Aho and Leon 1997
In this report, the design procedures for composite columns and beam-columns in the AISC LRFD (1993) and EC4 (1993) Specifications were assessed and compared. The development of a database for composite column and beam-column tests was then presented. Using the tests results documented in the database, two new design methods for composite members were derived.
In the AISC LRFD (1993) approach, modified material properties are provided for composite members, which are then treated and designed as all-steel sections. However, a stress-based design approach such as this was deemed to be inappropriate given the more complex behavior of composite members. The AISC LRFD (1993) procedure is also not satisfactory for coverage of issues such as confinement and slenderness effects. In addition, the interaction equations often underestimate the capacity of composite beam-columns.
The EC4 (1993) method utilizes individual material properties of the composite section in the design calculations. The confinement effect is taken into account for CFT members depending on the amount of eccentricity and slenderness. A plastic strength approach is applied in the beam-column design method. The interaction diagrams are also simplified with straight lines to avoid any iterative procedures.
The database contained material properties and capacities of specimens in column and beam-column tests from the literature. Steel-reinforced concrete specimens as well as circular and rectangular CFT specimens were included. However, cyclic tests and the tests with low a/D ratios were not covered in the database.
As a first step in developing new design methods for composite members, equations were proposed for the nominal axial strength and nominal moment strength based on calculation of plastic strengths. It was then intended to calculate the axial load capacity of columns by multiplying the nominal axial strength with a reduction factor. Two methods were utilized to calculate the reduction factor. In the first one, the slenderness equation of EC4 was taken as the reduction factor. Alternatively, the column curves in AISC LRFD (1993) were put into a form similar to the one in EC4. The predicted axial load capacities using the reduction factor equations were compared with the experimental values in the database. If good correlation was obtained, the applicability of the equations was checked against the beam-column test data. In the case of unsatisfactory correlation, either the equations were modified until they matched with both the column and beam-column test data or new equations were derived according to the beam-column data and checked for agreement with all of the tests.
Based on the analysis of the test data, seven different modified methods were proposed to replace the AISC LRFD (1993) design procedure. The predicted results were compared with the database and the method having the best correlation was determined. This method was then simplified and two new design methods, which employed the AISC LRFD (1993) column curve for the reduction factor of axial resistance, were derived. The first method was named the “Modified AISC Method” and it involved the replacement of the c1, c2, and c3 factors in AISC LRFD (1993) method for axial strength with the values 1.0, 0.85, and 0.3, respectively. For circular CFTs, factors increasing the capacity still further were used. In the case of beam-columns, a plastic strength approach was utilized using the simplified interaction equation with straight lines. The second method was called the “Plastic Method” and the plastic strength calculations were used for both columns and beam-columns. The axial load capacity for the columns was reduced depending on the slenderness, and confinement was accounted for in circular CFTs. For beam-columns, a simplified interaction diagram was used and the axial load capacity was reduced depending on the amount of eccentricity.
Reference
Aho, M. and Leon, R. T. (1997). “A Database for Encased and Concrete-Filled Columns,” Report No. 97-01, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia.