Seismic demand predictions of concrete-filled steel box columns

Steel columns partially filled with concrete at base have found their wide application in modern highway bridge systems in areas where severe earthquakes are likely to occur. The assessment of capacities in terms of ductility and ultimate strength as well as the estimation of demands of such structures are vital in any seismic design methodologies. This study presents demand prediction procedures for partially concrete-filled steel box columns. To this end, two methods are proposed: (1) a single degree of freedom system analysis procedure that uses a bilinear or trilinear force–displacement hysteretic model derived from results of a static pushover analysis; and (2) a finite element analysis procedure involving beam-column elements and individual cyclic stress–strain relations of concrete and steel (i.e. a fiber analysis procedure). Both procedures are applied to a number of specimens tested at Nagoya University by pseudo-dynamic testing procedure, and comparisons are made between test and analytical results. The results show that the maximum displacement demands computed from the fiber analysis and the method that involves with the trilinear hysteretic model exhibit good agreement with test results. Predicted residual displacement demands using both procedures do not agree well with test results, and alternatively, an empirical equation is proposed for the residual displacement demand in terms of the maximum displacement demand.