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23 The simplest and most primitive bridge is undoubtedly a plank or tree laid across a stream, supported by the land at each end and perhaps by a series of rocks placed in the stream. However, there are problems with beam bridges. Unless they are supported regularly, their reach is limited; and unless their supports are high enough, marine or other traffic cannot pass beneath. An early solution to these issues was the arch bridge, which takes advantage of natural laws of physics to support a deck high above the crossing. Gravity holds the elements of the bridge in place as they are pressed against each other by the downward force, which is distributed along the path of the arch to the ground. This distribution allowed for larger spans without intermediate support. Consequently , the arch bridge was useful for spanning distances greater than was possible with simple beams. Stone was an apt material for arch bridges, because it has compressive strength, meaning that it can withstand the pressure of pushing on it. There are two basic types of stone arches (and variations within those). The first, the corbel arch, is created by a series of stones or bricks laid on top of each other, with the ends extended in a kind of cantilever construction: three-quarters of the weight of each stone or brick supports the one-quarter overhang. Ultimately, like two staircases rising toward each other, the piles from opposite sides meet at the top. ARCH BRIDGES The second type of stone arch, the voussoir arch (also known as the “true arch”), is based on the parabolic placement of like-sized components. To build this bridge, a temporary structure called falsework was employed, on which the stones were laid until the keystone was placed in the center so that the structure could sustain itself. In discussions of stone arch bridges, some vocabulary is used that has particular meaning. Different sources use different definitions, and various of the sources cited in the bibliography offer their own definitions. I have sought a composite for purposes of this book. Rubble means irregular, uncut, rough stones or stone fragments . Ashlar indicates cut, square, or rectangular stone placed adjacent to other such stones. Random refers to stones placed in no particular order and of different shapes, either gathered from nearby or received that way from a quarry. Random placement is to be contrasted with coursed, in which the stones are placed in rows. A mosaic configuration is a pattern of different-sized stones fitted together. Thus, a “random rubble” bridge would consist of irregularly sized stones in no particular order, with smaller stones filling gaps, whereas “coursed rubble” would be different -sized stones placed in clearly discernible rows. Gauged stones have gone through a grinding process that reduces all of them to the same thickness. A parapet is a low protective wall or fence at the edge of a bridge. A spandrel is the flat vertical surface bounded by an arch on either side, with the tops of the arches bounding it at the top; it may be open (forming its own apparent arch) or closed (solid). The abutment is the support at each end of the arch. Arches may be pointed instead of rounded. Stone, of course, is not the only material suitable for the construction of an arch bridge. By the closing decades of the nineteenth century, steel became preferable to stone for constructing arch bridges because it could span longer distances and because it had an aesthetic appeal. Whereas stone bridges required the roadway to curve with the arch, steel permitted a flat deck, where traffic could go “through” the arch, or a flat deck on top of the arch. Concrete could also be used, as it was for two of the bridges in Newark’s Branch Brook Park that are featured in this section. Although the construction of large stone arch bridges entailed significant expense, smaller ones, often built by local people, were common in New Jersey in the eighteenth and nineteenth centuries. The availability of stone and rock made it relatively easy to span short distances across creeks and small rivers without theoretical calculations to determine the support requirements. Nonetheless, care had to be taken if the bridge were to last. It was not until Squire Whipple wrote his T H E B R I D G E S O F N E W J E R S E Y 24 [3.144...

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