Iron Bridges: Today, railroad truss bridges are built of steel, but during the period of most street railway construction, cheap, reliable steel wasn't available. So iron was the standard bridge-building materiel. In the earlier part of the 19th Century, cast iron, a material high in carbon as it came from the smelter, was commonly used. However, by the beginning of the streetcar era, wrought iron had become the favored building material.
Wrought iron is produced from cast iron by heating it to a temperature high enough to soften but not to melt it. In this state, carbon is removed, originally by being hammered by a blacksmith but later by going through a rolling mill. Wrought iron is malleable, allowing it to be shaped. As such, it's not as hard or resistant to compression as cast iron, but it's tougher and much less brittle.
The following pictures of the former Cleveland & Eastern 1899 interurban bridge, that still stands in Gates Mills OH, provides an example how this was accomplished. You will note (picture 1) that the truss is composed of members that form a series of triangles, thereby forming a rigid structure, resistant to bending.
As you look at the pictures you can see that, on this particular bridge, the major members are of three sorts:
The heaviest are those that most need to resist compression at which wrought iron is least effective. For this purpose, plates have been riveted together to from beams that are square in section (picture 2). Essentially, the bridge hangs from these members so that other members are mostly under tension and, thus, do not need to be as heavy.
However, because of the shifting weight as a car moves across the bridge, many members occasionally also have to withstand compression. Thus, they are somewhat heavier, and pairs are connected by small angular cross-braces (Picture 3).
Finally, as picture 4 shows, there are some members that consist only of thin bars of wrought iron. These members only have to withstand tension and, thus, can be made much lighter.
An interesting sidelight is that some of the members are not connected by riveting but are held together by short but heavy bolts through the connections (pictures 4 &5). This method of bridge construction was common at the time, allowing slight flexibility. The final picture shows how the railway grade leads onto the bridge.-- "All Aboard!"
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