Geotechnical plastic design

The OCC model shows that a type of plastic analysis that is helpful when analysing structures also helps in understanding the classification and the selection or rejection of soils as construction materials, and the problems of liquefaction and of dilation. The teaching, like Baker's structural teaching, should be based on plastic design theory and on approximate methods of analysis by upper and lower bounds. Whether structures are made of reinforced concrete, mild steel or soil, they will not have a disproportionate response to a small load increment or be at risk of progressive collapse if they can dissipate enough energy in a potential failure mechanism. The value of alternative load paths within a structure and of continuity and ductility, which was evident in bomb-damaged London, was also evident on 28 July 1945 in New York when a US Army Air Force B-25 bomber crashed into the Empire State Building between floors 79 and 80 in dense fog. The Empire State Building was built in 1930 with a 60 000 ton mild steel frame structure. The crew and some civilians were killed, but the energy of the crash was absorbed locally in plastic deformation. The B-25 started a fire but it did not penetrate into the core of the building. Newspaper photographs in 1945 showed a plane embedded in the Empire State Building (the tail portion stuck out 80 floors above the street), not a collapsed building. There was a difference between an aircraft structure and the building structure. The men flying the aircraft accepted that they were in a structure that could not itself dissipate the energy of all potential failure mechanisms. The Empire State Building was New York's icon, where people could expect engineers who built structures for public use to make them as safe as houses (and more safe than aircraft). But if the tests to determine soil constants and computer calculations in the design and construction of civil engineering structures, geotechnical or others, are compared with the material testing and design calculations and flight testing required for aircraft, it is doubtful if civil engineering structures are as safe as commercial aircraft.