In 1875 the American Society of Civil Engineers, in view of the repeated bridge disasters in this country, appointed a committee to report upon The Means of Averting Bridge Accidents. We might expect, when a society composed of some hundreds of our best engineers selects an expert committee of half a dozen men, that the best authority would be pretty well represented; and such was eminently the case. It would be impossible to have combined a greater amount of acknowledged talent, both theoretical and practical, with a wider and more valuable experience than this committee possessed. The first point taken up in the report is the determination of the loads for which both railroad and highway bridges should be proportioned. In regard to highway bridges, a majority of the committee reported that for such structures the standard loads should not be less than as shown in the following table:-- +-------------------+----------+----------+----------+ | | POUNDS PER SQUARE FOOT. | | SPAN. +----------+----------+----------+ | | CLASS A. | CLASS B. | CLASS C. | +-------------------+----------+----------+----------+ | 60 feet and less | 100 | 100 | 70 | | 60 to 100 feet | 90 | 75 | 60 | | 100 to 200 feet | 75 | 60 | 50 | | 200 to 400 feet | 60 | 50 | 40 | +-------------------+----------+----------+----------+ Class A includes city and suburban bridges, and those over large rivers, where great concentration of weight is possible. Class B denotes highway bridges in manufacturing districts having well-ballasted roads. Class C refers to ordinary country-road bridges, where travel is less frequent and lighter. A minority of the committee modified the table above by making the loads a little larger. The whole committee agreed in making the load per square foot less as the span is greater, which is, of course, correct. It would seem eminently proper to make a difference between a bridge which carries the continuous and heavy traffic of a large city, and one which is subjected only to the comparatively light and infrequent traffic of a country road. At the same time it should not be forgotten, that, in a large part of the United States, a bridge may be loaded by ten, fifteen, or even twenty pounds per square foot by snow and ice alone, and that the very bridges which from their location we should be apt to make the lightest, are those which would be most likely to be neglected, and not relieved from a heavy accumulation of snow. In view of the above, and remembering that a moving load produces a much greater strain upon a bridge than one which is at rest, we may be sure, that, as the committee above referred to recommend, the loads should not be less than those given in the table. We can easily see that in special cases they should be more.