Steel Frames In Fire Research Articles

Application of the direct iteration method for non-linear analysis of steel frames in fire

Based on finite element method, a direct iteration method capable of predicting the non-linear behavior of steel frames at elevated temperatures is proposed, in which the second-order effects of large deflections, the progressive softening of materials with temperature rise, gradual penetration of inelastic zone both over the section and along the member, non-uniform distribution of temperature within steel members are all taken into consideration. The secant stiffness matrix that relates the total element displacements to the corresponding total forces is used in deriving the basic finite element equations. Different from the commonly used Newton–Raphson method in which the deformations are calculated incrementally, the direct iteration method used in this paper allows the calculation of the total structural responses corresponding to a specified load level and temperature distribution by a straight iteration process. During this process, strains are used to check whether a point on a section is in the elastic domain or not. A computer program was developed to apply this method for analyzing steel frames in fire. To examine the validity of the method and the program, comparisons are made between the program predictions and test results available for steel frames both at room temperature and under fire conditions, in most cases, a satisfactory agreement is obtained.

Experimental studies of the behaviour of unprotected steel frames in fire

The deformation behaviour of unprotected steel framed structures exposed to real fire conditions has been studied experimentally and theoretically. A total of three tests on steel frames under different load level and different heating process are conducted, all the tested frames are single span and single story. In order to simulate a real fire condition, heating is attained by a specially built gas furnace. Behaviour of each tested frame in fire is described and comparisons are made between fire test results and results calculated by a 2D finite element computer program—NASFAF developed in the paper. The secant stiffness matrix which relating the total element displacements to the corresponding total element nodal forces is used in developing the program and the effects of geometrical non-linearity, “softening” of material at elevated temperature, gradual penetration of inelastic zone both along and across each member, non-uniform distribution of temperature are all taken into consideration. Creep effect is considered in an approximate way in the temperature-dependent stress–strain curves of the material.

Advanced analysis for the assessment of steel frames in fire

This paper describes the methodology of an advanced analysis technique for studying the large-displacement inelastic behaviour of building frames subjected to localised fire. The main feature of the proposed analysis is its use of one element per member to model each structural component and thereby obtain a realistic representation of material and geometrical nonlinear behaviour of the overall framework. Both ISO fire and natural fires can be simulated by the analysis model. The transient heat transfer is calculated by using a refined finite element mesh, whereas the structural responses are calculated with a nonlinear finite element technique based on an elasto-plastic beam–column formulation. Numerical studies are carried out on portal frames and multi-storey frames exposed to natural fires. The computed results are compared and contrasted with those from the conventional approach based on ISO standard fire, so that the advantage of using advanced analysis for direct assessment of the performance of steel structures in fire can be highlighted. Effects of fire load and ventilation resulting in localised fires in building frames are studied. The design implication of fire spreading in the storey of a building in which cooling and heating are taking place simultaneously is discussed.

Steel frames in fire: analysis

To apply fire protection to a structure, it is necessary to understand its behaviour in a real fire situation. The Building Research Establishment is developing a package of analytical methods to predict the behaviour of fire, temperature rise in the structure and the response of the structure at elevated. temperatures. This paper describes a finite element computer program studying the structural behaviour of steel frames at elevated temperatures. The effects of elevated temperature are dealt with by including the steel and concrete high temperature stress-strain relationships and temperature-induced strains of thermal expansion, creep for steel and concrete and transient thermal strain for concrete. Both uniform and nonuniform temperature distributions can be included in the analysis. A novel feature of this analysis is the inclusion of the behaviour of semirigid, beam-column connections. Features such as second-order geometrical nonlinearity, residual stress and initial deflection are also included. The program permits the calculation of the load-deflection response of steel frames at cold condition and their behaviour at elevated temperatures under constant loading. Examples are given to validate the assumptions made in the program and also to describe the behaviour of various structural members, in particular steel frames at elevated temperatures.

EFFECT OF MATERIAL PROPERTIES ON THE DEFORMATION OF STEEL FRAME IN FIRE

EFFECT OF MATERIAL PROPERTIES ON THE DEFORMATION OF STEEL FRAME IN FIRE