Seismic Retrofit of Lap Splices in Nonductile Square Columns Using Carbon Fiber-Reinforced Jackets

ACI Structural Journal, Nov/Dec 2006 by Harries, Kent A, Ricles, James R, Pessiki, Stephen, Sause, Richard

DISCUSSION OF EXPERIMENTAL RESULTS

Gravity load-carrying capacity

As noted previously, the axial load applied to the columns was maintained at a constant value throughout testing (refer to Table 3). Following all testing described, the columns continued to support this axial load. The presence of CFRP jackets will help to mitigate any expected degradation in axial load-carrying capacity because the jackets serve to stop concrete cover spalling and provide continued lateral support against buckling of the longitudinal reinforcing bars. Additionally, the presence of the jacket is known to increase the apparent concrete strength and axial deformation capacity of the column, although this effect is relatively small for the large square column sections tested (Carey 2003).

Lap splice slip and load-displacement response

Specimens F0 and L0 are essentially identical except for the 22d^sub b^ lap splice provided in L0. Specimen L0 did not attain the flexural capacity of Specimen F0. The behavior of Specimen L0 was governed by rocking resulting from slip along the lap splice. This rocking is evidenced by the notably pinched response of the hysteretic loops (refer to Fig. 4(a)). One may interpret the onset of lap splice slip in Specimen L0 as the point at which the stiffness begins to diverge from that of Specimen F0. This occurs at an applied lateral load of approximately 120 kN (27.0 kips), corresponding to a column base moment of 288 kN-m (212 kip-ft), approximately 55% of the capacity of the Specimen F0. The onset of lap splice slippage occurs at a lateral displacement of approximately 10 mm (0.4 in.).

Significant slip in Table 3 is defined as the point when the amount of slip exceeds the lug spacing of the spliced bars (Fig. 1, Region 4). This spacing is less than 0.7d^sub b^ (CRSI 1997). The transition from adequate bond stress being developed in the lap splice to slip of the splice results in the pinched hysteretic behavior evident in all of the L-series specimens. Significant slip in Specimen L0 begins to occur during the second cycle to 1.5δ^sub y^.

Specimens L1 and L2 appear to achieve and maintain their flexural capacity (refer to Fig. 4(b) and (c)). The pinching and loss of capacity indicating slip of the lap splice is not observed until the second and third cycles at 5δ^sub y^ for Specimens L1 and L2, respectively, as discussed as follows.

CFRP jacket behavior and strains

Specimens F2, L1, and L2 have essentially the same transverse retrofit jacket detail (4/2 plies). The provision of vertically oriented plies in Specimen L2 should have little effect on the observed transverse strains. The behaviors of Specimens L1 and L2 were limited by their lap splice failures, and these specimens did not achieve as great an overall displacement ductility as Specimen F2. Therefore, there was significantly less distress evident in the jackets of Specimens L1 and L2 than observed in Specimen F2. Indeed, the maximum transverse jacket strains observed in Specimens L1 and L2 were generally only approximately 1/3 those observed in Specimen F2. A more detailed discussion of the jacket behavior of Specimen F2 is presented in Sause et al. (2004).