Reliability, Brittleness, Covert Understrength Factors, and Fringe Formulas in Concrete Design Codes
Reliability, Brittleness, Covert Understrength Factors, and Fringe Formulas in Concrete Design Codes
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混凝土设计规范中的可靠性、脆性、隐性强度不足因素和边缘公式
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
Qiang Yu
中科院分区：
文献类型：
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作者：
Z. Bažant;Qiang Yu
本文分析了现行混凝土结构设计规范中含有隐性（或隐蔽）强度不足（或能力折减）因素的可靠性后果。由于材料特性的统计分散性、设计公式的误差和失效模式的脆性程度，这就妨碍了区分不同风险的不同组合，也使得结构可靠性（或生存概率）的任何预测都不可能。隐式公式误差因子由以下事实暗示：设计公式被校准为不通过支持实验数据的平均值而是通过支持实验数据的边缘（或外围、裕度）。隐蔽材料随机系数是设计所需的混凝土强度折减值与强度试验平均值的比值。作为补救措施，设计公式的隐性强度不足系数应公开，其变异系数（基于支持性试验数据）应明确，概率分布类型（例如，高斯或威布尔）（这也意味着概率截止）。或者，规范可以给出平均值公式，指定其变异系数和分布类型，并规定概率截止值或公开声明强度不足系数。质量控制所需的强度试验的平均值应根据规定的概率截止值和这些试验的变异系数，从所需的设计强度中计算出来。此外，有人建议，目前使用的经验不足的因素，主要占结构脆性（或缺乏延展性）的风险，应基于预期的最大动能，可以赋予结构。将考虑荷载和结构抗力随机性的可靠度积分推广到多个（统计独立）欠强度因子的情况。最后，它指出，目前假定的比例的拉伸和剪切强度的混凝土的抗压强度的平方根是现实的，只有平均值，但严重低估了分散的拉伸和剪切强度。
The paper analyzes the reliability consequences of the fact that the current design codes for concrete structure contain covert (or hidden) understrength (or capacity reduction) factors. This prevents distinguishing between different combinations of separate risks due to the statistical scatter of material properties, the error of the design formula, and the degree of brittleness of failure mode, and also makes any prediction of structural reliability (or survival probability) impossible. The covert formula error factor is implied by the fact that the design formula was calibrated to pass not through the mean but through the fringe (or periphery, margin) of the supporting experimental data. The covert material randomness factor is the ratio of the reduced concrete strength required for design to the mean of the strength tests. As a remedy, the covert understrength factor of design formula should be made overt, its coefficient of variation (based on the supporting test data) should be specified, and the type of probability distribution (e.g., Gaussian or Weibull) indicated (which then also implies the probability cutoff). Alternatively, the code could give the mean formula, specify its coefficient of variation and type of distribution, and either prescribe the probability cutoff or overtly declare the understrength factor. The mean of strength tests required for quality control should be figured out from the required design strength on the basis of a specified probability cutoff and the coefficient of variation of these tests. Furthermore, it is proposed that the currently used empirical understrength factor, which accounts mainly for the risks of structural brittleness (or lack of ductility), should be based on the expected maximum kinetic energy that could be imparted to the structure. The reliability integral taking into account the randomness of both the load and structural resistance is generalized for the case of multiple (statistically independent) understrength factors. Finally, it is pointed out that the currently assumed proportionality of the tensile and shear strengths to the square root of compressive strength of concrete is realistic only for the mean, but grossly underestimates the scatter of tensile and shear strengths.