Song, Han, and Yu 2010

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Analytical Study

A finite element analysis model was developed that would predict the load vs. deformation relationship for CFST stub columns. First, the columns are axially loaded at room temperature, followed by heating and cooling phases, where the load is left constant. Lastly, the load is increased until the critical load is reached. The uncoupled thermal analysis module in ABAQUS was used, and then the calculations are completed for a stress analysis. The nodal temperatures are used from the thermal analysis for the stress analysis. The temperature curve follows the ISO-834 standard fire curve, which includes both a heating and cooling phase. For the thermal analysis, the steel tube is modelled using 4-node shell element, whereas the concrete and bearing plate are modelled using 8-node brick elements. The steel and concrete are assumed to be in full contact, and the ‘mixed method’ is used to determine the temperature distribution. The fire curve is applied in multiple steps, and the temperature is calculated by an iterative method using equilibrium equations. Heat convection and radiation are utilized as boundary conditions: air to column heat transfer during heating, and column to air during cooling. Expressions for stress and strain are determined during the ambient, heating, cooling, and post-fire phases. For the load-deformation analysis, the elements change from heat transfer elements to stress elements. The bottom plate was fixed, and the nodes in the top plate were restrained laterally, however not restrained longitudinally. The temperatures for the square sections are slightly lower than that of the circular specimen, due to the larger area. The peak resistance for the circular section is unaffected under the heating, and the peak strain is increased by 356%, whereas for the square cross section it only increases by 200%. In general, the two shapes react similarly in different loading phases in terms of longitudinal stress, and the circular section shows a higher longitudinal stress. The confinement stress for columns with both shapes is negligible before the cooling phase, and increases significantly in the latter part of the cooling phase, as the temperature of the steel tube is lower than that in the center of the cross section. During the post-fire phase, the confinement stress in the circular specimen decreases initially, and then increases steadily until the end of the loading. In the square specimen, the concrete core has a low level of confinement, thus the confinement stress does not increase to the same level.

References

Song, T.-Y., Han, L.-H., and Yu, H.-X.(2010). “Concrete filled steel tube stub columns under combined temperature and loading.” Journal of Construction Steel Research, 66 (3), March, pp. 369-384 doi:10.1016/j.jcsr.2009.10.010