Method of solution
The effect of confinement is different for centric- and for eccentric
loading as indicated for linearly elastic materials in Fig. 9.
In case of centric load the stress distribution is uniform and
hence the confining stresses are equal. In case of pure bending,
at the compressed side of the cross-section the core material expands
laterally creating compressive confining stresses. The confining
stresses vary along the circumference of the core. To reach
an equilibrium in the cross-section, shear stresses arise between
the core and the confining FRP. In case of eccentric compression,
the in-plane stress state will be in-between the two previous cases.
For centrically loaded rectangular cross-sections the confining
(in-plane) stresses vary: high at the corner and low at the middle
of the cross-section (Fig. 10).
The core is concrete, which is a plastic material with no tensile
strength. When concrete starts yielding (and looses its tensile
resistance) the behavior is similar, however the calculation is more
complex.
To follow this behavior, we decided to use an ‘‘analysis oriented’’
model, which is based on a new, sophisticated 3D concrete
material law proposed by Papanikolaou and Kappos [24].