In real structures, the loading conditions of steel elements subjected to the action of a localized fire change with time, if their axial elongation is restrained. The mechanical action on these elements has a major influence on their deformation, which in turn influences the magnitude of the interaction forces between the heated elements and the cold structure. The stiffness of the structure to which compressed elements belong has a main influence on the evolution of their axial force, and subsequently on their critical temperature and on their fire resistance. This paper describes the experimental model used in the Laboratory for Structures and Strength of Materials of the Department for Civil Engineering of Instituto Superior Técnico (IST), Lisbon, Portugal, to study the critical temperature of compressed steel elements with restrained thermal elongation. A total of 168 tests on hinged bars were performed. Four different slenderness values, two eccentricities of the compression load and six levels of restraint to the axial elongation were tested. The test results and the results of a computer simulation using a finite element program show that neglecting the effect of thermal axial restraint may result in overestimation of the fire resistance of columns.
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