An-Najah Staff

Restrained steel beams, when exposed to fire, develop significant restraint forces and often behave as beam-columns. The response of such restrained steel beams under fire depends on many factors including: fire scenario, beam slenderness ratio, location of axial restraint at the supports, and high-temperature properties of steel. A set of numerical studies, using finite element computer program ANSYS, is carried out to study the fire response of steel beam-columns under realistic fire and restraint scenarios. Results from the parametric studies indicate that fire scenario, beam slenderness, location of axial restraint and high-temperature creep have significant influence on the behavior of restrained beams under fire conditions. Severe fires produce high axial forces at early stages of fire exposure; whereas in moderate fires, significant axial force develops only at later stages of fire exposure. Axial restraint enhances the fire resistance due to the development of tensile catenary action in restrained beams. Furthermore, restrained beams with low slenderness ratio exhibit better fire performance when the axial restraint at the support is located at the bottom flange.

At room temperature, and at service load levels, creep has little effect on the performance of steel structures. However, under fire conditions, creep becomes a dominant factor and influences fire resistance of steel members. Under fire conditions, significant forces develop in restrained steel beams and these forces induce high stresses in the steel section. The extent of creep deformations is affected by magnitude and rate of development of stress and temperature in steel. In this paper, the effect of high temperature creep on fire response of restrained beams is investigated. Current high temperature creep models are compared. Finite element model created in ANSYS was validated by comparing the predictions with fire test data. The validated model was applied to investigate the effect of load level, heating rate, fire scenario and fire induced axial restraint on the extent of creep deformations. Results from the parametric study indicate that the influence of high temperature creep increases with the increase in axial restraint, heating rate, and load level. Generally, neglecting high-temperature creep effect stiffens the structural response and leads to reduced deflections but larger restraint forces. Therefore, neglecting high temperature creep in fire resistance analysis of steel structures can lead to unconservative predictions.