Truss Bridges come in all shapes and sizes, although the vast majority are constructed of either Iron or Steel.
However, Arch Trusses don't necessarily need to be underneath the bridge deck, they can also be above the deck acting as parapets. Or, on longer spans, they will cross significantly above the road deck, which is then suspended from the arches. These are commonly referred to as Bowstring Bridges.
For short spans, or bridges designed to carry light loads, a truss need be no more than the size of a normal Parapet. In principle, they work in the same way as Girder Bridges, with the deck simply being carried between the Trusses rather than above them. It is the rigidity of the truss which carries the bridge, and provided that the deck is adequately supported , it can even be suspended beneath the trusses.
The shape of the Truss can also vary. For a single span, a straight rectangular truss is often the most cost effective, however an arched truss (above the deck) can also be used, as the truss needs to be at its deepest in the centre of the bridge where the bending moment is at its greatest. Multi span bridges more commonly use arched trusses, or even triangular trusses which peak over the piers and so act like Suspension Bridges, where the decks are effectively cantilevered out from the piers.
The longer the span, the deeper the truss needs to be, and this is also true if the bridge is carrying a busy road. While a quiet country lane crossing a small stream may only need a truss a couple of feet high, a busy trunk route across a major river will need Trusses as high as the lorries and buses using it. This can often lead to Box Trusses being used.
Box Truss bridges are those where the trusses form a box around the traffic deck. The most famous structure built from such trusses is probably the Forth Rail Bridge, but the Connel Bridge in the West Highlands is a scaled down version of this design - also built as a rail bridge!
Box Trusses can also come in a variety of shapes. The Connel Bridge is a variant of the cantilever approach, but Arched trusses and straight trusses can also be used, as can be seen at Boat O'Brig (right), where the road bridge in the foreground has an arched top truss and the rail bridge behind is trapezoid.