Matsui and Kawano 1988
The behavior of circular CFTs when they were used as truss members was investigated. The authors presented the results of the experiments conducted on trusses. Two kinds of parallel chord plane trusses were tested in two test series. The first type was constructed from concrete-filled tubular chords and the second type was constructed only of hollow steel tubes.
Experimental Study, Results, and Discussions
In the first test series, the specimens were loaded monotically with uniform bending moment. The trusses were simply supported at their ends for both in-plane and out-of-plane deformations. The D/t ratio for the chord tubes and web tubes were 26.3 and 14.32, respectively. The column slenderness ratio, L/rs, of the chords was 60 and the height between the parallel chords was 75.86 in.. The measured yield strength of the steel was 48.52 ksi and the measured compressive strength of the concrete was 4.98 ksi. The concrete-filled tubular trusses exhibited more ductile response compared to the specimens with hollow tubes. No lateral instability was observed in the concrete-filled trusses, which resulted in less reduction in strength after the peak load. However, rapid reduction in the post peak strength occurred in the hollow steel tube trusses due to lateral instability. From the compression and tension tests conducted on individual concrete-filled chord members (Kawano and Matsui, 1988), the buckling load was determined to be higher than the tensile yield strength by up to 25%. Consequently, it was concluded that the concrete in the steel tube made the buckling strength higher than the yield strength, thus preventing lateral instability.
The second test series included testing of cantilever truss beam-column specimens under constant axial load and cyclic horizontal load. The specimens were fabricated from the same materials as in the previous tests. They were braced against lateral deformations at the middle and at the ends. The applied axial load was equal to 20% of the yield strength found from the compression tests of similarly sized stub columns that were conducted in an earlier phase of this research. The concrete-filled trusses showed large and stable hysteresis loops, but the behavior of the hollow steel tube trusses was governed by lateral instability, and they showed much smaller hysteresis loops.
The authors reported that stable behavior of the trusses could be ensured if the buckling strength was greater than the yield strength. Consequently, the column slenderness ratio should be limited to increase the CFT buckling capacity. When selecting the limiting value of the column slenderness ratio, both the strain-hardening and Bauschinger effect should be taken into account because strain-hardening increases the strength of the steel, while the Baushinger effect tends to lead to a lower buckling strength. These effects both reduce the limiting column slenderness ratio.
Reference
Matsui, C. and Kawano, A. (1988). “Strength and Behavior of Concrete-Filled Tubular Trusses,” Proceedings of the International Specialty Conference on Concrete Filled Steel Tubular Structures, Harbin, China, August, pp. 113-119.