Overview

The designers of Greek temples or Gothic cathedrals were, rightly, not concerned with stresses stresses were so small in their buildings that there was, in general, no question of failure of the material. Nor, as a matter of interest, were they interested in deflexions, which was a subject thrown into prominence later with the engineering use of timber, and then the new materials iron and steel. A tree may sway in the wind, but an Egyptian obelisk Cleopatra's Needle does not, at least not visibly; nor does the church of Hagia Sofia.

A baulk of timber, however, will not only deflect (perhaps acceptably) under transverse load if the load is increased enough, the timber will fracture. In modern, and inexact, shorthand, fracture is governed by the attainment of some limiting stress, and the exploration of the unknown territory of stress began in the seventeenth century. The engineering definition of stress is simple and precise, but the notion of stress caused (and continues to cause) some difficulty both in understanding and in application to structural analysis.

There are mathematical difficulties in the description and handling of stress, but a first and fundamental physical problem is that stress cannot be measured. The breaking strength in tension of a stone or timber specimen can certainly be determined, and Galileo in his Discorsi of 1638, of which more will be said, shows what appears to be a stone cylinder under test. The weight necessary to cause fracture would give a measure of the absolute strength of...