Research On The Fire Performance Of Concrete Filled Steel Tubular (CFST) Triple-limb Laced Columns

Fire resistance experiments including6concrete filled steel tubular (CFST)triple-limb laced columns and4CFST single-limb columns have been conducted toresearch the influence of load ratio and tube type on the fire performance of CFSTtriple-limb laced columns, the fire performance of which is compared with that of CFSTsingle-limb columns. The results indicated that the three chords of the laced columnscould work together well in fire, and lost bearing capacity at the same moment when thelaced column reached the fire resistance. Load ratio has a significant influence on thefire resistance of the laced columns. With the increase of load ratio, the fire resistance ofthe columns decreases. Besides, the fire resistance of the laced columns produced withstainless steel tubes is obviously better than that of the ones made of carbon steel tubes.For the CFST members with a large slenderness ratio, it is also showed that if the watervapor from concrete couldn’t escape from vent hole smoothly when heating, the fireperformance of the members would be seriously affected.In actual projects, sufficientvent holes should be set on the steel tubes to ensure CFST members not to occur ansudden destruction due to water vapor.A finite element (FE) model, which has been verified by the fire resistanceexperiments, is established to studythe mechanic characteristics of CFST triple-limblaced columns subjected to ISO834standard fire including a heating phase and acooling phase, including temperature distribution, failure mode, axial deformation, loaddistributions on each chord as well as on steel tubes and core concrete. The failuremechanism and influence factors when the laced columns reach the limit state both inheating and cooling phase are also discussed based on the FE model. It shows that theCFST triple-limb laced columns can reach the limit state both in the heating phase andcooling phase of a fire, And three failure modes (the overall bending failure, the overalltorsion failure and flexural-torsional failure) can occur with the change of geometricdimension.The parametric analysis results indicate that load ratio, chord dimension andslenderness ratio have a significant influence on the fire resistance of the laced columns.The fire resistance of the columns decreases with the increase of load ratio andslenderness ratio, or with the decrease of the chord dimension. In addition, heating time ratio and load ratio have large influence on the failure time in cooling phase. For thesituation that the laced columns could reach the limit state in cooling phase of thestandard fire, the larger the heating time ratio or the load ratio is, the shorter the failuretime in cooling phase under other same conditions.Based on the results of parametric analysis, a simplified calculating method of fireresistance and failure time in fire cooling phase is proposed. The calculating resultshave a good agreement with those obtained by the fire experiment and FE analysis, andcan provide a reference for engineering design.