混凝土轴压柱的抗火可靠指标初探

Reliability Index of Axially Loaded Concrete Columns in Fire

火灾作用下混凝土构件的承载能力总体不断下降且具有明显随机性,采用抗火可靠指标可较好地评估构件的火灾安全性。本文针对混凝土轴压柱的高温承载能力极限状态,考虑材料强度变异性导致的柱高温极限承栽力的随机性,以及高温下柱荷栽效应组合的随机性,采用JC法计算给出了ISO834标准升温曲线作用下不同受火时间对应的柱抗火可靠指标,讨论了主要参数对该可靠指标的影响规律;并根据我国现行建筑防火规范,通过分析和参照对比,初步提出了混凝土轴压柱抗火可靠指标的目标建议值。研究认为:(1)截面尺寸越小,柱抗火可靠指标随受火时间增加而下降的速率越快;(2)现行规范给出的截面(200×200)mm、(240×240)mm、(300×300)mm 3种柱在各自耐火极限时刻的抗火可靠指标较为接近,但截面(370×370)mm柱的抗火可靠指标却明显偏低;(3)混凝土轴压柱的目标抗火可靠指标建议取为1.6。

Load bearing capacities of concrete members are stochastic in fire and generally decrease with increasing temperature,so fire safety of concrete members should be evaluated by using fire-resistance reliability index.Considering the bearing capacities limit state of axially loaded concrete columns,the randomicity of bearing capacities due to variation of concrete and steel bar＇ s strengths,and the randomicity of loading effects combination in fire,the JC method is adopted to calculate the reliability indexes of concrete columns subjected to ISO834 standard fire.The influences of parameters on reliability indexes are also discussed. Then,according to the national code for fire protection design of buildings,a target fire-resistance reliability index is put forward for axially loaded concrete columns through analysis and comparison.It can be found that;（a） the less dimension the column＇s cross-section is,the more rapid the fire-resistance reliability index decreases with increase of heating time;（b） the fire-resistance reliability indexes at critical time for the three concrete columns,as being given in the national code with cross sections of 200 mm×200 mm,240 mm×240 mm and 300 mm×300 mm,are close to each other,but the fire-resistance reliability index for the concrete column with a cross section of 370 mm×370 mm is much lower;and（c） the target fire-resistance reliability index for axially loaded concrete columns is suggested to be 1.6.