JOURNAL OF STRUCTURAL ENGINEERING / SEPTEMBER 1999 / 1009 STRENGTH AND DUCTILITY OF CONCRETE ENCASED COMPOSITE COLUMNS By Sherif El-Tawil, 1 Associate Member, ASCE, and Gregory G. Deierlein, 2 Member, ASCE ABSTRACT: Concrete encased composite column design provisions of the American Concrete Institute Code (ACI 318), AISC-LRFD Specification, and the AISC Seismic Provisions are reviewed and evaluated based on fiber section analyses that account for the inelastic behavior of steel and concrete, including the effects of strength and confinement on the concrete’s stress-strain properties. Trial column designs are analyzed to evaluate their strength and ductility as a function of the ratio of structural steel to gross column area, the nominal compression strength of concrete, and confinement of concrete by seismic hoop reinforcing. The analyses highlight known differences in the calculated nominal strength requirements between the ACI 318 and AISC-LRFD provisions and suggest a review of criteria used to establish the limits of the provisions. Compared to columns with low- to medium-strength concrete, columns with high-strength concrete = 110 MPa) are shown to rely to a greater ( f ' c degree on hoop reinforcement to provide the necessary ductility for seismic design. INTRODUCTION Due to the traditional separation of structural steel and re- inforced concrete design and construction, concrete encased composite steel-concrete columns have not received the same level of attention as steel or reinforced concrete columns. This is evident by incomplete and sometimes conflicting provisions for concrete encased composite columns in current design codes and standards. For example, provisions for concrete en- cased composite columns in the American Concrete Institute (ACI) Code (ACI 318) (‘‘Building’’ 1995) and the AISC- LRFD Specification (Load 1993) have been shown to give significantly different values of calculated member strengths as each treats composite columns through extensions of criteria for reinforced concrete and steel members, respectively [e.g., Furlong (1983), Griffis (1992), and El-Tawil et al. (1995)]. Moreover, until the recent publication of the NEHRP Rec- ommended Provisions (NEHRP 1994, 1997) and AISC/LRFD Seismic Provisions (Seismic 1997), there were no codified cri- teria in the United States for the design of composite columns in regions of high seismicity. Recent applications of concrete encased composite columns as a cost-effective alternative to steel columns in mid- to high-rise buildings and developments related to high-strength concrete and seismic design motivate the review of composite column behavior and current design provisions. The use of high-strength concrete for seismically designed composite columns is of particular concern because of the in- herent brittleness of high-strength concrete that may reduce member ductility. Test results indicate that high-strength re- inforced concrete members possess sufficient ductility for seis- mic applications provided that the concrete is well confined (Muguruma and Watanabe 1990; Muguruma et al. 1991; Sheikh et al. 1994; Cusson and Paultre 1994, 1995). However, uncertainty remains as to what constitutes adequate confine- ment for reinforced concrete and encased composite columns and whether current specifications are adequate (Collins et al. 1993). For composite columns, additional concerns include whether strain compatibility is maintained between the con- 1 Asst. Prof., Dept. of Civ. and Envir. Engrg., Univ. of Central Florida, Orlando, FL 32816-2450. E-mail: el-tawil@mail.ucf.edu 2 Assoc. Prof., Dept. of Civ. and Envir. Engrg., Stanford Univ., Stan- ford, CA 94305-4020. Note. Associate Editor: Takeru Igusa. Discussion open until February 1, 2000. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on December 4, 1998. This paper is part of the Journal of Structural Engineering, Vol. 125, No. 9, September, 1999. ASCE, ISSN 0733-9445/99/1009–1019/ $8.00 + $.50 per page. Paper No. 19768. crete and structural steel and the difficulties in detailing seis- mic hoop reinforcement around the steel core. On the other hand, the encased steel core provides redundancy to carry load following deterioration of the concrete that may occur under strong earthquakes. The purpose of this paper is to review design criteria for concrete encased composite columns with emphasis on seismic behavior and the use of high-strength concrete. Strength and ductility of composite columns are studied using a fiber anal- ysis technique that accounts for the inelastic stress-strain re- sponse of steel and concrete. Considered is the change in com- posite column behavior as a function of the ratio of structural steel to gross column area, the nominal compression strength of concrete, and concrete confinement by reinforcing bar ties. The discussion is limited to short columns where slenderness effects are not considered. Slenderness effects are discussed elsewhere (El-Tawil and Deierlein 1996). OVERVIEW OF DESIGN PROVISIONS Basic strength design provisions for concrete encased com- posite columns in the United States are included in the ACI 318 Code (‘‘Building’’ 1995) and the AISC-LRFD (Specifi- cation (Load 1993). The AISC-LRFD composite column pro- visions can be traced to a Structural Stability Research Council (SSRC) task group report (SSRC 1979) that proposed a design model consistent with the then current AISC/LRFD allowable stress specification. The SSRC report stipulated that columns with a ratio of encased steel area to gross area of A s /A g > 0.04 should be designed as composite and those with A s /A g < 0.04 should be designed as reinforced concrete. Accordingly, the AISC-LRFD Specification limits its definition of and require- ments for composite columns to members with A s > 0.04A g . Although the ACI 318 Code has not applied a comparable restriction of A s < 0.04A g , this limit is implied by the seismic design provisions for composite structures described below. The AISC/LRFD Seismic Provisions for Structural Steel Buildings (Seismic 1997) has a new section, entitled Part II — Composite Structural Steel and Reinforced Concrete Build- ings, that includes requirements for the seismic design of com- posite and hybrid structures. Modeled after provisions origi- nally published in the NEHRP Recommended Provisions for Seismic Regulations in New Buildings (NEHRP 1994), the AISC Seismic Provisions are referenced in the NEHRP Rec- ommended Provisions (NEHRP 1997) and the seismic require- ments of the proposed International Building Code 2000 (IBC 2000) (International 1997). The AISC/LRFD Seismic Provi- sions include requirements for concrete encased composite columns that build upon the AISC-LRFD and ACI 318 pro- visions and adopt the ratio of A s /A g = 0.04 as a criterion to