principles of the rehabilitation strategy are outlined in the following sections with reference to two main issues: (1) design of column confinement; (2) design of exterior beam–column joints and wall-type column shear strengthening。

5。1。 Confinement  of columns

Both the experimental activity and theoretical assessment of the ‘as-built’ structure showed that the columns’ cross-sectional dimensions and the amount of longitudinal steel reinforcement were inadequate to satisfy the demand generated by the biaxial bending associated with the axial load; the weak column–strong beam condition led to the formation of plastic hinges in the columns。 With a view to a ‘light’ strengthening intervention, it was decided to increase the ductility of the plastic hinges at column ends without changing their position rather than establishing a correct hierarchy of strength by relocating them。 Indeed, in this latter case, flexural strengthening of the columns with proper anchorage at their ends would have been necessary。

This objective was pursued by GFRP columns’ confinement, which allows the ultimate concrete compressive strain to be enhanced。 This corresponds to an increase in the curvature ductility that, assuming a plastic hinge length not significantly affected by the retrofit intervention, leads to a proportional increase in the plastic hinge rotation capacity。

In order to compute the axial strain of the FRP-confined member, the equation provided by the latest guideline developed by the Italian National Research Council, CNR-DT 200/2004 [14] was used

where the ultimate axial strain for FRP-confined concrete, nccu, is computed as a function of design compressive concrete strength, fcd, and the effective lateral confining pressure, fl,eff ( fl,eff = keff fl, where keff is the coefficient of effectiveness depending on the cross-section shape and FRP config- urations, and fl is the confining lateral pressure depending on the geometric strengthening ratio, qf = 2tf · (b + d)/b · d (tf is the FRP thickness, b and d are cross-section dimensions), the FRP modulus of elasticity and design strain)。 The equations to compute the coefficient of effectiveness, keff, and the confining lateral pressure, fl, are given in CNR-DT 200/2004 [14]。 Considering that calculations are referring to an existing structure, the design compressive concrete strength was assumed as the average compressive concrete strength obtained by field tests, fcm = 25。5 MPa。

To quantify the amount of FRP to be installed, the central column, C3, was selected for calculations since it carries the maximum axial force due to the gravity loads ( P = 409 kN at first story); thus, it has the minimum rotational capacity。 In Table IV, theoretical results in terms of

concrete ultimate axial strain provided by Equation (5), along with the ultimate curvature (calculated based on section analysis), are reported for one, two and three plies of uniaxial GFRP or CFRP confinement (with unit weight of 900 and 300 g/m2 and thickness of 0。48 and 0。166 mm/ply, respectively)。 The last two columns summarize increases in the ultimate rotation and the percentage rotation increase with respect to the original configuration, Aabs。 The ultimate rotation values were computed with reference to Equation (3)。

On the right-hand side of Figure 6, the moment–curvature relationship is plotted for the original C3 column cross-section (continuous line) under the axial load acting at first story ( P = 409 kN, due to only the gravity loads); the dashed line shows how the moment–curvature relationship changes as one ply at a time of GFRP confinement is added。 The same graph is plotted in the

Table IV。 Influence of GFRP and CFRP confinements on concrete ultimate axial strain, ultimate curvature and ultimate rotation。