Nonlinear analysis and design of high-strength concrete filled steel tubular columns under nonuniform fires

The main objective of this study is to investigate the behaviour of concrete-filled steel tubular (CFST) columns with ultra-high strength concrete (UHSC) and high strength steel (HSS) under fires through finite element method (FEM). High-strength materials can be used in CFST columns to reduce member dimensions, lowering material consumption and foundation loads. However, their fire behaviour has been insufficiently examined, especially under nonuniform fire exposure. In this study, thermal-structural models of the composite columns are developed using the SAFIR software. The numerical results are compared with published test data on temperature distribution, axial displacement, lateral displacement, failure modes and failure time. Then, parametric analyses are conducted to investigate the influence of various factors such as concrete strength, steel strength, different fire exposures, column length and load ratio on the fire performance of the CFST columns. In addition, the tabular design approach in European code EN1994-1-2 and Australian/New Zealand code AS/NZS 2327:2017 for designing uniformly exposed CFST columns is evaluated. It is observed that the prescribed design values in the table for R30 and R60 fire ratings under load ratio lower than 0.47 appear insufficient for the allowable slenderer columns. The applicability of the tabulated data for a load ratio up to 0.47 is further checked for the columns with UHSC of 120 MPa and HSS of 690 MPa, which is the strength limits recommended in AS/ NZS 2327:2017.