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Posted: Tue May 18, 2010 1:31 pm Post subject: Gems of Structural Engg - J. F. Baker - (Post-18)
John Baker (1901 - 1985)
John Fleetwood Baker was born on 19 March 1901. Baker was a son of J.W. Baker and Emily Fleetwood. He was educated at Rossall and Clare College, Cambridge and married Fiona Mary MacAlister Walker in 1928. He rose as a British scientist and structural engineer.
After graduation, Baker worked with the Air Ministry on structural problems of airships. At 28, in 1929, he contracted tuberculosis. Upon recovering, he became a Technical Officer with the Structural Steel Research Committee, which was investigating why measured stresses in structural steel bore little resemblance to theoretical ones. During his time in this post, he developed the plastic theory of design, a revolutionary method of design of steel structures which gives a lower bound on the collapse load, and is hence always safe. Until then, all design of steel structures was based on elastic theory of design, which gives an upper bound on the collapse load.
In 1933 Baker became Professor of Engineering at Bristol University, after which he was scientific adviser to the Design and Development Section of the Ministry of Home Security from 1939 to 1943. In this time he created the Morrison indoor shelter, using his plastic theory of structural analysis. From 1943 to 1968, he was Professor of Mechanical Sciences and Head of Department at Cambridge University Engineering Department. During this time the department more than tripled in size, from 24 lecturers to 111. He used the structure of the Forth rail bridge as the basis of explanations of structural design theory to his students.
During his time at Cambridge, plasticity theory was used to design the new Baker Building of the department, making it the first building in the world designed by this method.
In 1963 he became an honorary graduate as Doctor of Science at the University of Edinburgh. He was made a life peer as Baron Baker, of Windrush in the County of Gloucestershire on 1 February 1977.
Plasticity Theory of Design
Steel can either behave elastically or plastically. Elastic deformation is reversible, and with the removal of load the material will return to its original shape, position and stress distribution. Plastic deformation is not reversible, and with the removal of load the material will assume a different shape, position and stress distribution to the one it held originally.
Plasticity theory is based on plastic behaviour, and calculates a lower bound on the load that a structure can carry (the load at which it collapses will not be lower than that calculated). This allows a structure to be designed so it will always be able to carry the chosen magnitude of load, even if the exact way it does so is not understood.
Elasticity theory depends on guessing the way in which a structure works, and the loads it will be subjected to, and designing it to carry those loads in the assumed manner. This ensures it is safe if the structure is well understood, but it may not be safe if the structure carries the loads in a different manner. Therefore it gives an upper bound on the collapse load (the load at which it collapses will not be higher than that calculated). Elastic design is sensitive to deformation of the structure, and only works for small deflections.
During the 1950s Baker and W.Prager of Brown University published a two volume account of the history of steel structures, with plastic theory integral to it. By the 1960s it was being taught in the undergraduate engineering course at Cambridge University, and Baker presented an undergraduate lecture on the principles of design of the Morrison shelter as an interesting introduction to his theory of plastic design of structures; in 1968 this lecture was attended by Prince Charles, and it can be summarised as follows:
It was impractical to produce a design of indoor bomb shelter for mass production that could withstand a direct hit, and so it was a matter of selecting a suitable design target that would save lives in many cases of blast damage to bombed houses. Examination of bombed buildings indicated that in many instances, one end wall of a house was sucked or blown out by an nearby blast, and the floor of the first storey pivoted about its other end (supported by a largely intact wall) and killing the inhabitants.The Morrison shelter was therefore designed to absorb the energy of the upper floor of a typical two storey house undergoing a partial collapse of this form. The shelter was designed to absorb this energy by plastic deformation, since this can absorb two or three orders of magnitude more energy than elastic deformation. Its design enabled the family to sleep under the shelter at night or during raids, and to use it as a dining table in the daytime, making it a practical item in the house.
Although plastic theory is theoretically superior to elastic theory, it is still not in common usage today. Elastic theory, in a codified form, is used for the design of most steel structures, mainly due to the relative ease of use of elastic theory and the computerisation of the calculations. Plastic theory is used in some structures and specialist applications, specifically in bridge design.
A distribution method of stress analysis (Reports and memoranda) (1935)
Enterprise versus Bureaucracy : The Development of Structural Air-Raid Precautions during the 2nd World War (1978)
John Fleetwood Baker Plastic Theory - Morrison Shelter
The most spectacular application of Baker’s plastic theory was to the ‘Morrison Shelter’. At the start of the Second World War almost all the standard forms of air raid shelter were unsatisfactory. Baker designed a new type of indoor shelter, the size of a dining table, beneath which a whole family could shelter.
These shelters were designed to squash down plastically by no more than 12 inches if the house collapsed as a result of a bomb.
This video clip shows Baker lecturing on his famous invention, which saved countless lives and for which he won £3000.
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