ANALYSIS OF STEEL STRUCTURE SUBJECTED TO POST EARTHQUAKE FIRE LOAD

Abstract

Structural steel creates a stronger building with far less material and cross-section than concrete. However, the previous studies shown strength loss of steel structures under fire loading. In practice, fire is followed by earthquake aftershocks and there are few studies on behavior of structural steel under post earthquake fire. This thesis work presents the post-earthquake fire response of a steel structure. Experimental result reported in literature are used for validation analysis using ANSYS software program and further, parametric studies are performed on influential parameters such as inter story drift, temperature intensity, cross-section size and length of column, duration of fire and boundary condition to get insight on post-earthquake fire behaviour of steel structure. FE analysis results showed post-earthquake fire affected residual performance of a steel structure. Axial and lateral deformations increased by 87% and 98% respectively. As, cross-section size increased, also, axial and lateral deformations percentage decreased by 74% and 84% for Pinned-Fixed connection and 54% and 70% for Pinned-Pinned connection compared to CS1 from CS2 to CS4 under ISO 834 loading. In the case of duration of fire, it is observed that axial and lateral deformations for time load steps plotted percentage change increment is 30% for Pinned-Fixed and 55% for Pinned-Pinned connection. For restrained ends with ISO 834 loading, axial and lateral deformations are raised from 95% to 96% and 98% to 98.3% related to Fixed-Fixed from Pinned-Fixed to Pinned-Pinned support conditions. When the length of the column increased, the axial and lateral deformations percentage change elevated by 41% and 67% for Pinned-Fixed and 42% and 70% for Pinned-Pinned 55% related to 3m from 4 to 5meters. But, buckling load factors declined from 3% to 15% related to 3m length. Fire and Fire plus Earthquake Loading result comparison with ISO 834 for 60min duration of fire shows the percentage change for axial deformation increased by 58% related to fire load. It can be concluded that smaller axial and lateral deformation but higher buckling resistance and larger force developed in the modelled column when it is not subjected to an inter-story drift prior to the fire event. On the other hand, the modelled columns experienced larger axial and lateral deformation but smaller buckling resistance and smaller force developed when it is pinned-pinned and pinned-fixed connected smaller cross section size and elevated temperature used, and duration of fire and length of column increased. From these outcomes, it can be concluded that the sever level of earthquake load have a major effect on the fire performance of steel columns than light level.