Experimental and Numerical Studies of CFT Columns and Composite Beams toward Performance-Based Fire Safety Design

Abstract

Nowadays technical demands from project clients and construction industry are becoming more and more diversified and growing. Many advanced design methods and constructional technologies are increasingly being applied to meet the demands. Among many building design technologies, fire design is one of the most under-developed and its improvement is urgently needed. However, in spite of the fact that the structural fire design guides in Korea is still at a relatively low-level of fire engineering, associated experimental and analytical fire studies are still lacking and reliable fire test database accumulated from systematic full-scale testing of composite members are very scarce. This study, therefore, was intended to develop thermal and structural analysis technologies for the composite column and beam members and to establish both rational and practical approach under the framework of performance-based fire safety design. Thermal and structural behavior of rectangular concrete-filled steel tubular (CFT) columns under fire condition was first investigated by an experimental program to augment the reliable fire test database. One of the key observation in this testing program was that early local buckling of steel tubes of CFT columns tends to induce load transfer from steel tube to concrete, and eventually triggered concrete crushing, or complete loss of the load bearing capacity of the columns. This implies that the limit state of local buckling as well as overall flexural buckling should be incorporated for a rational fire design procedure. Test results also showed that the limiting temperature method in current design codes and existing empirical design formula overestimate the fire resistance of CFT columns. In order to supplement costly and time-consuming fire testing, nonlinear finite element modeling technique to simulate the thermal and structural response of CFT columns under fire condition was also developed and verified against fire tests. Comparison of fire behavior between numerical simulations and test results indicated that the numerical modeling in this study can predict the fire resistance of CFT columns reasonably. Based on the experimental and numerical studies of CFT columns, design methods to improve the approaches in current design codes were also suggested and verified: (1) To predict more accurately the temperature distribution of CFT sections under standard fire condition, new formula was proposed based on test-backed finite element heat transfer analysis. (2) Limitations of the current simple calculation model for rectangular CFT columns were critically evaluated and a modified simple calculation model was proposed considering the effects of column length, sectional width, and steel tube thickness. The applicability of the proposed method was validated against the fire test results. (3) To determine critical compressive strength of steel tubes in CFT sections at elevated temperatures as governed by local buckling, theoretical and analytical studies were conducted. Critical buckling curve of steel tubes in CFT sections at elevated temperatures was developed based on the parametric thermo-mechanical coupled analysis of CFT stub columns. Next, fire behavior and resistance of various composite beams including H-shaped composite beam, partially encased beam, and slimfloor beam were investigated by the standard fire tests. The test results showed that partially encased and slimfloor beams can be a promising alternative to conventional H-shape composite beams which requires costly and cumbersome fire protection works. Numerical study to describe the thermal and structural responses of these composite beams was also conducted. Comparison with test results showed that the nonlinear finite element modeling developed in this study can predict the fire behavior of various composite beams reasonably. The fire behavior of CFT columns and composite beams subjected to parametric fire conditions was also investigated. It was found that the CFT columns under the parametric fires should resist the additional compressive load due to the thermal contraction of steel tube by temperature decrease during the cooling phase. In the case of composite beams, it was observed that different ventilation conditions affect the fire behavior significantly and, especially, slow fire-growing condition with lower maximum gas temperature may induce the larger maximum and residual deflection of composite beam. Experimental and analytical studies including the proposed design methods in this research can be used to improve current irrational prescriptive fire design practice for composite members under the framework of performance-based fire safety design.