BEHAVIOR OF SLENDER RC COLUMNS WITH INITIAL DEFORMATION IN FIRE

Abstract

Reinforced concrete columns formed with high-strength concrete have been developed for use as axial force carrying members. They have a reduced cross section compared with conventional “short” columns and are known as “slender” columns. These columns realize larger building interior spaces and offer architectural space with good visibility. However, under fire conditions, bending stiffness of RC columns decreases considerably due to thermal degradation of the cover concrete. It is anticipated that slender RC columns would be more vulnerable to buckling failure than to cross sectional failure. The authors have proposed an evaluation method based on tangent modulus theory for the buckling strength of straight RC columns subjected to centric axial load in fire, and shown that this proposed method offered a good approximation of both experimental fire-resistance time and failure mode. However, existing members have initial deformation formed in manufacturing process. For this reason, it is anticipated that slender RC columns with initial deformation fails earlier than straight ones. This paper presents the main results from an analytical study of slender RC columns with initial deformation shaped sine wave under fire conditions, leading to a discussion of fire performance, which includes deformation behavior and fire-resistance time. All analytical models consist of concrete with a compressive strength of approximately 120N/mm2 and have a square section where the length of one side is 250mm and 5000mm in height. From thermal stress analysis in consideration of geometrical nonlinearity, the followings are obtained: 1. Fire-resistance time of slender RC columns whose the initial deformation α/h was less than approximately 1/103 decreased sharply. 2. There are two failure modes of slender RC columns with initial deformation which are buckling and three-hinged mode. 3. The time, while columns resist an axial force after they start to bend at the time evaluated by the analytical method based on tangent modulus theory, is approximately 10~15 minutes. 4. The larger transient strain was, the shorter fire-resistance time was because the large additional moment occurred with bending deformation increases in fire. 5. The directions of initial deformation have little influence on fire-resistance time. 6. The mechanical boundary conditions at edge of columns have large impact on fire-resistance time.