Behaviour of High Strength Steel-Composite Columns Under Combined Actions

Prepared by: Joyce Lee
Supervisor: Associate Professor Brian Uy

Introduction

The use of concrete filled steel columns in high rise buildings have increased significantly throughout the world. Its growing popularity is due to substantial economical savings composite columns can provide over their steel counterparts. The utilisation of composite columns reduces cross sectional dimensions which leads to material costs and space savings. Current applications of composite construction include the Latitude Building on George St, Sydney, illustrated in Figure 1, where a transfer truss is used to support loads from the upper levels.

Figure 1: Latitude Building, Sydney

This ongoing research aims to compare the ultimate strengths of high strength steel-composite and hollow steel columns through a series of experiments, and evaluating these results. The objective is to consider whether current design guidelines for these columns under biaxial bending are appropriate. If not, suggested modifications will be made.

Experiments

There are 8 specimens of short columns to be tested under both compression and biaxial bending. Their material and cross sectional properties are shown in Table 2.

Materials Testing

A series of materials test were carried out to determine the yield strength of steel and compressive strength of concrete. The results of these tests are illustrated in Table 2.

Table 2: Properties of Columns

Figure 3: Cross section details of column

Figure 4: Test setup

Experimental Procedure

A detailed picture of the test setup can be seen in Figure 4. The experiments were conducted at the Heavy Structures Laboratory in Randwick. Specially designed knife edges were used to ensure pinned ended supports were provided to either side of the column.

Results

The test results obtained were analysed and illustrated in Figures 5 and 6.

Axial Load – Deflection
By comparing the results between a concrete filled steel section and a hollow section, it is evident that a concrete filled column has a higher ultimate load strength. Also, the difference in downward slopes of the graphs demonstrates the increased stiffness obtained in a concrete filled column and the improved post peak load behaviour.

Axial Load – Strain
These graphs are useful in determining when yielding and local buckling of steel plates take place.

Failure Modes
Figure 7 presents the difference in failure modes of the hollow and concrete filled sections. Due to the restrained effect provided by the concrete, the concrete filled column buckles outward instead of inward as in the case of a hollow section. This is a desirable effect as it delays the onset of local buckling in the steel plate.

Figure 5: Load-Deflection results for (a) hollow steel section and (b) concrete filled steel section

Figure 6: Load-Strain results for (a) hollow steel section and (b) concrete filled steel section

Figure 7: Failure mode of (a) hollow steel section and (b) concrete filled steel section

Conclusions

It is illustrated through this research that composite columns have greater strengths and stiffness over steel hollow sections – a definite advantage for multistorey building construction.

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