"Very thin shells may be in danger of locally. The problem hardly arises for civil engineering applications over moderate spans, but may be very important if the spans are very large. Local buckling of a thin shell will occur at a typical stress\sigma_{cr} = kE \frac{t}{R}where E is [and R is the minimum radius of curvature of the shell]. The value of the constant k varies from author to author, but a reasonable value is about 0.25. Thus for a concrete shell for which E is about 20 000 N/mm2, and for which \frac{t}{R} is as small as 1/1000, the critical stress is determined as say 5 N/mm2."