Page 27 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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History and Overview 7

only provide an incentive for developing good designs, but would also result in the social equivalent
of Darwinian natural selection, where the worst engineers were removed from the profession.
The durability of ancient structures is particularly amazing when one considers that the choice
of building materials prior to the Industrial Revolution was rather limited. Metals could not be
produced in sufficient quantity to be formed into load-bearing members for buildings and bridges.
The primary construction materials prior to the 19th century were timber, brick, and mortar; only
the latter two materials were usually practical for large structures such as cathedrals, because trees
of sufficient size for support beams were rare.
Brick and mortar are relatively brittle and are unreliable for carrying tensile loads. Consequently,
pre-Industrial Revolution structures were usually designed to be loaded in compression. Figure 1.4
schematically illustrates a Roman bridge design. The arch shape causes compressive rather than
tensile stresses to be transmitted through the structure.
The arch is the predominant shape in pre-Industrial Revolution architecture. Windows and roof
spans were arched in order to maintain compressive loading. For example, Figure 1.5 shows two
windows and a portion of the ceiling in King’s College Chapel in Cambridge, England. Although
these shapes are aesthetically pleasing, their primary purpose is more pragmatic.
Compressively loaded structures are obviously stable, since some have lasted for many centu-
ries; the pyramids in Egypt are the epitome of a stable design.
With the Industrial Revolution came mass production of iron and steel. (Or, conversely, one
might argue that mass production of iron and steel fueled the Industrial Revolution.) The availability
of relatively ductile construction materials removed the earlier restrictions on design. It was finally
feasible to build structures that carried tensile stresses. Note the difference between the design of
the Tower Bridge in London (Figure 1.6) and the earlier bridge design (Figure 1.4).
The change from brick and mortar structures loaded in compression to steel structures in
tension brought problems, however. Occasionally, a steel structure would fail unexpectedly at
stresses well below the anticipated tensile strength. One of the most famous of these failures
was the rupture of a molasses tank in Boston in January 1919 [5]. Over 2 million gallons of
molasses were spilled, resulting in 12 deaths, 40 injuries, massive property damage, and several
drowned horses.

FIGURE 1.4 Schematic Roman bridge design. The arch shape of the bridge causes loads to be transmitted
through the structure as compressive stresses.