Comparisons of concrete-encased composite column strength provisions of ACI code and AISC specification

In the ACI-318 code (1999) and AISC-LRFD specification (1993), different approaches are used for the design of concrete-encased composite columns. The calculated member strengths based on these two design provisions may show significant difference in some cases. The objective of this study is to investigate the difference between these two approaches and to evaluate the accuracy of their strength predictions by comparing to 78 physical test results done by previous researchers. This comparative study indicates that the ACI-318 approach generally gives closer predictions than the AISC-LRFD does. The statistical results show that the ACI-to-experimental capacity ratio has a mean value of 0.90 with a coefficient of variation (COV) of 15% and the AISC-to-experimental capacity ratio has a mean value of 0.73 with a COV of 21%. Also investigated herein are the difference of design philosophy between the design provisions, the failure mode of the tested specimens, the column strength interaction diagram, and the effect of steel ratio on the accuracy of the strength predictions.