach to the use of precast components, in order to overcome the traditional idea that their design should “emulate” that of cast-in-place structures. Stone and others [5] drew on this initial idea, while experimental evidences were provided by the PRESSS (PREcast Seismic Structural System) research program, carried out at UCSD (California University, San Diego) from 1990 to 1999 [6] During this program extensive experimental tests were performed over models of a 5 storey building (in a 2:3 scale) subjected to seismic loads, providing different types of nodes among the precast elements, both for moment frames and shear walls resisting systems. It has been widely demonstrated through both experimental and theoretical studies that the use of unbonded prestressing tendons as a connection system significantly improves the seismic performance of precast frames, that can so be conveniently used as a competitive option to cast-inplace frames. With this procedure it is possible to attain the required flexural rotational and dissipative capacity as well as the self-centering capacity, that is the ability of the structure to return towards its initial configuration upon unloading
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