彎矩構架合成梁高溫行為之有限元素分析

Abstract

傳統鋼結構耐火性能設計僅考慮單一簡支梁之防火時效，但結構中梁構件於高溫下之結構行為受周圍結構構件所束制，因此與單一簡支梁之火害行為不同。由於火害實驗費用昂貴，有限元素分析可協助研究結構構件之火害行為，並提供實驗中因受限而無法測量之數據。本研究以有限元素分析軟體建立兩組分析模型，C1與C2，模擬彎矩構架合成梁子結構於升溫階段之行為，並與實驗溫度與變形結果對比以驗證分析模型之準確性，最後探討彎矩與軸力之變化。兩組模型之柱構件尺寸不同，模型C2之束制構件對受火梁所提供之軸向束制勁度為模型C1的2.8倍，而旋轉束制勁度為3.3倍。本研究之模型可合理預測合成梁於高溫下之溫度變化與結構行為。分析結果顯示束制合成梁之防火時效高於單一簡支合成梁，相較於簡支合成梁，模型C1之防火時效提高41%，而模型C2提高51%。束制合成梁構件遭受火害時，熱膨脹變形因受到束制而產生負彎矩與軸壓力，模型C2之最大負彎矩為215 kN-m，是模型C1的1.17倍；模型C2之最大軸壓力為870 kN，是模型C1的1.69倍。梁翼板因無法傳遞軸壓力易產生局部挫屈行為，模型C2因承受較大軸壓力而使其局部挫屈行為發生之時間早於模型C1。模型溫度穩定後，材料強度因高溫而降低使梁中心點位移變大，合成梁所承受的負彎矩與軸壓力亦開始降低，直至模型達到CNS 12514-1所規定之性能基準。

Traditionally, fire engineering design for steel structures is based on isolated structure members. However, the structural behaviours of beam members in a building are influenced by the adjacent structural elements when subjected to fire. Thus, the fire performance is dramatically different to the isolated beams. Standard fire tests are expensive to be performed. Finite element analysis can be used to assist the understanding of the structural behaviour of restrained beams in fire. In addition, detail information can be provided where instrumentations is limited. In this research, two finite element models, C1 and C2, have been developed to evaluate the thermal and structural behaviour of composite beams in moment frames subjected to fire. Temperature variation and deformation of the models have been validated with experimental results. Bending moment and axial force developments have been discussed. The difference between model C1 and C2 is the restraint stiffness of the composite beam provided by different column size. The axial restraint stiffness provided by the restraint members of model C2 is 2.8 times that of model C1 and the rotational stiffness is 3.3 times. The models developed in this research are able to predict the thermal and structural behaviour of composite beams in fire with reasonable accuracy. The results show a significant improvement in fire resistance is achieved by providing restraints. In comparison to the simply supported composite beam model, the fire resistance for model C1 has increased by 41% and model C2 has increased by 51%. When restrained composite beams are subjected to fire, the thermal expansion of the beams is restrained, resulting in hogging moments and compression forces. The maximum hogging moment produced in model C2 was 215 kN-m, which is about 1.17 times that of model C1. The maximum compression force induced in model C2 was 870 kN, which is about 1.69 times that of model C1. Local buckling occurs if the beam flanges cannot transfer the induced compression forces. Since the compression force in model C2 was larger than model C1, the occurrence time of local buckling was earlier. Once the temperature stabilized, the midspan deflection started to increase rapidly due to reduction of material strength at high temperatures. At the same time, the hogging moment and axial forces began to reduce until the limiting criteria of CNS 12514-1 has been reached.