CFT Members Subjected to Combined Loading

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Typically, beam-column tests are differentiated from eccentric loading tests by the magnitude of the induced moment and the type of failure. Eccentric load tests refer to tests where a moment was introduced to accentuate initial out-of-straightness, or tests in which the moment was of relatively small magnitude (e.g., less than 20% of the ultimate moment capacity). The eccentricity hastens the onset of buckling in a typical failure. Beam-column tests, on the other hand, have moments of significantly larger magnitude, warranting careful consideration of the interactive failure due to combined moment and axial load. These moments may be introduced transversely, via loading of a connected member, or by a number of other methods.

General Behavior

Several key parameters influence the behavior of beam-columns. Among them are the D/t ratio, the axial load ratio (P/Po), and the L/D ratio or the slenderness of the member. The D/t ratio determines the point of local buckling, and it affects the ductility of the section. A smaller D/t ratio will delay the onset of local buckling of the steel tube. Tubes with high D/t ratios (above approximately 50) will often exhibit local buckling even before yielding of the section occurs. A low D/t will provide greater ductility, illustrated by the long plateau in the moment-curvature diagrams for such columns (Tomii and Sakino, 1979a). Tomii and Sakino (1979a) also showed that beam-columns with low D/t ratios (24 and 33) can sustain the maximum moment after local buckling. Beam-columns with higher D/t ratios (44) began to lose capacity as the curvature increased, although only under large axial loads did the capacity drop significantly. Ichinohe et al. (1991) determined that for circular CFT specimens with a D/t less than 53, the moment increases after local buckling without strength degradation.

The axial load ratio is a second important parameter in CFT beam-column behavior. The relationship between the moment and axial load, as illustrated by interaction diagrams, is typically a curve that bulges outward for low axial compressive loads (i.e., with the maximum moments exceeding Mo) and then approaches Po approximately linearly, showing a rapid decrease in the moment capacity for high axial compressive load ratios. As axial compression is added to a CFT member in bending, the contribution of the concrete begins to increase, utilizing the composite action of the section to a greater extent. The axial load increases the strength of the concrete in a manner similar to the confinement effect discussed in Section 2. As with the D/t ratio, the axial load ratio has an effect on ductility. Large values of P/Po lead to rapid moment capacity deterioration and brittle failures. If the steel on the compression side has buckled (which is more likely as the D/t ratio increases), a more brittle failure typically ensues. It can be seen that a combination of a high D/t and high P/Po leads to undesirable modes of failure. Because of this, Tomii and Sakino (1979a), for example, limited their studies to beam-columns with P/Po less than or equal to 0.5.

Finally, the L/D ratio will have a significant effect on the performance of the member. A number of authors (e.g., Chen and Chen, 1973; Bridge, 1976; Tomii and Sakino, 1979a; Fujimoto et al., 1996; Tsuda et al., 1996) have presented interaction diagrams (both from experimental and analytical research) that show the moment-axial load relationship for different L/D ratios. For a given cross section, the maximum moment (Mo) will remain approximately the same with an increase in length, but the maximum compressive axial force (Po) will drop markedly.

With respect to material strength, Varma et al. (2000) conducted beam-column tests on square CFTs with high strength materials. He observed a reduction of initial stiffness and moment capacity with an increase in D/t ratio. The yield strength of the steel and the axial load ratio did not have any significant influence on the initial stiffness. However, when the yield strength of the steel increased, the moment capacity was enhanced. The amount of improvement was larger for low D/t ratios due to better resistance to local buckling. The steel strength also had little influence on the ductility seen in the CFT beam-columns. However, both high axial load levels and large D/t ratios caused a reduction in ductility, often more severe than is seen with normal strength materials. Failure took place due to local buckling of the flanges and extensive concrete crushing. In most of the cases, yielding of steel tube occurred prior to the specimen achieving its peak load. In the post-peak region, local buckling moved to the webs and tension cracking of the corners took place. Nakahara and Sakino (1998) also determined that, compared to using normal strength concrete, introduction of high strength concrete reduces ductility.

Cyclic Behavior

Concrete-filled steel tube beam-columns typically perform better under cyclic loading than comparable hollow tubes and reinforced concrete members. CFT members show very full hysteresis loops indicating large energy dissipation. Compared with a reinforced concrete member with the same slenderness ratio, steel ratio, and axial load ratio, the CFT exhibits a higher value of ultimate axial load and a higher amount energy dissipation (Huang et al., 1991).

As with monotonic loading, when high strength concrete was used for CFT beam-columns subjected to cyclic loading, Nakahara and Sakino (2000a) concluded that the response is governed partially by the level of axial loading on the member. Decreasing the D/t ratio increases ductility when the axial load ratio is high. However, they found that the effect of D/t ratio on ductility is less evident in the case of low axial load ratio, and less evident than for monotonic loading. Varma et al. (2000) tested square CFT columns with high strength materials under cyclic shear and also found that the D/t ratio had a less significant effect on ductility than for monotonic loading, but that high axial load levels caused a reduction in energy dissipation capacity and ductility. However, the secant stiffness of the specimens was larger when the axial load level was high due to higher contribution of concrete.

Failure Mechanism

The concrete in a CFT beam-column subjected to cyclic flexural loading contributes little strength in bending, but it does increase the capacity of the section by delaying the onset of local buckling of the steel tube (Kawaguchi et al., 1991). In their tests of beam-columns, Prion and Boehme (1989) noticed a pinching of the hysteresis loops similar to their beam tests due to the opening and closing of the concrete cracks during steel yielding (see Section 3). In the cyclic shear tests on square CFT columns with high strength materials (Varma et al., 2000), steel yielding in tension and concrete crushing occurred either prior to or at the same time as local buckling of the flanges, which took place near the peak load. Following that stage, local buckling of the webs and corners of the tube occurred and the specimens failed with tension cracking of the steel tube corners. After local buckling in the web, the strength of the specimens started to deteriorate more dramatically than lower strength specimens.