The typical moment-thrust interaction of a short column represents the
cross-sectional strength. In order to study the behavior of RC and FRPRC
columns, fiber element models were developed by discretizing the
section into a number of integration layers. The concrete was modeled
by a stress-strain curve. Steel reinforcing bars were modeled as elastic
material. The FRP reinforcing bars were assumed to be linear-elastic
with the same modulus of elasticity and strength in tension and
compression. A perfect bond was assumed between the concrete and
reinforcement, FRP, or steel. It was also assumed that the proper
spacing and adequate size of FRP ties can provide the same level of
confinement as steel ties. The tensile strength of the concrete was
ignored, making the analysis slightly conservative. The reinforcement
was placed in one layer on the tension and compression sides of the
section. The effect of the creep and shrinkage of the concrete and the
creep and relaxation of the reinforcement were ignored.
The interaction curves were constructed for each strengthened
column (Fig.8). For each curvature, the maximum compressive
strain of the section was assumed, and an iteration was carried out
until the equilibrium of forces in the section was satisfied. The
process was repeated for increments of curvature until the failure of
the section was triggered by one of the following conditions:
• crushing of concrete at its ultimate strain 0.0035
• rupture of the reinforcement in compression or tension.
The level of the axial load was then increased until the column
failed and in pure compression. The moment-thrust interaction
diagram of the short column was constructed from the maximum
moments at each axial load level.