Seismic Design Criteria for Circular Lap-Spliced Reinforced Concrete Bridge Columns Retrofitted with Fiber-Reinforced Polymer Jackets

ACI Structural Journal, May/Jun 2005 by Elsanadedy, Hussein M, Haroun, Medhat A

A statistical study was carried out on (V^sub u-exp^/V^sub u-th^), (Δ^sub u-exp^/Δ^sub u-th^), and (µ^sub Δu-exp^ /µ^sub Δu-th^) ratios. For the different confinement and bond-slip models, statistical parameters were computed for the tested-to-calculated ratios. It should be noted that Samples CF-R1 and CF-R2 were excluded from the statistical study because they were designed incorrectly by the manufacturer according to a jacket strain of 0.004 instead of 0.001 as required by Caltrans.12 Based on the best-fit models, summaries of statistical analysis of the (V^sub u-exp^/V^sub u-th^), (Δ^sub u-exp^/Δ^sub u-th^), and (µ^sub Δu-exp^/µ^sub Δu-th^) ratios are listed in Table 5 for Samples CF-R3 to CF-R10.

Seismic retrofit design of lap-splice columns will be based on a demand ductility that is set by current seismic design codes. The capacity-to-demand ratio should be always greater than 1.0. To implement safety factors in the retrofit design methodology, the predicted ultimate displacement and ductility for the retrofitted bridge column should be reduced and then compared to the demand values. Accordingly, retrofit design safety factors for both ultimate displacement and ultimate ductility should be knockdown coefficients, which, of course, have to be based on statistical lower bounds for (Δ^sub u-exp^/Δ^sub u-th^) and (µ^sub Δu-exp^/µ^sub Δu-th^) ratios. Retrofit design factors for both ultimate displacement and ultimate ductility are based on statistical lower bounds for (Δ^sub u-exp^/Δ^sub u-th^), and (µ^sub Δu-exp^/µ^sub Dgr;u-th^) ratios in terms of minimum value and (m - σ).

Following its design for ductility capacity-to-demand ratio greater than 1.0, the retrofitted lap-splice bridge column should be protected against unfavorable brittle modes such as shear friction failure at the column base. Accordingly, the demand lateral load should correspond to maximum feasible extreme estimates of flexural strength developing at the plastic hinge location after column retrofit. Therefore, the retrofit design factor for maximum lateral load should be based on statistical upper bounds for (V^sub u-exp^/V^sub u-th^) ratio in terms of maximum value and (m σ).

Based on modified Xiao's bond-slip model along with Hosotani's confinement model, statistical lower and upper bounds of (V^sub u-exp^ /V^sub u-th^), (Δ^sub u-exp/Δ^subu-th^), and (µ^sub Δu-exp^/µ^sub Δu-th^) ratios were computed and presented in Table 5 for circular composite-jacketed columns. In addition, recommended retrofit design factors are listed in Table 5.

RETROFIT DESIGN METHODOLOGY

The following methodology is proposed for seismic retrofit design of circular reinforced concrete columns with poor lap-splice details using FRP jackets. The retrofit design methodology is divided into five major steps: the first is to attain the design properties of the materials as discussed in a companion paper by the authors;13 the second is to seismically assess the existing column. The third step involves the design of the FRP jacket for flexural ductility enhancement as the most stringent of requirement for confinement of the compression concrete within the plastic hinge zone, antibuckling constraint, and requirement for clamping on the lap-splice region. Step 4 involves checking jacket design to mitigate high diagonal compression stress levels in the jacketed column and to preclude shear-friction failure at the column base. The last step is to design the extent of the FRP jacket. These steps are detailed as follows.