Chapter 7, Problem 7.25QPP

In May 1970, a 70 ft tall, 20 ft diameter concrete grain silo was constructed at a site in Eastern Canada (Bozozuk, 1972b). This cylindrical silo, which had a weight of 183 tons, was supported on a 3 ft wide, 4 ft deep ring foundation. The outside diameter of this foundation was 23.6 ft, and its weight was about 54 tons. There was no structural floor (in other words, the contents of the silo rested directly on the ground). The silo was then filled with grain. The exact weight of this grain is not known, but was probably about 533 tons. Unfortunately, the silo collapsed on September 30, 1970 as a result of a bearing capacity failure. The soils beneath the silo are primarily marine silty clays. using an average undrained shear strength of $500{\text{lb/ft}}^{\text{2}},$ a unit weight of $80{\text{lb/ft}}^{\text{3}},$ and a groundwater table 2 ft below the ground surface, compute the ASD factor of safety against a bearing capacity failure, then comment on the accuracy of the analysis, considering the fact that a failure did occur.