The truss bridge is among the oldest types of modern bridges. Originally made of wood, truss bridges represented a radical change in bridge design and engineering principles that had been used since for centuries. Iron became available as an inexpensive engineering material in the 19th century. This new material required new and innovative design and engineering. Because of their strength and economical use of materials, many iron and steel truss bridges were built in the United States from the mid-1800s through the 1930s. Widely used by the railroads, truss bridges contributed to the westward expansion of the United States.
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Truss Bridge Design
Though there are many different types, all truss bridge designs are based on the same principle. Straight structural elements of the bridge are connected as triangles that form the skeletal structure of the bridge. Truss bridges are designed to be self-supporting and many have little, if any, support structure beneath them. The trusses transfer and absorb the compression and tension of loads moving across the bridge. Connected series of trusses are capable of supporting great weight across long distances.
The Pratt Truss
Designed in 1844 by the Pratt Brothers, the Pratt truss was among the first design to make the transition between wood and metal trusses. There are variations of the Pratt truss, but the structure is typically constructed with vertical members and diagonal members sloping downwards towards the centre of the bridge forming a V-shape. The diagonal members commonly cross in the centre section of the bridge. The Pratt truss bridge is one of the most common railroad truss bridge designs.
Deck Truss Bridge
The deck truss bridge is easily recognisable because its truss structure supports the road or deck from below. Wooden deck truss bridges are common on hiking and biking trails. Steel deck trusses can be found spanning wide spaces, such as canyons. Steel deck truss is a common highway and highway overpass bridge design. The Adams Street Bridge over the Chicago River in Chicago, Illinois, is a classic example of a deck truss bridge.
The Warren Truss
The Warren truss was patented in 1848 and is unidentified by upper and lower horizontal beams connected by diagonal cross-members that form a series of alternately-inverted, equilateral triangles or V shapes across the span. There are no vertical elements to the structure. This design has been incorporated into airframe design on some small aircraft fuselages.
Bowstring Arch Truss
The bowstring arch truss, also known as a tied arch design, is commonly used for suspension bridges. The main arch is connected to the deck by vertical members. The arch slopes away from the centre of the bridge and typically use Pratt or Warren type trusses. In a bowstring arch, vertical force is transferred from the vertical beams along the length of the arch to a horizontal member connecting the ends of the arch.
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