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

Many reinforced concrete frame structures built prior to the 1970s were designed only for gravity and wind loads. Compression-only reinforcing details used in the columns of these structures are often associated with nonductile lap splice failures when subject to lateral load. The objective of this study is to investigate the use of carbon fiber-reinforced polymer (CFRP) jackets as a seismic retrofit measure for deficient lap splices.

Three full-scale building column specimens with lap spliced longitudinal reinforcing bars were tested under combined axial and cyclic lateral load. The columns were intentionally designed such that the lap splices would fail prior to achieving the flexural capacity of the column. One column was tested without retrofit as a control specimen, while the others were retrofitted with CFRP jackets. Comparisons are also made with results of tests of similar columns without lap splices.

It is shown that with a CFRP jacket retrofit, the nominal flexural capacity of the column may be achieved. The ductility of the repaired column, however, is limited by slip of the spliced bars resulting in a splitting failure in the lap splice region. Additionally, the behavior of the lap splice is accurately modeled and predicted using existing lap splice behavior models regardless of the presence of the retrofit jacket.

Keywords: column; lap splices; polymer; seismic.

(ProQuest Information and Learning: ... denotes formulae omitted.)

INTRODUCTION

In many regions of North America there exist numerous reinforced concrete frame structures constructed in the 1950s, 1960s and 1970s for which gravity load effects dominated the designs. Many of the construction details used in these structures are now recognized to be associated with nonductile structural failure modes when subjected to seismic loading. As a result, the performance of these structures is considered suspect under moderate to severe seismic loading.

A significant deficiency in these nonductile frames is often the columns (Pessiki et al. 1990). Due primarily to poor confinement of the concrete, these columns are unable to undergo significant lateral deformations while maintaining their lateral-load carrying capacity, and relatively brittle modes of column failure, accompanied by soft story structural failure mechanisms, are possible. Providing additional concrete confinement increases the ductility of these columns. Deficient or poorly located lap splices in buildings have also been shown to result in nonductile failure modes of columns; such details are the focus of this study. Related studies investigating other nonductile column behaviors are reported elsewhere (Harries et al. 1998; Pessiki et al. 2001; Sause et al. 2004).

RESEARCH SIGNIFICANCE

The study reported herein investigates the use of carbon fiber-reinforced polymer (CFRP) composite jackets as a method of seismic retrofit of existing nonductile reinforced concrete building columns having inadequately detailed lap splices. The results indicate that although improvements in ductility can be achieved, these are limited by eventual slip, which is only marginally affected by the confining effects of the CFRP jackets.

EXISTING NONDUCTILE COLUMN DETAILS AND BEHAVIOR

Typical reinforced concrete column details that have been identified as leading to nonductile behavior under seismic loading have been identified and discussed previously (Pessiki et al. 1990; Sause et al. 2004). Details associated with nonductile behavior include:

1. Large spacing of often small diameter ties resulting in poor confinement of the core concrete and lap splice, and inadequate lateral support of the longitudinal reinforcing bars;

2. Poor tie configuration resulting in loss of confinement and loss of longitudinal reinforcing bar support after cover spalling;

3. Inadequate shear capacity due to insufficient transverse steel resulting in large shear cracks and potential splitting cracks; and

4. Lap splices with insufficient length and poor confinement located in regions of high bending moment.

Inadequate detailing of reinforced concrete columns, particularly inadequate size, spacing, and detailing of ties, results in inadequate column ductility. Depending on the nature of the column deficiencies, the frame geometry, and the loading on the column, different modes of column failure may occur. Four nonductile failure modes are classified (Sause et al. 2004) as: 1) axial-flexural failure in the column hinge region; 2) ductile shear failure in the column hinge region; 3) brittle shear failure; and 4) lap splice failure.

Lap splice failures occur when the longitudinal bars of the column develop more force than the lap splice can resist. The failure may result from inadequate confinement in the lap splice region or lap splices that are not long enough to develop the tensile strength of the lapped longitudinal bars. Lap splice failures are characterized by either splitting cracks of the concrete along the interface between lapped bars or shearing of the concrete between bar lugs and relative sliding of the lapped bars.