Numerical simulation for structural steel member or framed structure at elevated temperature

 Author: Iu, Chi-kin Jerry Title: Numerical simulation for structural steel member or framed structure at elevated temperature Degree: Ph.D. Year: 2004 Subject: Hong Kong Polytechnic University -- DissertationsStructural analysis (Engineering) -- Mathematical modelsMetals at high temperatures Department: Dept. of Civil and Structural Engineering Pages: xviii, 267 leaves : ill. ; 30 cm Language: English InnoPac Record: http://library.polyu.edu.hk/record=b1781122 URI: http://theses.lib.polyu.edu.hk/handle/200/1552 Abstract: In the past, the numerical fire analysis is based on the plastic zone approach to investigate the thermal effect on the structural members. However, this method requires much computation time and effort in the numerical integration. Also the numerical convergence is slow by comparing with those in plastic hinge method, which can simulate the yielding behaviour. Therefore, fire analysis based on plastic method is a trend of development. Basing on plastic method, this fire study presents an accurate and robust geometric and material nonlinear formulation to predict the structural behaviour of unprotected steel members at elevated temperatures. This finite element formulation of beam-column elements is based on the plastic hinge approach to model the elasto-plastic strain-hardening material behaviour. To allow for the large deflection effect, the Newton-Raphson method is employed to solve the nonlinear governing equations throughout the thermal history of fire. A new stiffness formulation for determining member resistance is derived for the efficient modeling of geometric nonlinearity. Degradation of material strength due to increasing temperature is simulated by a set of temperature-stress-strain curves in accordance with both ECCS and BS 5950 Part 8. Thermal creep deformation is allowed for, only implicitly. In addition, when the cooling effects on the behaviour of a structure are modeled, the strain reversal effect is thus included by consideration of the permanent strains in the analysis. It is felt that, owing to the above, this numerical analysis including the above nonlinear and thermal effects is helpful to the fire safety design of a structure, including plane and space multi-storey steel framed building. The difficult in the plastic hinge method is to model the stress-strain relationship at different temperatures levels by using the spring stiffness. Thus the highlight of this research project is the total spring stiffness formulation, which simulates the effect of load redistribution when a member yielded, so as to include the thermal-time dependent effect. Also this fire study modifies the incremental spring stiffness to include residual strength after effective yield stress at elevated temperatures, which is important to the structural behaviour of a building when exposed to fire. Moreover, during cooling phase, the stiffness formulation incorporating permanent deformation is developed. Thus, the incremental-iterative procedure for unloading path to determine permanent deformations is then evaluated.

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