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Structural performance of confined high strength concrete columns

Friis, J. (2003). Structural performance of confined high strength concrete columns. (Unpublished Doctoral thesis, City, University of London)


The mechanical and physical properties of high strength concrete make it an attractive building material, especially for components resisting high compressive loads such as the lower columns in high-rise buildings. However, due to the brittle nature of this material, concern exists about the ductility of high strength concrete columns. This thesis investigates the possibility of improving the ductility of slender high strength concrete columns by means of hoop reinforcement.

A total of twelve full-scale columns with 250 mm square cross-sections was experimentally tested under either uniaxially or biaxially eccentric compression. The columns had effective lengths of either 4 m or 8 m, and were transversely reinforced by hoops with spacings ranging from 200 mm to 50 mm. Observations were made on failure mode, axial loads, deflections and strains. The tests indicated that a dense reinforcement cage had some effect of enhancing both the strength and the pre-peak ductility of the columns. The maximum compressive concrete strains at failure were significantly less than the 3.5 mm/m used in traditional stress block design of normal strength concrete sections. For none of the columns was the hoop reinforcement found to yield at the time of strength failure.

A new confinement model, which is equally valid for normal and high strength concrete, was developed. According to this model the effect of confinement on the complete stress- strain behaviour of concrete can be expressed directly through its influence on the strength and corresponding strain. Empirical equations for estimating both of these quantities are presented. By modifying a well-known method for calculating the effective confining pressure the stress-strain model is shown to be equally capable of describing test results obtained under passive and active confinement conditions.

A computer program, which incorporated the findings from the investigation into the modelling of confinement effects, was developed for the analysis of slender high strength concrete columns. From a parametric study, backed up by a survey of published information on eccentrically loaded reinforced concrete columns, it was demonstrated that the unconfined concrete cover plays a major role in negating the structural benefits of confinement, and that this is especially the case for high strength concrete columns. For a given column slenderness and load eccentricity the deflections at strength failure were found to be largely independent of the concrete strength. However, in terms of load capacity high strength concrete was shown to be most effective in short columns subjected to nearly concentric compression.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics > QA75 Electronic computers. Computer science
T Technology > TA Engineering (General). Civil engineering (General)
Departments: School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
School of Science & Technology > Engineering
[thumbnail of Friis thesis 2003 PDF-A.pdf]
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