the pound prices of the two types, the said difference usually, but not always, being in favor of the latter.

ELECTRIC RAILWAY BRIDGES

This type of structure is found on interurban roads and elevated railways. The economic problems of the deck and floor-system are similar to those in steam-railway structures, except that the construction is much lighter. The economic lengths are somewhat greater because the loads are smaller, but the economic principles involved are identical.

HIGHWAY BRIDGES

Decks

For highway bridges there are two general types of deck, viz., timber and concrete. In respect to first cost of both the deck itself and the material required to support its weight, the timber type is always the cheaper, the saving increasing with the span length. In times past the timber deck per se was very much less costly than the concrete deck, but to-day it is otherwise, because the price of timber has risen more rapidly than that of any other material employed in bridge construction. Whether at the present time in any particular case timber or concrete per se for decks is the cheaper will depend somewhat upon the locality and the availability of supply for the various materials. The solution of the question has to be determined specially for each case, as it arises, but the difference in first cost for the two types will seldom be found great. The real economic question involved is one of comparative weights and costs of the materials required to carry the deck.

When, however, maintenance and renewals are taken into consideration, the concrete deck will nearly always be found to be the more economical for short spans and those of moderate length. There is, though, one factor of vital importance that cannot well be considered in the economic comparison, viz., the danger from fire. This is so important as to rule out the use of timber in bridge decks for all cases in which it is practicable to raise the money required for the concrete construction, excepting only that creosoted wooden blocks on a reinforced concrete base are permissible on account of being almost fireproof, or at any rate very slow-burning.

An exception might be made for the movable span in a bascule bridge because the tipping of the floor to a vertical, or nearly vertical, position is somewhat objectionable, although it is not impracticable to design a concrete floor with a concrete pavement thereon in such a manner as to withstand effectively the said tipping. In case the timber floor be adopted, it would be necessary to take the utmost precaution against injury by fire.

Pavings for Roadways

The type of paving to adopt depends upon whether any timber is to be used in the deck. Planks make the cheapest kind of flooring, but