Bridge and Webb 1993
The experimental study contained in this paper was prompted by the desire to use thin-walled steel tubes filled with concrete for the columns in a 43-story high-rise in Melbourne, Australia. The use of CFTs proved economically superior to equivalent steel or steel-reinforced columns. Not only were the material costs less, but there was no need for concrete formwork and the construction of the steel framework could proceed several stories above the filling of the steel tubes. The steel tubes were filled by pumping the concrete into the bottom and then up the steel tube. As many as six stories were filled at a time. The most economical CFT would be a column in which only enough steel to support the framework during construction was required. The authors conducted the tests based on this efficiency of thin-walled tubes.
Experimental Study, Discussion, and Results
The tubes used in the construction of the aforementioned structure had D/t ratios on the order of 120, in excess of the limits specified by the Australian Standard AS4100-1900 and Eurocode 4 [see Roik and Bergman (1993) for the Eurocode 4 D/t limits]. Therefore, the authors proposed an effective diameter, De:
to account for local buckling and conducted experimental tests to verify the accuracy of using an effective diameter.
Two CFT tests and two HT tests were conducted using high-strength concrete with suitable pumping characteristics. The CFTs were loaded into the post-ultimate stage. Local buckling began prior to the point of ultimate load, although at initially small magnitudes of deformation. Beyond the ultimate load, the entire steel tube buckled locally near midheight and axial shortening was concentrated in this region.
The axial load capacity using the effective diameter for the steel tube was expressed by:
where Ae is the effective area of the steel tube using the effective diameter, De. The results of this calculation provided a conservative estimate of the strength and the authors confirmed the appropriateness of their equation. The corresponding tests on the HT sections showed that the steel was only contributing 10-12% of the compressive strength of the column. Aside from supporting the construction loads prior to being filled with concrete, the tube was primarily used to contain the concrete.
Reference
Bridge, R. Q. and Webb, J. (1993). “Thin Walled Circular Concrete Filled Steel Tubular Columns,” Composite Construction in Steel and Concrete II, Proceedings of the Engineering Foundation Conference, Easterling, W. S. and Roddis, W. M. (eds.), Potosi, Missouri, June 14-19, 1992, ASCE, New York, New York, pp. 634-649.