Please join us on Monday, June 8 for our next HEMI Seminar, held in collaboration with CISMMS.
Speaker: Zdeněk Pavel Bažant
McCormick Institute Professor and Walter P. Murphy Professor of Civil Engineering and Materials Science
Department of Civil and Environmental Engineering
Robert R. McCormick School of Engineering and Applied Science
Northwestern University
Title: Critique of lower-margin equations in concrete design codes and their detrimental effect on structural safety and reliability software
Co-authors: Houlin Xu, Jia-Liang Le, George Deodatis. Yang Zhao, Anh Nguyen
Abstract: The design equations of all design codes have traditionally been formulated as ”lower-margin” equations, set at the lower margin of the test data cloud (which lies, depending on structure size, 25% to 40%, below the data mean, in the case of shear strength of RC beams). The load factors are applied to these lower-margin equations while the offset of the mean and the variance of the database remain buried in the code committee documents. Moreover, probabilistic modeling of the mechanics of failure processes, which determines structural strength, has been incorrectly employed, and the probability density function (pdf) required to extrapolate to 10−6 failure probability has been chosen arbitrarily, often as the lognormal pdf for mathematical convenience. Although the lognormal pdf may be an acceptable approximation for a database of concretes with very different strengths, it is shown to be physically impossible to model the strength distribution of one-and-the-same concrete (i.e. a concrete of the same design strength and composition). These traditional concepts have rendered the current failure probability predictions of the structural safety and reliability software for reinforced concrete structures meaningless. However, experience of many decades shows that the frequency of structural failures has not been excessive. The explanation is that many designs must have had excessive safety margins, thus becoming uneconomical, while the benefit of sophisticated commercial software applicable to the randomness of applied loads gets wasted. A sine qua non of the remedy is that the values of the coefficient of variation of the database and of the offset of the database mean from the code equation accompanying each design code equation must be revealed. This could be done in the code Commentary.
Bio: Professor Zdeněk Bažant is McCormick School Professor and Walter P. Murphy Professor of Civil Engineering and Materials Science in the Department of Civil and Environmental Engineering at Northwestern University’s Robert R. McCormick School of Engineering and Applied Science. He is a world leader in research on scaling in the mechanics of solids. He has authored nine books dealing with concrete creep, stability of structures, fracture and size effect, inelastic analysis, scaling of structural strength and probabilistic mechanics of quasi-brittle structures. He is a member of the US National Academy of Sciences, US National Academy of Engineering, American Academy of Arts and Sciences, Royal Society of London, Austrian Academy of Sciences, Academy of Engrg. of Czech Rep., Italian National Academy, Spanish Royal Academy of Engineering, among others. His honors include 9 honorary doctorates.
