wrought-iron, or steel, of suitable form and arrangement to ensure strength and stability. Not only must the piers be strong enough to carry the weight that may be brought upon them vertically, but they must have sufficient width of base to ensure lateral steadiness. The design should admit of facility of erection, with a minimum of scaffolding, and the pieces should be of convenient length and weight for transport. The lower length of river piers, or portion liable to be in contact with flood-water, should be of solid masonry, to resist the action of the water, or of any débris brought down by the current. More than one fine viaduct has been swept away for want of due attention to the latter precaution.
[Fig. 97] illustrates a type of pier composed of cast-iron columns, well braced and stayed with wrought-iron. The ends of the columns and all contact surfaces should be properly turned and faced by machinery to ensure true and perfect joints, and the socketed ends should be turned and bored to fit closely. The latter is important, and if not carefully carried out, a slight sliding movement of the flanges may take place, and throw undue strain on the bolts.
[Fig. 98] shows a very similar pier, constructed entirely of wrought-iron or steel.
Each of the above-described piers has a liberal amount of taper or batter, both in the front and transverse elevation.
The size and number of the columns, and the dimensions of the braces or stays, will depend upon the height of the pier and the weights and strains to be sustained.
Many important and lofty viaducts have been erected on this principle of iron piers springing from masonry foundations, more particularly across deep rugged ravines abroad, where iron piers offered the only practical, substantial means of dealing with what appeared otherwise an impossibility.
