BRIDGE CONSTRUCTION, American. The application of scientific principles to the construction of bridges is more complete to-day than ever before. This statement applies to the specified requirements which the finished struc ture must fulfil, the design of every detail to carry the stresses due to the various loads im posed, the manufacture of the material compos ing the bridge, the construction of every mem ber in it, and finally the erection of the bridge in the place where it is to do its duty as an instrument of transportation.
A close study of the economic problems of transportation in the United States and the experimental application of its results led the railroad managers to the definite conviction that, in order to increase the net earnings while the freight rates were slowly but steadily mov ing downward, it was necessary to change the method of loading by using larger cars drawn by heavier locomotives, so as to reduce the cost of transportation per train mile. While these studies had been in progress for a number of years and there was a gradual increase in the weight of locomotives, it is only within the years 1885-90 that the test was made, under favorable conditions and on an adequate scale, to demonstrate the value of a decided advance in the capacity of freight cars and in the weight of locomotives for the transportation of through freight. The test was made on the Pittsburgh, B. & Lake Erie Railroad, which was built and equipped for the transportation of iron ore from Lake Erie to Pittsburgh and of coal in the opposite direction.
When the economicproposition was fairly established, it was wonderful to see how rail road managers and capitalists met the situation, by investing additional capital for the newer type of equipment and for the changes in road bed and location necessarily involved by that in the rolling stock. Curves were taken out or diminished, grades were reduced, heavier rails were laid and new bridges built, so that prac tically some lines were almost rebuilt. The process is still going on and money by the hun dred millions is involved in the transformation and equipment of the railroads. Some impres sion of the magnitude of the change in equip ment may be gained from the single fact, that one of the leading railroads has within a few years expended more than $20,000,000 for new freight cars alone, all of which have a capacity of 100,000 pounds.
The form of loading for bridges almost universally specified by the railroads of this country consists of two consolidation locomo tives followed by a uniform train load. These loads are frequently chosen somewhat larger than those that are likely to be actually used for some years in advance, but sometimes the heaviest type of locomotives in use is adopted as the standard loading. The extent to which the specified loadings have changed in late years may be seen from the following statement based on statistics compiled by Ward Baldwin and published in the Railroad Gazette: Of the railroads whose lengths exceed 100 miles, located in the United States, Canada and Mexico, only 2 out of 77 specified uniform train loads exceeding 4,000 pounds per linear foot of tracks in 1893, while in 1901, only 13 out of 103 railroads specified similar loads less than 4,000 pounds. In 1893, 37 railroads speci fied loads of 3,000 pounds and 29 of 4,000 pounds, while in 1901, 4,000 pounds was speci fied by 50, 4,500 pounds by 14 and 5,000 pounds by 17 railroads. The maximum uniform load rose from 4,200 in 1893 to 7,000 pounds in 1902.
In a similar manner in 1893 only one rail road in 75 specified a load on each driving wheel axle exceeding 40,000 pounds, while in 1901 only 13 railroads out of 92 specified less than this load. In 1893 only 21 of the 77 rail roads specified similar loads exceeding 30,000 pounds. The maximum load on each driving wheel axle rose from 44,000 pounds in 1893 to 60,000 pounds in 1901.
In 1916 on the majority of railroads the pre vailing practice is to design the bridges for axle loads of 60,000 pounds and practically no bridges are being built for lighter axle loads than 50,000 pounds. Bridges designed in ac cordance with present standards of details will safely carry an overload of 50 per cent in regu lar service without restriction in speed, and a still larger load with restricted speed.