Some engineers entertain the notion that for short deck-spans an economy can be effected by using open-webbed girders instead of plate-girders. If there be no objection to increasing the depth considerably, some metal can be saved in this way; but, if the same depth must be employed for both types, there is but little, if any, saving in weight, provided that the detailing be done properly—besides the pound price of the manufactured steel is a trifle greater for the open-webbed structure.

Some years ago there were designed for a transcontinental line a number of plate-lattice-girder spans. Their raison d'être was supposed to be primarily their ability to pass water through them when submerged, but secondarily, economy. The designer claimed that they effected a saving of metal amounting to about fifteen hundred (1500) pounds for an eighty (80) foot, single-track span, and that the pound price for their manufacture was no greater than that for ordinary plate-girder work. The author once used plate-lattice girders for the cross-girders of the Union Loop Elevated Railroad of Chicago, but his object was simply to evade a troublesome clause in the city ordinance. The webs of these cross-girders were solid near mid-span and at the ends, and were open near the quarter points, while those of the railroad girders previously mentioned were solid at the ends and open over more than the middle half of the total length. As far as the author's experience goes, it takes just as much metal to build the webs open, and the pound price for the finished metal is a trifle greater than it is for ordinary plate-girder construction. The fact that this same railroad, when drawing up a set of standard plans a few years later, discarded the plate-lattice girders is a pretty sure indication that the advantages claimed for them were more imaginary than real. It is true, of course, that in case of submergence they would pass a certain amount of water through their webs; but it is seldom that a railroad company will build a bridge of any kind so close to the high-water mark as to run any risk of its being submerged.

In respect to the economics of deck and through riveted spans, it may be stated as a general proposition that, although the former sometimes require more metal than the latter, they effect a great saving in the cost of the piers, and hence are to be adopted whenever permissible. Deck spans are cheaper per se when the ties can rest on the chords. This arrangement works well with double-track bridges having trusses spaced about twenty feet centers with two lines of stringers.

The question of the comparative economics of pin-connected and riveted spans is treated at length in the next chapter.

In respect to the comparative economics of the various kinds of trusses, it might be stated that a very few of them have stood the test of time, all freak and expensive styles having been discarded, the only types used