Uniaxial Interaction Charts for Fully Encased Composite Columns

No Thumbnail Available



Journal Title

Journal ISSN

Volume Title


Addis Ababa University


Nowadays, steel-concrete composite construction is used to meet performance and functional requirements of mid to high-rise structures as well as large span structures. These structures acquire the structural and constructional advantages of both the concrete and steel. Among composite members is a composite column. A steelconcrete composite column is a compression member, comprising either a concrete encased steel section or a concrete filled tubular steel section. The resistance of a composite column to combined compression and bending is determined using an interaction curve of its cross-section. Developing interaction curves requires rigorous section analysis. For this reason, different practice codes incorporated simplified analysis and design procedures. Among which, the Eurcode 4 provides a simplified method for composite sections and columns satisfying certain requirements. According to this method, the axial force-moment interaction curve of a composite cross-section is obtained by assuming full plastic stress distribution. Furthermore, the Eurocde 4 approximates the entire interaction curve by a polygon made up of four or five points on the interaction curve. Despite these simplifications and approximations, the analysis of a composite section is yet computationally demanding. In addition to this, for composite sections that violate the code requirements a more rigorous analysis is mandatory. This has been a major disincentive for using composite frames. Despite their advantages, the topic of composite columns is given few attentions in the Ethiopian construction industry. Moreover, neither design aids nor analysis tool haven’t been developed yet to assist structural engineers in analysis and design of composite columns. In this thesis, more accurate uniaxial interaction curves are developed for “I” and “H” steel sections that are fully encased in concrete. The stress resultants were evaluated starting with strain distributions in the ultimate limit states that were adopted from the Eurocode 2. The stress-strain laws of materials were taken from Eurocode 2 and Eurocode 3. The stress resultants of the concrete and structural steel involved double integrals of the stress over the compressed regions of the concrete and structural steel section as well as over the tensioned regions of the steel section. These integrals were iv then transformed into line integrals by using Green’s theorem. Finally, the line integrals were solved using Gauss Quadrature which is numerically exact method with the adopted material laws. Double counting of the concrete area in compression zone which is replaced by the structural steel section and the reinforcement was avoided. The uniaxial curves developed were verified against the outputs of the software called MASQUE. The comparisons indicate that the developed interaction curve is almost identical with that of MASQUE output. To increase the applicability of this study, especially in the Ethiopian market, a computer program UICISEC with a friendly graphical user interface is incorporated. Finally, a design example for a column length subjected to biaxial bending was carried out according to the Eurocode 4 simplified method. Here, the uniaxial interaction curves of this study were utilized. Key Words: composite column, fully encased composite section, strain distribution in the ULS, section analysis, stress resultants, column length



composite column; fully encased composite section; strain distribution in the ULS; section analysis; stress resultants; column length