This chapter is mainly a reproduction of a joint paper by Mr. H. Malcolm Priest and the author, lately presented to the Engineers' Society of Western Pennsylvania.

For many years past bridge engineers have held differing opinions concerning the advantages of continuous trusses as compared with the corresponding non-continuous ones. Some claimed a great saving in weight of metal from continuity while others felt sure there was none. The author had been under the impression that the advantage claimed for the continuous truss was mainly due to the ignoring of the effect of reversing stresses; and, as will be seen later, in this opinion he was partly right and partly wrong.

Under certain conditions it is not bad practice to use continuous trusses, and under others it is, irrespective of the question of economics. When the foundations of the piers are solid rock or other very hard material, continuity is permissible; but when they are piles or comparatively soft material without piles, it is better to forego any possible saving of metal rather than to run the risk of unequal settlement of piers and the consequent upsetting of stress distribution throughout the trusses from end to end of structure.

The most notable example of continuous trusses in America, or, as far as the author knows, anywhere else in the world, is the Sciotoville Bridge over the Ohio River on the line of the Chesapeake and Ohio Northern Railroad. At this location continuous trusses were permissible, for the reason that the pier foundations are solid rock at no great distance below the bed of the stream. That structure consists of two continuous, double-track-railway spans of 775 feet each. It was designed and engineered by Dr. Gustav Lindenthal, Consulting Engineer, and was completed in 1917.

Desiring to have this comparison of weights of metal for continuous trusses conform as closely as possible with actual conditions, the author assumed the outlines of the Sciotoville structure, and contrasted it with a bridge of two simple spans, each made five (5) feet shorter, so as to allow for the distance between centers of pedestals on the middle pier, using practically the same panel-lengths as those of the Sciotoville Bridge and economic truss-depths for the simple spans based upon the information given in "Bridge Engineering."

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