WEIGHTS OF BRIDGES 20. Formulæ. In order to obtain the stresses due to the weight of the structure, the latter quantity must be known. As this weight can be determined only after the structure has been designed, it is evident that an assumption as to the weight must be made. The best method is to use the actual weight of a similar structure of like span which has been built. As the necessary data for this is not always available, it is customary to use formulæ to derive an approximate weight of sufficient accuracy for purposes of computation. Table II gives some of the most reliable formula;.
In order to give an idea of the relative weights of steel in different classes of bridges, let it be required to compute the dead weight of a 100-foot span of, each class. For heaviest highway bridges to carry heavy interurban cars: w=600+180+ 27 X16 + 16 12100 \ 11 1 358 lbs. per linear ft.
For heavy pin-connected highway. bridges to carry heavy farm or traction engines: w — 34 +22 X 16 + 0.16 X 16 X 100 + 0.7 X 100 = 710 lbs. per linear ft. For light country highway bridges to carry 100 pounds per square foot of floor surface: w = 250 + 2.5 X 100 = 500 lbs. per linear foot.
If the total weight is required, the weight of the wooden floor must be added. Take, for example, the last bridge: The weight per linear foot for a railroad truss bridge of 100-foot span is: w = 650 + 7 X 100 = 1 350 lbs. per linear foot.
This is about the same as that for a heavy interurban bridge The reason for this is that in addition to the heavy rolling stock of the electric road, the heavy highway traffic must be provided for. A deck girder of 100-foot span weighs: w = 124 + 12 X 100 = 1 324 lbs. per linear foot.
21. Actual Weights of Railroad Spans. In case actual weights can be obtained, a more exact analysis can be made. The weights of bridges indicated in the accompanying tables and diagrams, are based on actual constructions recently erected. These bridges rep resent the very best modern practice of engineers and manufacturers The weights of through truss-spans made of medium steel and designed for E 50 loading, are given in Fig. 15. The weights include the weight of the ordinary open steel floor, and they also include the weight of the ties and rails, which is taken at 400 pounds per linear foot per track.
The weight of steel in medium steel deck plate-girder spans designed for E 50 loading, is given in Table III.
The spans are the distance center to center of bearings; and the weights do not include the weight of the ties and rails, which is to be taken at 400 pounds per linear foot per track. Intermediate spans may be interpolated.
22. Actual Weights of Highway Spans. The actual weights of highway spans for heavy interurban trolley-cars and traffic,should preferably be computed from the formuhe of Shaw or Waddell (Table II). The weights of country bridges, including floor, may be taken from the diagram of Fig. 16.
23. Classes of Loads. Those weights just given constitute what is called the dead load of the bridge. The traffic which passes over the bridge is called the live or moving load. In addition to the two classes mentioned, is the effect of the wind, which is designated as the wind load. These loads vary with the class of bridge, be it highway or railway, and with the purpose for which it is intended.
24. Live Loads for Highway Bridges. Highway bridges are usually divided into several classes according to the traffic, which may be that of heavy interurban cars, light trolley-cars, farm engines, road rollers, teams, human beings, or some combination of these loadings. The standard specifications of J. A. L. Waddell or of Theodore Cooper are obtainable for a very small sum. Their pur chase is advised, and the reader is referred to them for further infor mation.
The trusses of country highway bridges are usually designed for a live load of 100 pounds per square foot of roadway. This may be considered good practice; and it is the law in some States. The floor system of these same bridges should be of sufficient strength to sustain 100 pounds per square foot of roadway, or a 12-ton farm engine having 4 tons on the two rear wheels, which are 12 inches wide and 6 feet apart, and 2 tons on each of the front wheels, which are 6 inches wide and 5 feet apart. The axles of this engine are spaced 8 feet center to center.