A truss is a triangulated framework of elements that act primarily in tension and compression. It is a light-weight yet very stiff form of construction. Truss girders were common in early steel bridge construction as welding had yet to be developed (pre 1930s), and rolled sections and plate sizes were of a limited range.
They are considered expensive to fabricate today, being labour intensive, and maintenance issues have to be carefully addressed, e.g. ease of access. However, they can still show dvantages in particular applications such as footbridges and railway bridges. Typical spans in one form or other can range from 40m to 500m.
Trusses may be used as girders below the deck level, or as through girders with the deck at the bottom chord level. Such through-truss girders minimise the effective construction depth, and the length of approach embankments. Hence, they are particularly suited to footbridges and railway bridges. The light-weight, yet stiff, nature of truss bridges also makes them ideal for demountable "Bailey" bridges.
Trusses can also be used as arch elements, examples of these are the Sydney Harbour Bridge, and as cantilever elements, the Forth Rail Bridge in Scotland, a further example of this is the Brinnington rail bridge in Manchester. They also have applications as stiffening girders on major suspension bridges.
Brinnington rail bridge, Manchester, UK
The Yeonjong Bridge in Korea is clearly a suspension bridge, but it is a classic example of how a truss is used as the main deck stiffening girder. However, in this example the stiffening girder is also subjected to large compressive forces as it this bridge the world’s longest self-anchored suspension bridge.
Yeonjong bridge, Korea