Kappos and Konstantinidis nonlinear concrete model - con_hs

This is a uniaxial nonlinear constant confinement for high-strength concrete model, developed and initially programmed by Kappos and Konstantinidis [1999]. It follows the constitutive relationship proposed by Nagashima et al. [1992] and has been statistically calibrated to fit a very wide range of experimental data. The confinement effects provided by the lateral transverse reinforcement are incorporated through the modified Sheikh and Uzumeri [1982] factor (i.e. confinement effectiveness coefficient), assuming that a constant confining pressure is applied throughout the entire stress-strain range.

Four model calibrating parameters must be defined in order to fully describe the mechanical characteristics of the material:

Compressive strength - fc
This is the cylinder (150x300 mm) compressive stress capacity of the material. Its value typically varies from 50 MPa up to 120 MPa. The default value is 78 MPa.

Tensile strength - ft
This is the tensile stress capacity of the material. It can usually be estimated as , where kt varies from 0.5 (concrete in direct tension) to 0.75 (concrete in flexural tension), as suggested by Priestley et al. [1996]. When this value is reached, the concrete is assumed to abruptly loose its tensile resistance, without any sort of tension softening. The default value is 4.6 MPa (see note below).

Modulus of elasticity - Ec
This is the initial elastic stiffness of the material. Its value usually oscillates between 35000 and 45000 MPa. The default value is 40742 MPa.

Specific weight -
This is the specific weight of the material. The default value is 24 kN/m3.

Notes

1. The need for a special-purpose high-strength concrete model raises from the fact that this type of concrete features a stress-strain response that differs quite significantly from its normal strength counterpart, particularly in what concerns the post-peak behaviour, which tends to be considerably less ductile.
2. On occasions, depending on the structural model and applied loading, crack opening may introduce numerical instabilities in the analyses. If, on some of those instances, the user is interested in predicting, for example, the top displacement of a building (i.e. global response) rather than accurately reproducing the local response of elements and sections (e.g. section curvatures), then tensile resistance may be simply ignored altogether (i.e. ft=0 MPa), and in this way stability of the analysis will most certainly be achieved in easier fashion.