The above examples of under-line and over-line bridges are given more with a view of illustrating some of the many different descriptions of flooring, rather than to point out or suggest the type of main girder to carry the load. The description and size of the main girders can be varied to suit the span of the bridge, the requirements of the traffic, and the opinion of the designer. For spans up to 50 feet it will generally be found that web-plate girders are both simpler and cheaper than lattice or truss girders; at the same time, there are occasions where plate girders can be advantageously adopted for very much larger spans, as, for instance, in the example given in [Fig. 145], where the deep plate girders form a most efficient screen.
[Figs. 160] to 194 give diagram sketches of a few out of the many forms of open, or truss, girders which have been adopted for large spans. There are many types from which to make a selection, each one possessing its own special features and
advocates. In working out the details of any, or all of them, there are some points which should always be kept in mind when deciding the distribution of material in the main booms. Rain-water, or moisture of any kind, is the great enemy of wrought-iron or steel work, and therefore the plates, angles, tees, or channel sections, should be so arranged as to afford the least possible facility for the collection or lodgment of water. With open, level booms, as in [Figs. 137], [139], [140, 144], and [145], the rain-water cannot collect, but runs off at the sides, and the plates are quickly dried by the sun and wind. With trough booms, as in [Fig. 158], the collected rain-water can only get away through holes drilled for the purpose in the bottom plates. These holes are liable to become choked up, but even when open they rarely carry off all the accumulated water; some of it remains to corrode the plates, and is only dried up by evaporation. The inside of trough booms should be constantly inspected, and the exposed plates more frequently painted than the rest of the girder. In a similar manner, in small double-web lattice girders, with the lattice-bars inserted between two angle irons, as in [Fig. 159], the rain-water finds its way into the spaces at A, A, in spite of the most careful packing or filling with cement or asphalte. Numbers of small girders of this latter type have had to be taken out after a comparative short life, in consequence of the great corrosion and wearing away of the lower ends of the lattice-bars and angle irons into which they were inserted.
It is most essential, also, that all portions of the girder-work should be conveniently accessible for inspection and painting. Complicated connections, and parts which are difficult to examine, are liable to be overlooked, or, at the best, only painted in a very imperfect manner. Neglected corners soon create deterioration, the paint scales off, corrosion commences, and the working section is gradually reduced. A discovered weakness in some of the important parts points to an early condemnation of the entire structure. The difficulty of access to the interior of box or tubular girders, especially those of small or moderate dimensions, is a great objection to that type of girder. Experience has pointed out that open girders, free and exposed to the light and air, can be so much more effectually inspected and painted.
