| Structural steel is frequently used in high rise building construction due to its low weight-to-strength ratio and high ductility characteristics. However, steel suffers fast degradation of its strength and stiffness properties at elevated temperature, and hence, steel structural members are to be provided with a certain level of fire protection to achieve required fire resistance. The current approaches for evaluating fire resistance of steel members have many limitations and do not take into consideration critical factors governing fire response. For instance, a restrained steel beam develops significant fire induced restraint forces when exposed to fire and these forces transform the behavior from beam to that of a beam-column. However, current design provisions do not take into consideration the effect of fire induced restraint forces, and thus beams are continued to be designed for flexural capacity only under fire conditions.;To obtain test data on the influence of thermal gradients on steel beam-columns, fire resistance tests on four beam-columns were conducted as part of this study. The test variables included magnitude and direction of thermal gradients, load level, insulation scheme, and fire scenarios. Results from the fire tests show that beam-columns develop significant thermal gradients under uneven fire exposure and these gradients alter the failure mode from that predicted using conventional P-M curves.;Test data were utilized to validate finite element models created using ANSYS for tracing thermal and structural response of beam-columns under fire conditions. The finite element models account for various critical factors, namely high temperature material properties, fire induced restraints and thermal gradients, and the different strain components (including high temperature creep), that have significant influence on the fire response of steel beam-columns. Once validated, the models were used to carry out detailed parametric studies to quantify the influence of critical parameters, such as load, end restraints, and fire scenario, on the fire response of beam-columns. Results from the parametric study showed that fire resistance of restrained steel beams is adversely affected by increasing load ratio and fire severity, while fire resistance gets enhanced by increasing axial and rotational restraint stiffnesses. Results from the parametric studies and from fire experiments were utilized to develop two simplified calculation methodologies at sectional and global levels for tracing the response of steel beam-columns under fire. At sectional level, a simple methodology for adjusting the conventional plastic P-M diagrams to account for the effect of fire induced thermal gradients is derived. At global level, an engineering methodology is developed for evaluating the fire response of restrained steel beams utilizing equilibrium and compatibility principles. The proposed approach accounts for load, boundary conditions, fire scenarios, and beam geometry, and can be applied for evaluating fire resistance of restrained steel beams utilizing either deflection or strength limit state.;A comparison of results show that the proposed approaches provide better fire resistance estimates of restrained beams than that predicted by current provisions in codes and standards. The simplicity of the proposed approaches makes them attractive for undertaking a rational fire design under a performance-based environment Application of these rational approaches in the design process of beam-columns will contribute to reliable and cost effective fire response predictions. |
