Abstract
The plastic energy dissipation (PED) and ductility are important parameters of RC structures in seismic zones. Several factors influence these parameters, mostly negatively, such as symmetrical reinforcing of RC section, over or under reinforcing, prestressing of concrete and gravitation loads. The influence peculiarities of reinforcing on its PED and ductility were analysed in a previous paper of this author. In this paper the influence of RC section initial stresses due to prestressing and gravitational loading on its plastic deformations is presented. Based on experimental results, it is assumed that the concrete modulus of elasticity increases linearly versus the compressive strength of concrete. Accordingly the ultimate elastic and ultimate plastic deformations were calculated for each concrete class. The values of the compression section ductility as a function of the concrete strength are obtained. It is shown that this function is parabolic. For the prestressed concrete section, the ductility factor µ is limited to 3 ≤ µ < 6 (unconfined section). Since the compression steel strength can not exceed 400 MPa (as demonstrated by the steel-concrete common deformation equation), high-strength steel (HSS) for the prestressed concrete (PC) bending section does not contribute to its PED in the compression zone. In addition, when the tensile zone is analysed, the initial stresses in the HSS lead to significant decreases of the section PED and ductility. Also there is the problem of determination of the HSS conditional yield deformation. It is necessary to take into account that the PC section frequently is over-reinforced. The same problems arise when the RC section is under initial stresses due to gravitational (static) loading. The influence of gravitational stress on RC section PED under seismic forces is investigated applying the non-symmetric cyclic analyses. In this case the hysteretic loops area is reduced and accordingly the ductility factor decreases.
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