Bridges and viaducts consisting of arches of masonry or brickwork form the most substantial and permanent works of construction for railway purposes; once properly built, the expenditure on future maintenance or repairs is merely nominal. For viaducts the span of the arching must be regulated by the height of the viaduct. The greater the height the larger the span. In one case 30-feet spans may be suitable, whereas in another it may be more economical to introduce spans of 60 feet or more, and so reduce the number of lofty piers. From a cost point of view there is, however, a limit to the span of arching, and, except for special cases, where expenditure is of secondary importance, large spans are very rarely adopted. Arches of large spans, no doubt, have been built both in masonry and brickwork, and have been a complete success in every way except expense. Unfortunately, the quantity and weight of materials in arching, and the corresponding cost, increase
very rapidly as the span increases, and for openings of more than 60 or 70 feet girder-work becomes much cheaper than arching.
[Figs. 92] and [93] are examples of viaducts having piers of masonry, with girders to carry the roadway. In the one case the roadway is carried on the bottom flange of the girders, and in the other on the top. The latter arrangement affords greater facility for securely bracing the girders together, while for the former it is claimed that the girders form a massive parapet, which would serve as a protection in the event of an engine or vehicles leaving the rails.
In the early days of railways, many large viaducts were constructed having masonry piers, and timber trusses to carry the roadway. Much ingenuity was displayed in designing the trusses, and in the introduction of cast-iron joint-shoes and wrought-iron bracings. Many of these wooden superstructures served well for several years, but they were always exposed to the imminent risk of destruction from fire, and however carefully the logs may have been selected, the decay of the timber was only a question of time. The deterioration of one piece was equivalent to the weakening of the entire truss, and the renewal of any part was both difficult and costly. The shrinkage of the timber, and the working at the joints, caused the trusses to deflect considerably under a passing load, and although the actual strength of the structure may not have been much impaired, the creaking and depression had anything but a reassuring effect. Timber superstructures for anything but small spans are rarely adopted now, except for temporary works, or on lines abroad, where the transport on girder-work would be very costly, and where good timber is very cheap and abundant. Even in the latter case the wooden superstructure is generally looked upon as a temporary expedient, to be replaced at no very remote date with iron or steel girders, when the materials can be conveyed over the entire completed line.
[Figs. 94, 95, and 96] are sketches of three types of timber trusses as constructed in viaducts of several spans.
There are many localities, especially abroad, where suitable stone is most difficult to obtain, and very expensive to work and convey. In such cases it is compulsory to use as little of it as possible, and to resort to iron or steel both for the girders and a large portion of the piers. The piers may be made of cast-iron,