Consequences of Ignoring or Mis-Judging the Size Effect

Except for huge unreinforced structures, Weibull’s statistical size effect is weak in concrete structures. The size effect source is principally energetic, caused by stress redistribution due to a large fracture process zone size or large cracks formed before maximum load. It is shown that an unbiased statistical analysis of the existing database for shear of R.C. beams without stirrups supports the energetic size effect theory, and that the size effect, albeit milder, afflicts beams with stirrups, too. Known though has this type of size effect been for two decades, it has been mostly ignored in design codes as well as practice. What are the consequences? —overdesign of many small structures but, more seriously, unacceptable risk for large ones. A tolerable failure probability of engineering structures is 10 per lifetime, and collapse statistics indicate that this has indeed been true for small structures. Probabilistic analysis calibrated by a large statistical database confirms this level of failure probability for shear of reinforced concrete beams without stirrups < 0.2 m deep. However, if the size effect is ignored (as in ACI code), the failure probability is shown to increase drastically—to 10 for beams 1 m deep (and nearly as much for the unrealistic formulae of CEB, fib and JSCE underestimating the size effect). This finding roughly matches several statistics showing that very large structures have been collapsing with a frequency roughly 10-times greater than small ones. This is unacceptable. Now that no longer just a handful of theoreticians, but entire scientific societies and concrete fracture committees (IA-FraMCoS, ASCE-EMD, ACI Comm. 446), are convinced of the inevitability of energetic size effect in brittle failures of concrete structures, the engineering societies ignoring or severely underestimating the size effect in their design codes, and perhaps even design firms, are exposed to legal risk when another collapse occurs.