Although in years long gone by the author did considerable work in the line of examination and repairs of old bridges, he feels that he can no longer consider himself an expert therein. It is a case of tempora mutantur; for the methods employed today in bridge repairing are essentially different from those that were in vogue some two or three decades ago. For this reason the author appealed to several old friends in his specialty, who are authorities in this class of work, to furnish him data for the writing of this chapter. Several of them generously complied with his request, viz., Mr. Chas. F. Loweth, Chief Engineer and formerly Bridge Engineer of the Chicago, Milwaukee, and St. Paul Railway System, Mr. Carl S. Heritage, Bridge Engineer of the Kansas City Southern Railway Company, of which line the author is, and for some two decades has been, the Consulting Engineer, and Messrs. J. G. Chalfant and V. R. Covell, respectively County Engineer and Deputy County Engineer of the County of Allegheny, Penna. For their truly valuable aid the author desires to extend to these gentlemen his hearty thanks and his deep appreciation of their kindness and courtesy.

A rule of practice which the author established for his own guidance fully a quarter of a century ago seems to have found favor with the profession, viz., that any old bridge, which, in either main members or details, would be overstressed by the actual live loads passing across it, or likely soon to traverse it, not more than fifty per cent in excess of the standard intensities of working stresses employed in designing new structures, may safely be allowed to remain in use. If overstressed much more than this, it should be removed and employed at some other location where the traffic is light, or else scrapped. Exception was made in the case of plate-girder spans; for these could be relied upon to give ample warning of failing strength by rivets working loose. A plate-girder span when greatly overloaded will not collapse suddenly as will a pin-connected or even an open-webbed-riveted span.

A favorite economic expedient of the author's used to be to convert two old duplicate bridges into one and put in a new one at the crossing left vacant. This scheme was specially applicable on long lines of railway where standard I-beam and deck-plate-girder spans were used.

The more crudely a bridge was designed the more difficult it is to rein-

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