Energy balance for restrained steel columns in fire

Resumo

The behavior of restrained steel columns in fire is analyzed in the paper, by quantifying the evolution
of the mechanical and thermal energies from ignition until collapse. An appropriate structural model is adopted,
to simulate real columns in buildings, and is analyzed by means of the voxels-based Rayleigh-Ritz method. This
numerical method accounts for stability effects, spread of plasticity, degradation of the steel’s mechanical
properties with heating and non-uniform temperatures distributions at any stage of heating. The column’s behavior
in fire consists of thermal buckling followed by plastic collapse. From a Thermodynamics point of view, the heat
given to the column is transformed into thermal and mechanical forms of energy stored in the column, and in
energy conducted to the surrounding frame. The energy transformations during this process must respect all
principles of Thermodynamics. To this end, a new energy balance expression is proposed, and the most relevant
energy parts related to the mechanical problem are computed for an illustrative example. A new quantity – the
thermal absorption capacity denoted by H – is proposed to quantify the resistance of columns in fire.