Plasticity and original Cam Clay

Plastic design of a structural system ensures that it will safely absorb all the work done in any damage to it. Before coming to more general discussion of plastic design, a particular example will be described that Professor J. F. Baker quoted in his opening lecture on structures to first-year undergraduates in 1948. He had designed a bomb shelter in World War II (Figure 5.1(a)) for houses in the UK where a family would sleep under the kitchen table during a night bombing raid. The risk to the family was that a nearby bomb burst could break the brick walls of the house and cause the roof and floors to fall. Wood is weak, and such a table and everyone below it would be crushed. Baker (1954) designed a strong shelter made of mild steel. The members were bolted together in the kitchen, with a steel plate to serve as a table top during the day, and steel mesh sides to prevent rubble from coming in from a collapse of the house in a night raid (Figure 5.1(b)). He had already developed an early plastic design method for steel portal frames in his pre-war research, and used this research in the shelter design. The present author has a friend who, as a boy on the night that the blast of a flying bomb made his family's house collapse, was saved by this type of shelter.