to 3, KL3, with a corresponding confidence factor, CF, equal to 1。 The significant damage limit state, (LSSD), was investigated to assess the structural capacity at both 0。20g and 0。30g PGA level in directions PX –NX and PY –NY , respectively。 The pushover curves on the FRP-retrofitted structure for each direction analyzed are reported in Figure 8。 The theoretical results in terms of maximum base shear, Fmax, top displacement, dmax, and absolute inter-story displacement are summarized in Table III。 Seismic demand was computed with reference to the same design spectra analyzed in the ‘as-built’ configuration (see Figure 4) scaled at 0。20g and 0。30g PGA level。 The results in terms of maximum top displacement required for each investigated PGA level and direction are summarized in Table III, which shows that the FRP retrofitted structure is able to satisfy the LSSD in each direction at both 0。20g and 0。30g PGA level。 In particular,来*自-优=尔,论:文+网www.chuibin.com

PY_FRP NY_FRP PX_FRP NX_FRP LSSD

-0。15 -0。10 -0。05 0。00 0。05 0。10 0。15

Top displacement [m]

Figure 8。 Pushover curves for the assessment of the FRP-retrofitted   structure。

Figure 9。 FRP-retrofitted structure elastic and inelastic demand spectra vs capacity    diagram。

verification is also satisfied in direction NX where the maximum gap in terms of displacement demand was recorded for the ‘as-built’ structure: capacity is increased up to 0。1076 m (0。0626 m in the ‘as-built structure’) while demand at the target seismic level intensity, 0。30g, is equal to 0。0927 m。 These results are illustrated in Figure 9 where the seismic demand and structural capacity of the FRP-retrofitted structure are determined (by using the CSA) in direction NX , for the two levels of ground motion analyzed。 However, in the case of the retrofitted structure at the 0。30g PGA level, the most critical verification is in direction NY for which the capacity

As Built Structure (PX) FRP Retrofitted (PX)

Figure 10。 Theoretical seismic performance comparison at 0。3g PGA between ‘as-built’ and FRP-retrofitted structure。

displacement is 0。0908 m, whereas the demand is equal to 0。0906 m。 In order to show the increase in deformation capacity provided by FRP confinement in each direction, LSSD verification at 0。30g PGA by using the CSA is reported in Figure 10: on the left-hand side, the ‘as-built’ structure

is analyzed, while on the right the theoretical predictions for the FRP-retrofitted structure are plotted。

Figure 10 clearly shows that the column confinement provides the structure with significantly enhanced ductility, allowing it to achieve the theoretical demand by only playing on the plastic branch of the base shear top displacement curve。

7。 TESTS ON THE RETROFITTED STRUCTURE

Once testing of the ‘as-built’ structure was completed, prior to laminates installation, unsound concrete was removed in all the parts of the elements where crushing was detected。 The original cross-sections were then restored using non-shrinking mortar。 In addition, all cracks caused by the first round of testing were epoxy injected。 Then, according to the design of the retrofit illustrated above, the eight square columns were all confined at the ends by using two plies of GFRP uniaxial laminates, each with a unit weight of 900 g/m2。 At each story, GFRP confinement was extended for 800 mm from the beam–column interface。 In some  cases,  this  length  was  increased  up  to 1000 mm to account for the more extended concrete damage (see Figure 11(a))。 Beam–column joints corresponding to the corner square columns (C2, C5, C7 and C8) were strengthened using two plies of quadriaxial GFRP laminates having each a unit weight of 1140 g/m2。 This joint reinforcement was extended on the beams by 200 mm on each side in order to U-wrap it and to ensure a proper bond。 The joint strengthening intervention scheme along with the joint internal and external view after the retrofit is presented in Figure 11(a)。 The GFRP panels on the joints were not connected to the  columns。  Indeed,  the  continuity  of  external  reinforcement  can  vary the strength hierarchy of the connection and reduce the contribution of fixed end rotation to the rotation capacity of the column。 Therefore, the plastic hinge length of the rehabilitated columns was assumed comparable with that of unconfined columns。 The shear strength scheme of column C6 and an overview of the whole structure after the retrofit intervention are presented in Figure 11(b)。 Once FRP retrofitted, the structure was first tested with a PGA level of 0。20g, to have a direct comparison with the previously executed experiment, then with a PGA level of 0。30g。 The experimental activity showed that the retrofitting intervention provided the structure with a very significant enhanced deformation capacity with respect to the ‘as-built’ configuration, which almost totally lacked the appropriate capacity to resist even the 0。20g PGA level of excitation。 After the vertical elements and the joints were wrapped with glass fibers, the retrofitted structure was able to withstand the higher (0。30g PGA) level of excitation without exhibiting significant damage。 After tests, FRP was removed and it was shown that the RC core was neither cracked nor damaged。 The comparison of the column damage after tests on both the ‘as-built’ and the FRP-retrofitted structure is  reported  in  Figure  12。  The  experimental  outcomes  are  summarized in Table I for both PGA levels of 0。20g and 0。30g for directions X and Y , respectively; very similar behavior emerges between the ‘as-built’ and retrofitted structure at the same seismic level intensity (0。20g)。 Indeed, the maximum base shears recorded were 195 and 211 kN in direction X , and 276 and 287 kN in Y , for the ‘as-built’ and retrofitted configuration, respectively (increase of about 8 and 4%, in directions X and Y , respectively)。 The same trend was recorded in terms of top displacement: the maximum difference recorded was about 9% in direction Y (0。1031 m vs 0。1125 m for the ‘as-built’ and  retrofitted  structure,  respectively)  confirming  that,  as masses and strength do not significantly change, the retrofit intervention does not modify the structural response。  Moreover,  on  increasing  the  seismic  level  intensity  up  to  0。30g,  the  maximum  base