Triaxial effects in concrete-filled tubular steel columns
The work described in this thesis consists of three phases:
Phase I - Uniaxial analysis: A computer program has been written
in a form suitable for calculating the failure loads for a large number of
eccentrically-loaded, circular or rectangular concrete-filled columns,
using the uniaxial material properties and the part cosine-wave assumption.
The experimental failure loads of 22 eccentrically-loaded square and
rectangular columns are compared with the computed loads, and a satisfactory
agreement is found, i.e. triaxial effects are negligible for the
Phase II - Stub columns: The elasto-plastic biaxial stresses in
the steel are calculated from observed strains for 14 of the available
tests on concentrically-loaded stub columns, using the generalised flow-law
for plastic solids. The triaxial stresses in the concrete are then
calculated from simple statics. It is found that near failure the
longitudinal compression in the steel is approximately equal to threequarters
of the uniaxial yield stress, and the hoop tension is half the
longitudinal compression in magnitude, while the longitudinal compression
in the concrete core is twice the strength of uncontained concrete.
The equivalent longitudinal 'stress-strain relationships' for the
steel and concrete are represented by equations, and a formula is given
for predicting the failure load of concentrically-loaded stub columns.
Phase III - Triaxial analysis: Moment-load-curvature characteristics
are experimentally determined for 35 circular columns comprising 7 tube
thicknesses with 5 levels of axial load for each thickness0 Triaxial
effects are found to be insignificant when the axial load is less than
40 per cent of the sum of the uniaxial compressive strengths of the steel
and concrete. For higher axial loads, triaxial effects are taken into
account by using the equivalent stress-strain relationships of Phase II
in a semi-rational analysis.
All the experimental moment-load-curvature characteristics are
numerically simulated on a digital computer.