Interaction model for unprotected concrete filled steel columns under standard fire conditions

This paper extends the Rankine method to the analyses of reinforced concrete filled steel (RCFS) columns and plain concrete filled steel (PCFS) columns at elevated temperature. The advantage of this approach is that it gives structural engineers a physical feel of the adequacy of a column in terms of its plastic squashing capacity and elastic buckling capacity. The method hinges on the concept of modified buckling coefficient, which is equally applicable for both axially and eccentrically loaded columns. A novel feature is that the fire resistance of a column is expressed in terms of its plastic squashing capacity and elastic buckling capacity at ambient temperature. The mean of agreement between the Rankine predictions and test results for both RCFS (61 columns) and PCFS (95 columns) is satisfactory. The paper includes a worked example to illustrate the flexibility of the approach, and also comments on the accuracy when the Rankine method is applied to other types of columns such as steel or reinforced concrete columns.