Research on fire resistance of the bottom frames of commercial residential buildings

The bottom frames of commercial residential buildings collapse more easily than pure frames in fire, for reasons that fire load of the bottom frames used as commercial buildings is relatively large, and the ratio of dead load to the total loads is also large, axial deformations and flexural deflections caused by high temperature produce adverse effect on arch mechanism for transmitting the vertical loads of masonry walls. Separated loading method for applying temperature loads to concrete and steel bars, which is fit for fire resistance analysis of common reinforced concrete structures and elements in ABAQUS, is proposed firstly in this article. An anisotropic elasto-plastic material model for masonry is introduced to simulate upper masonry walls. The full collapse process from temperature rises to buildings destroy in fire is analyzed. A parametric study is conducted to investigate the effects on fire resistance of the bottom frames of commercial residential buildings when the bottom frames expose in fire due to the change in effective load ratio, section size and reinforcement ratio related to seismic classes. The study shows that the failure mode of the bottom frames of commercial residential buildings under fire conditions is mainly due to column collapse for high axial compression ratio. The damage of arch mechanism causes the vertical loads on middle column increase 1.05~1.25 times as that in normal temperature. The dimensions of column for bottom frames of commercial residential buildings in non-seismic fortification zone and low degree seismic fortification zone should be lager than 400mm to meet time limits of fire resistance.