Metal bridges and viaducts over railroad tracks frequently show excessive corrosion in the floor system and laterals, due to smoke and gas from locomotives, also because of the fact that the solid floors of such bridges do not permit the steel work beneath to dry out quickly.

Metallic overhead bridges having a scant clearance, so that the stacks of the locomotives come close to the steelwork, frequently show excessive wear from the sandblasting effect of cinders issuing from the engine exhaust, particularly when the location is on an ascending grade where the locomotive is worked hard under the bridge.

Possible deterioration of the structure of the metal itself, by fatigue, has in some quarters been a matter of apprehension; but it now seems to be recognized that no such internal deteriorating action takes place where the bridge has not been subjected to excessively high stress. If crystallization is found in the metal of a structure, it probably was there at the time the structure was built, and is due to improper methods of manufacture of the material.

It may, therefore, be taken as a certainty that iron and steel bridges, if not reduced in section by rust, etc., and if not shaky on account of inadequate bracing, are fully capable of carrying the figured loads at reasonable limiting unit stresses, provided they are carefully inspected and properly maintained.

Strengthening of Light Bridges

Strengthening of light bridges may be either a matter of reinforcing minor details which are found to limit the carrying capacity of the structures, or may consist of heavy reinforcing in an attempt to increase the strength thereof throughout. The minor strengthening can usually be done at small expense; and it is an economical method of getting considerably greater life out of bridges. Heavy reinforcing may or may not be an economical proposition, as it involves work being done in the field, which is costly, and the maintenance of traffic during the time the work is in progress, which involves some risk to traffic and is usually expensive. On very large bridges where the cost of replacing is great, some extensive strengthening operations have been carried out economically.

In making plans for reinforcing bridges, it is usually preferable to add new material to the structure so that the present structure is not temporarily weakened, rather than to remove parts and substitute heavier ones, though the latter expedient has sometimes to be resorted to. The descriptions of the points at which low classification usually occurs suggest in themselves how these might be strengthened.

In plate girders the top and bottom flanges may be reinforced by additional cover plates, particularly at points where the web is spliced and not effective for carrying its proportion of the bending stress. Where there are no cover plates on the girders, cover plates of desired length can be added. On plate girders where there are two or more cover plates, additional cover plates would be nearly the full length of the girder and expensive to apply. Plate girders can be doubled up to make deck spans, using three or more girders per span at small expense, thereby using up light girders and providing bridges of large carrying capacity.

Where waterways or other under-crossing conditions permit, timber bents can be placed under spans to strengthen them.

Where the rivets in the flanges of girders show low classification. larger rivets can be substituted for existing rivets, or, where the rivet spacing permits, additional rivets can be driven.

Where the web plates give a low classification, additional stiffeners can be placed in the panels near the ends of the girders to provide extra support for the web and thereby increase its classification.

Shelf-angles can be strengthened by placing short vertical stiffeners beneath them. Where web splices with low classification occur, these can be replaced with wider splice plates having additional rows of rivets in the splice.