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10 1.3.5 Compression Members tional to the strength of the transverse confining steel. Mugu- ruma et al. (1990) demonstrated very high axial ductilities The concept of providing transverse reinforcement to using high-strength transverse reinforcement and reported concrete compression members is intended to improve the yielding of transverse reinforcement having yield strengths of strength and ductility. As a concrete column is compressed axi- 198 ksi shortly after the peak axial load is achieved. Yong et al. ally, it expands laterally. This lateral expansion is resisted by the (1988) observed two peaks on their axial load-deformation transverse reinforcement and a lateral confining pressure in the responses; the high-strength transverse reinforcement did concrete core is developed. Concrete strength and deformabil- not yield initially but had yielded at the second peak. Mugu- ity are enhanced by the resulting state of multi-axial compres- ruma et al. (1991) suggest that high-strength transverse rein- sion (Richart et al. 1928 and countless researchers since). forcement offers better control of longitudinal bar buckling Current design philosophy for compression members than normal strength confining steel. Cusson and Paultre equates the expected loss of axial load carrying capacity due (1994) report improvements in strength and ductility due to to cover spalling to the expected strength gain of the remain- high-strength confining steel only for well-confined columns. ing core due to the presence of confining reinforcement. This Improvements in axial column behavior with high-strength approach was developed and calibrated for columns fabricated transverse reinforcement have also been reported by Bjerkeli with what today may only be considered normal, or moderate et al. (1990), Nagashima et al. (1992), Razvi and Saatcioglu strength, concrete (fc < 8000 psi) and normal strength confin- (1994), and Nishiyama et al. (1993). Studies that have specif- ing steel (fy = 60 ksi). There is a perceived need for greater ically used A1035 transverse reinforcement have provided confinement for high-strength concrete than what is required similar conclusions (Restrepo et al. 2006, Stephan et al. 2003, for normal strength concretes (ACI 363 1992). Strength and deformability of concrete are known to be inversely propor- El-Hacha and Rizkalla 2002). tional; therefore, more confinement is required in order for high-strength concrete columns to reach levels of deformation 1.3.6 Bond and Development expected of well-detailed normal-strength concrete columns. In general, the degree of improvement in both axial capacity Bond characteristics of ASTM A1035 reinforcing bars and ductility due to the provision of confinement is inversely should not be expected to differ significantly from those of proportional to the unconfined concrete strength (Pessiki conventional reinforcing steel grades since neither the steel et al. 2002, Carey and Harries 2005). The use of high-strength modulus nor bar deformations differ (Ahlborn and Den transverse reinforcement represents one manner by which Hartigh 2002, Florida 2002). Studies that have reported load- this additional confinement may be realized. slip relationships for A1035 steel have not concluded that Confining pressures are generated from tensile forces in the these differ in any significant manner from similar relation- transverse reinforcement that result from lateral expansion of ships established for A615 bars (Ahlborn and DenHartigh the axially loaded concrete. As the lateral expansion is depend- 2002, El-Hacha et al. 2002 and 2006). Limited evidence ent on the mechanical properties of the concrete, the lateral (Sumpter 2007 and Zeno 2009) suggests modestly improved strains, particularly in high-strength concrete, may be insuffi- bond behavior that is believed to be associated with the rib cient to engage the higher confining pressures made possible geometry resulting from the rolling of the tougher A1035 by the use of high-strength transverse reinforcement (Martinez material. Nonetheless, this effect is modest and cannot be et al. 1982, Pessiki and Graybeal 2000). An additional, related generalized across material heats. consideration is that the transverse strains that engage the Due to the higher bar stress to be developed, A1035 bars confining reinforcement must be limited to ensure continued require a longer development length (ldb). However, simply resistance to shear. The maximum permitted transverse strain increasing development length without providing confine- in this regard is often reported as 0.004 (Priestley et al. 1996). ment is an inefficient means of developing greater stresses Previous research offers differing conclusions with respect (Seliem et al. 2006 and 2009, El-Hacha et al. 2006). With long to the use of high-strength transverse reinforcing steel. Ahmed development (or splice) lengths, the bond stress at the "front" and Shah (1982) demonstrated analytically that high-strength of the development length is exhausted before the bond stress transverse reinforcement may enhance the ductility of a col- along the entire development length can be developed umn while having little effect on its strength. Martinez et al. (Viwathanatepa et al. 1979). (1982) propose limiting the strength of transverse reinforce- Confining reinforcement around development regions or ment, based on their results showing that the higher steel splices is required to control the splitting cracks associated with strength was not fully utilized. Pessiki and Graybeal (2000) a bond failure (Seliem et al. 2009). With higher strength steel, also conclude that the yield capacity of high-strength transverse greater bar strain and slip will occur prior to development of reinforcement cannot be developed. Polat (1992) reported that the bar. The associated displacement of the bar lugs drives the ductility and strength enhancements were less than propor- splitting failure beyond that where yield of conventional bars