An-Najah Staff

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, load level, degree of 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, load and restraint scenarios. The finite element model is validated against experimental data, and the importance of high-temperature creep on the fire response of steel beam–columns is illustrated. The validated model is used to carry out a set of parametric studies. Results from the parametric studies indicate that fire scenario, load level, degree of end-restraint and high-temperature creep have significant influence on the behavior of beams under fire conditions. The type of fire scenario plays a critical role in determining the fire response of the laterally-unrestrained steel beam within a space subframe. Increased load level leads to higher catenary forces resulting in lower fire resistance. Rotational restraint enhances the fire resistance of a laterally-unrestrained steel beam, while the axial restraint has detrimental effect on fire resistance.

A performance based approach is developed for assessing the fire resistance of restrained beams. The proposed approach, based on equilibrium and compatibility principles, takes into consideration the influence of many factors including fire scenario, end restraints, connection configuration (location of axial restraint force), thermal gradient, load level, beam geometry, and failure criteria in evaluating fire resistance. The validity of the approach is established by comparing the predictions from the proposed approach with results obtained from rigorous finite element analysis. The applicability and rationality of the proposed approach to practical design situations is illustrated through a numerical example.

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.

Predicting the response of restrained beams under fire conditions is complex owing to the development of fire-induced forces and requires finite-element or finite-differences analysis. In this paper, a simplified approach is proposed for predicting the fire-induced forces and deflections of restrained steel beams. The method applies equilibrium equations for obtaining critical fire-induced forces and then utilizes compatibility principles for obtaining temperature-deflection history of the beam. Effect of end restraints, thermal gradient, location of axial restraint force, span length, and load intensity are accounted for in the proposed approach. The validation of the approach is established by comparing the predictions from the proposed approach with results obtained from rigorous finite-element analysis. The applicability of the proposed approach to practical design situations is illustrated through a numerical example.