Rectangular composite columns can be transformedinto equivalent square composite columns. The methodis that the areas of steel and concrete of the rectangularsection are equal to those of the square section,respectively. The longitudinal stress fzy in the squaresteel tube should be replaced by a modified longitudinalstress f′zy of the rectangular steel tube, i.e4.3. Longitudinal Stress-Strain Relationship of Core ConcreteAfter the longitudinal load by the steel tube is obtained,the force taken by the core concrete can be calculated bydistracting the load taken by the steel tube from the totalload. Based on the experiments, the numericalseparation method was used to get the relationshipbetween the confined concrete and unconfined concreteas Zhang (2000) and repeated in Eqn 45
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