Axial stress-strain characteristics and confinement mechanism of concrete-encased steel composite columns: An analytical model

Concrete-encased steel (CES) composite columns are frequently employed in high-rise buildings owing to their superior structural efficiency. The axial compression behaviour of CES composite columns is examined analytically at the constitutive level in this paper. For each material inside the composite sections, including highly confined concrete, partially confined concrete, unconfined concrete, steel webs, steel flanges, and longitudinal reinforcement, uniaxial stress-strain relationships are independently analysed. Then, a comprehensive analytical model is suggested. An innovative confinement model is suggested to reproduce the full range of the axial stress-strain response of CES columns by introducing the concept of the generalised variational principle for the mechanical analysis of composite sections under confinement in steel sections. The model considers the real-time variation in the confining pressure on the concrete during compression of a CES column and can accurately and dynamically calculate the axial stresses in the web of the section under complex stresses and the effective lateral confining pressure provided by the section steel. A database of CES axial compression members is created to confirm the correctness of the analytical model. The database is drawn from the published literature and encompasses a broad range of geometrical and material properties. Comparing the analytical model with the database shows that the model can accurately anticipate the axial bearing capacity, post-peak response, and stress-strain properties of CES axial compression members. Finally, the steel section-induced confinement effects are evaluated, and recommendations are made.