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TheStructuralEngineer April 2013
Feature Toys that save millions
'Toys that save millions' - a history of using physical models in structural design Bill Addis MA (Cantab), PhD Consulting engineer Synopsis Small-scale physical models have been used to determine the most efficient form of pure compression and tensile structures since the 17th century - and to predict structural behaviour of some full-size structures since the mid-19th century. This paper traces the development of model testing in the design of building structures from its origins in the design of bridges and ship hulls in the 19th century, to the design of gravity and arch dams in the early 20th century, through the early days of designing concrete shell roofs in the 1920s and 1930s, to its most sophisticated use in the 1960s and (in conjunction with computer modelling) the 1970s. The great strength of model testing was, and is, to deal with new types of structures without design codes and even without precedent.
Introduction Scope of the paper Building designers have been using models for at least 2500 years. This paper addresses only those aspects of model testing that structural engineers have used during the design stages of projects to determine quantitative results regarding structural form, deflections, stresses, bending moments, wind loads, dynamic behaviour, ultimate strength, stability, collapse loads and factors of safety. The author has discussed many of these issues in greater detail, as well as some of the philosophy underlying model testing, previously1-6. The paper does not look at the use of models to convey the geometry of buildings to clients or builders, or to check the viability of construction and assembly, or as part of scientific research into structural behaviour, or to learn about structural behaviour and gain a physical feeling of the behaviour of structures.
Why is the topic important? The contribution of structural theory to the development of structural engineering has long been recognised, and is the subject of many excellent studies7-10. The contribution of model testing, both to the design of many extraordinary and innovative structures and, more generally, to progress in structural engineering has, on the other hand, not been duly recognised and has been addressed by very few historians of structural engineering11-13. It is the author’s view that the two aspects are of equal importance, especially since the late 19th century, and that much research is needed to redress the current imbalance.
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Figure 1 Sketch by Christopher Wren showing line of a hanging chain within the masonry of the dome of St. Paul’s Cathedral, London (c. 1680)
Why did engineers do model testing? The task of the structural engineer is twofold:
• To provide dimensions of components and their relative disposition, as well as materials specifications, to enable a contractor to begin construction • To raise the confidence of the engineer and builder such that a proposed structure can be constructed and, once constructed, will work as intended The purpose of testing scale models of structures was to help the designer achieve these two goals when calculations using structural theory were judged to be inadequate or would have been too complex or time-consuming. This situation often arose when new, unprecedented types of structure were being developed, and was encountered particularly during the 20th century in the design of thin reinforced concrete shells and lattice shells, of both reinforced concrete and timber.
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