A number of additional economical benefits stem from the use of CFTs. The tube serves as formwork in construction, which decreases labor and material costs. In moderate- to high-rise construction, the building can ascend more quickly than a comparable reinforced concrete structure since the steelwork can precede the concrete by several stories. The cost of the member itself is much less than steel and roughly equivalent to reinforced concrete on a strength per dollar basis for low to medium strength concrete (Webb, 1993). When compared to steel moment resisting frames, in unbraced CFT frames, the amount of savings in steel tends to grow as the number stories increases (Morino et al., 1996).On the other hand, relatively simple beam- to-column connection details can be utilized for rectangular CFT members. This also results in savings for the total cost of the structure and facilitates the design process. In addition, the steel tube and concrete act together to provide natural reinforcement for the panel zone, which reduces the material and labor costs of the connections. With the use of high-strength concrete, CFTs are stronger per square foot than conventional reinforced concrete columns (Webb, 1993).In high-strength applications, smaller column sizes may be used, increasing the amount of usable floor space in office buildings. The smaller and lighter framework places less of a load on the foundation, cutting costs again. These advantages have secured an expanding role for this versatile structural element in modern construction.
A primary deterrent to widespread use of CFTs is the limited knowledge regarding their behavior. A number of factors complicate the analysis and design of concrete-filled steel tubes. A CFT member contains two materials with different stress-strain curves and distinctly different behavior. The interaction of the two materials poses a difficult problem in the determination of combined properties such as moment of inertia and modulus of elasticity. The failure mechanism depends largely on the shape, length, diameter, steel tube thickness, and concrete and steel strengths. Parameters such as bond, concrete confinement, residual stresses, creep, shrinkage, and type of loading also have an effect on the CFT’s behavior. Axially loaded columns and, in more recent years, CFT beam-columns and connections, have been studied worldwide and to some extent many of the aforementioned issues have been reconciled for these types of members. However, researchers are still studying topics such as the effect of bond, confinement, local buckling, scale effect, and fire on CFT member strength, load transfer mechanisms and economical detailing strategies at beam-to-CFT column connections, and categorization of response in CFTs and their connections at all levels of loading so as to facilitate the development of performance-based seismic design provisions. It should also be noted that, despite a recent increase in the number of full-scale experiments, the majority of the tests to date have been conducted on relatively small specimens, often 6 inches in diameter or smaller (see Tables 1 through 6). This is due to the load limits of the testing apparatus and the need to run the tests economically. Whether these results can be accurately extrapolated to the typically larger columns used in practice remains a pertinent and debatable question, although recent research in Japan has begun to address this important issue (Morino et al., 1996).
