EFFECT OF SIZE ON SAFETY FACTORS AND STRENGTH OF QUASIBRITTLE STRUCTURES: BECKONING REFORM OF RELIABILITY CONCEPTS

• Main Authors: Zdenek P. Bazant L, Sze-dai Pang
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: materials (e.g. • rocks; fiber composites. sea ice; tough ceramics; wood; cohesive soils. etc; Sea ice
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.694.3944; http://www.civil.northwestern.edu/people/bazant/PDFs/Papers/P195.pdf

The effect of structure size D on structural strength has two causes: I) energy release due to stress redistribution caused by a large fracture or a large fracture process zone, and 2) extreme value statistics of random material strength. While, in perfectly ductile failures, the probability density function (pdf) of failure load is Gaussian. with a size-independent mean and coefficient of variation (CoV) decreasing as D · l12, in perfectly brittle failures the pdf is Weibull. with a size-dependent mean but size-independent CoV. In quasibrittle structures, there is a continuous size-dependent transition. which can be described by a composite pdf approaching Gaussian for vanishing sizes and Weibull for infinite sizes. Since, for the same mean and same CoV, the point of acceptable failure probability such as 10.7 is for Weibull pdf about twice as far from the mean than it is for Gaussian pdf, the understrength (capacity reduction) factors in concrete design codes, as well as reliability indices (Cornell's, Hasofer-Lind's), must be revised. making them size dependent. Comparisons with test data show that large covert understrength factors are implied in the current code specifications for brittle failures of concrete structures, and also that an irrational size effect is hidden in an excessive load factor for self-weight. All these features preclude at present any meaningful probabiUstic reliability estimates when failure is quasibrittle. A restructuring of safety provisions in concrete design codes. and a revision of the concept of reliability indices of Cornell and Hasofer-Lind. are proposed. An energetic-statistical theory ' of size effect. applicable to concrete, as well as other quasibrittle