This is an upright frame, usually made in wrought iron or steel strutted by diagonal thrust beams against the engine-house wall or other solid abutments, the height to the bearings of the guide pulleys being from 80 to 1 00 ft.
The substructure consists of (a) the piers and end piers or abutments, the former sustaining a vertical load, and the latter having to resist, in addition, the oblique thrust of an arch, the pull of a suspension chain, or the thrust of an embankment; and (b) the foundations below the ground level, which are often difficult and costly parts of the structure, because the position of a'bridge may be fixed by considerations which preclude the selection of a site naturally adapted for carrying a heavy structure.
On the other hand, a girder imposes only a vertical load on its piers and abutments, and not a horizontal thrust, as in the case of an arch or suspension chain.
Brunel constructed the towers and abutments for a suspension bridge of 702 ft.
Two main towers in the river and two towers on the shore abutments carry the suspension chains.