This paper presents the results of an experimental work carried out to study the strength and behavior of concrete filled steel square tubular beam-columns under constant vertical and varying horizontal loads. The effect of filled concrete on the strength and behavior of the beam-columns is discussed. It is shown that the concrete filled steel tubular columns even whose width-thickness ratios are above a limiting value possess the good earthquake resistant properties, and the new limiting width-thickness ratio of a concrete filled steel tube can be relieved to 2 times that for a hollow tube.
