Analysis and modelling of the seismic behaviour of high ... - Ingegneria

4. SEISMIC RESPONSE OF PARTIAL-STRENGTH COMPOSITE JOINTS
The two material constants are linked to the Mohr-Coulomb constants, viz. the
cohesion and the angle of internal friction, by matching the fictitious tensile strength
ft and the biaxial compressive strength f’bc of concrete according to transformation
formulae in Chen (1982). Moreover, a non-associated flow rule is exploited. The
strain-hardening behaviour of concrete is governed by means of the stress-strain
law of concrete in uniaxial compression or uniaxial tension, complemented with
appropriate post-peak softening rules. In detail, the tension-softening behaviour of
concrete related to its progressive fracturing or tension-stiffening behaviour owing
to the presence of reinforcements is reproduced with exponential decay curves
(Stevens et al, 1991). Confining effects owing to transversal reinforcements and
profiled-steel sheeting are considered in the compression regime by means of the
model of Mander et al. (1988). The concrete model does not embody the specific
fracture energy Gf, to overcome mesh-dependent results (Hilleborg et al, 1976).
However, as the concrete slab is moderately reinforced both in the longitudinal and
transversal direction, the mesh-dependency is small. Moreover, the FE analyses
account for steel nonlinearities using the von Mises yield criterion. Isotropic
hardening is assumed for the analyses. In the analysis the measured stress-strain
properties of the materials obtained by tensile test were used. The elastic modulus
and the Poisson’s ratio were assumed as E=210000 and ν=0.3, respectively.
Longitudinal rebars in the slab are assumed to be made with a hardening elasto-
plastic material and modelled using discrete two-noded beam elements for 3D
models. The discrete representation of the reinforcements is adopted because the
influence of bond-slip is of interest. Thereby, dimensionless bond-link elements are
adopted to connect concrete and steel nodes. In detail, the bond stress-slip relation
is modulated according to the law proposed in Stevens et al. (1991). Friction
between the structural steel and the concrete slab is not modelled because it has
little influence on the substructure responses. Elastic and inelastic convergence
studies have been conducted to evaluate and arrive at the final mesh for the finite
element models.
The reaction force vs. the controlled displacement is illustrated in Figure 4.39,
where the numerical simulations are compared to the envelope curve of the cyclic
experimental response. One may observe that experimental data and numerical
prediction are in a good agreement. Under sagging bending moment the specimen
yield strength is well captured as expected; moreover the numerical simulation
captures very well the hardening branch of the experimental response, both in term
of strength and stiffness; this indicates a satisfactory behaviour of the numerical
model. The model clearly shows the evolution of the distribution of the principal
stresses of compression in the slab for the specimen subjected both under sagging
and hogging bending moment.
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