Uniaxial Interaction Charts for Fully Encased Composite Columns
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Date
2015-12
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Addis Ababa University
Abstract
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
Description
Keywords
composite column; fully encased composite section; strain distribution in the ULS; section analysis; stress resultants; column length