Arch bridges
In the traditional form, a steel arch has a similar structural action to old masonry arch bridges. The arch springs from the foundations and exerts horizontal thrusts on them. The arch elements act primarily in compression. The deck may either be supported on struts, resting on arch below, or it may be suspended on hangers from the arch above.
A tied-arch or "bow string" arch is a particular development of the arch form. The horizontal thrusts from the arching action are resisted by tension members between the arch springings. Effectively the deck acts as a tension tie, and is supported by hangers from the arch above. This form is suited to the soft soils of riverbanks, where the ground cannot withstand the large horizontal thrusts from arching action.
In recent years, arches and tied-arches have become a little more common, partly because the use of an arch from which to hang the deck allows the construction depth of a suspended deck to be kept shallow, even at longer spans, and partly because arches make a clear architectural statement. Arches are sometimes skew to the line of the deck and sometimes the arch planes are inclined for dramatic visual effect.
