By using GFRP laminates, the ultimate rotation increase goes from 98% for one GFRP ply installed and becomes about 138 and 169% for two and three GFRP plies, respectively (see Table IV)。 Since the design goal was to allow the structure to withstand a 0。3g PGA level and given that the theoretical analysis indicates that a 48% structural deformation capacity increase was necessary to pursue this objective, it was estimated that an increase in the local rotation capacity of the plastic hinge at least twice that of the original member could have been necessary。 Importantly, the local increase in the rotation capacity is not proportional to the increase in the global deformation capacity。 Thus, based on such considerations, the first trial in the design of the GFRP confinement was chosen as two plies of laminates with a unit weight of 900 g/m2 applied to all the square columns and extended by a length greater than that of the effective plastic hinge (about 380 mm)

computed by Equations (3) and (4)。 Furthermore, in order to validate the design choice, a nonlinear static pushover on the FRP-retrofitted structure was provided at the end of the design process by using the SAP2000 [8] analysis program。

5。2。 Design of shear strengthening: beam–column joints and wall-type column

To avoid the attainment of shear strength of exterior joints caused by increasing the ductility of columns, which is brittle and could be detrimental to the global performance, further FRP was designed on beam–column joints corresponding to the corner square columns C2, C5, C7 and C8。 The original shear strength of the exterior joints was computed by using equations provided   by

Ordinance 3431 [13]。 Using this seismic guideline, the principal tensile stress of an exterior joint,

ont, may be determined by using the following equation:

where N is the axial force in the upper column, Ag is the horizontal joint area, Vn is the acting shear on the文献综述 joint due to the contributions of both shear force on the upper column and tensile reinforcement on the beam and  fc is the compressive concrete strength。 By assuming ont equal to

0。3√ fc, it was possible to compute, for each exterior joint of the structure, the horizontal ultimate

shear force and the corresponding shear strength, v0,max(Vn/Ag), under which tensile joint failure is achieved。

Theoretical results, in terms of original joint shear strength, v0,max, with reference to the external joints at each story, along with the axial force due to only gravity loads, are summarized in Table V。 Since theoretical simulations of the first round of tests predicted shear stresses on the exterior joints comparable with those reported in Table V (e。g。 1。87 and 2。01 MPa vs 1。82 and

Table V。  Shear strength of the un-strengthened and GFRP-retrofitted corner   joints。

GFRP-retrofitted joint shear strength vmax  (MPa)

Floor Exterior

joint column Axial force,

N (N) Original joint shear

strength v0,max  (MPa)

1 ply

2 plies

3 plies

1st story C5 59 100 1。92 3。40 4。46 5。34

C8 44 280 1。82 3。26 4。48 5。47