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 without consideration of seismic loads. Compression-only or gravity-only reinforcing details typically short, poorly confined lap splices used in the columns of these structures are often associated with nonductile lap splice failures when the column longitudinal reinforcement is subjected to tension due to bending moment as the frame responds to lateral loading. The objective of this study is to investigate the use of CFRP jackets as a seismic retrofit measure for such deficient lap splices.

The research reported in this paper was part of a comprehensive study of nonductile reinforced concrete building columns, conducted at Lehigh University, that investigated each of the aforementioned modes of failure (Walkup 1998; Patel 2000; Harries et al. 1998; Pessiki et al. 2001; Sause et al. 2004). It is noted that this study addresses reinforced concrete building columns, as opposed to bridge pier/ columns addressed by many other studies. Building columns typically have different geometry and reinforcing details than bridge pier/columns, and the ratio of serviceto- ultimate load capacity is generally greater.

LAP SPLICE BEHAVIOR

Lap splices require sufficient bond between the reinforcing steel and surrounding concrete to develop the capacity of the spliced bars. As noted previously, lap splices may exhibit one of two modes of failure, namely:

1. Splitting of the cover concrete along the weak plane formed by the lapped bars resulting in a loss of bond; or

2. Shearing of the concrete between bar lugs resulting in the pullout of the bar from the concrete mass.

Deficient lap splices are unable to develop the full tensile capacity of the spliced bars. Lap splice design equations (ACI Committee 318 2005; CEB 1990) are based on providing sufficient bond stresses to develop the bars. These equations are aimed at preventing the potential splitting cracks (Mode 1, shown previously), which will result in a significant loss of bond. Upper limits on the bond stresses for design ensure that a pullout failure (Mode 2, shown previously) will not occur (ACI Committee 318 2005; ACI Committee 408 1990; Hamad and Mansour 1996; Orangun et al. 1977). The upper limit on the bond stress is important in that it implies that the capacity of a lap splice may be controlled by pullout, which results from the shearing of the concrete surrounding the rolled-in deformations (lugs or ribs) of the reinforcing steel.

Bond stress-slip relationship

Figure 1 shows an idealized bond stress-slip relationship for a lap splice (CEB 1990). The values of parameters proposed for use with this model are given in Table 1. These values generally agree with other bond stress-slip models. The values presented are considered valid for reinforcing steel having typical lug geometry and for good bond conditions, implying that the reinforcing steel is clean, splices are generally in contact, the concrete is well consolidated, and curing was performed to minimize shrinkage effects. The definition of confined concrete in relation to concrete cover provided and/or transverse reinforcement is given in Table 1.