In the present PhD thesis, the thermal and mechanical phases of the fire analysis of spatial steel beamlike structures are addressed. An iterative procedure for the determination of air temperature in the void space between the fire-protective boards and the steel cross-section surfaces is proposed, accounting for the heat transfer with convection and radiation. A novel mathematical model for the planar heat transfer over the steel cross-section, protected with an intumescent coating is also presented. The model considers all main phenomena of intumescent coatings such as a progressive expansion, and time and temperature dependent thermal properties of the coating. These are determined from the rules of mixtures as the ratios of temperature dependent properties of material phases; virgin, intumesced and charred material phases are considered. The ratios of each material phase are based on the current progress of the chemical reaction of pyrolysis. The reaction is modelled by the Arrhenius equation. A fully new mathematical model is also presented for the determination of the mechanical response of spatial steel frames in fire. The recently developed model for the dynamical analysis of spatial beam-like structures is based on Reissner%s geometrically exact beam theory which properly considers multiplicative properties of spatial rotations. The first spatial derivatives of the velocities and the angular velocities are chosen as the basic unknowns of our new numerical model. The model has been expanded to account for temperature dependent mechanical properties of steel, and considers thermal deformations, Harmathy%s model of viscous creep, plastic hardening and softening of material and several non-linear stress-strain relations. The model has been validated against experimental and numerical results from literature. The effects of numerical, mechanical and thermal parameters have been assessed. The results show that the present thermal and mechanical models well enable us to determine the mechanical response of spatial steel frames in fire realistically and accurately.