Numerical modelling of composite steel and concrete double-skin and double-tube square cross-section columns subjected to fire

The fire resistance of composite steel and concrete double-tube and double-skin square cross-section columns has been previously evaluated through experimental tests carried out at the University of Coimbra. The results of these experimental tests were used to calibrate a finite element numerical model. This numerical model allowed a parametric study for developing new simplified fire design formulas for these types of columns. The heat transfer modelling used an implicit analysis where different thermal parameters have been tested. In the mechanical modelling, an explicit analysis was used considering the mechanical properties at high temperatures proposed by different authors for steel and concrete. In the parametric study, the influence of the relative slenderness, loading level and axial and rotational restraining levels in the column's critical and ultimate collapse times were analysed. Columns with slenderness higher than 0.5 showed lower ultimate collapse times than columns with relative slenderness between 0.25 and 0.5, despite the axial and rotational restraining level applied to them. Loading levels above 40% in the columns led to columns' premature ultimate collapse. The increasing axial and rotational restraining level benefited the columns' critical times, however columns showed similar ultimate collapse times when using restraining levels above K1.