ENGINE LOADS 44. Use of En gine Loads. It was formerly the custom for railroads to spec ify that the engine to be used in computing the stresses in their bridges should be one of their own which was in actual use. The engines of differ ent roads were usual ly different both in regard to the weight on the various wheels and in regard to the number and spacing of the wheels. Of late years, consider able progress has been made towards the adoption of a typical engine load ing as standard. These typical engines (see Fig. 17, Article 25) vary only in re gard to the weights on the wheels, the number and spacing of wheels being the same in all engines.
The distance between wheels is an even number of feet, instead of an odd number of feet and inches and frac tions thereof. For examples of load ings which are in almost universal use, consult the specifications of Cooper or Waddell.
The labor of computation of stresses when engine loads are used is consid erably lessened by the use of the so called engine diagrams. Fig. 85 gives a diagram which has been found very convenient. The first line at the top represents the bending moment of all the loads about the point to the right of it. All the loads are given in thou sands of pounds, and all the moments are in thousands of pound-feet. The practice of writing results in thousands of pounds—or, in case of moments, in thousands of pound-feet or pound inches—is to be recommended, as it saves the unnecessary labor of writing ciphers. Throughout this text this practice has been extensively followed, the stresses being written to the near est ten pounds or one-hundred pounds as the case may be. For example, 6 433 may be written 6.43 or 6.4, the few pounds which are neglected mak ing no appreciable difference in the design. The distances are in feet.
As an example of the use of the first line at the top, suppose that it is de sired to find the moment of all the loads to the left of a certain point when wheel 6 (the numbers of the wheels are placed inside of the circles representing the wheels) is just over the point. The moment will
be 1 640 000 pound-feet, which is obtained by reading off the 1 640 just to the right of the line through wheel 6.
When using the first line for values at sections in the uniform load, the values given represent the moment of all wheel and uniform loads about the points in the line or section to the left of the value given. For example, if it is desired to find the moment about a point in line 2, it will be 19 304 000 pound-feet, the value 19 304 appearing to the right of the line.
The line of figures below the wheels indicates the distances between any two wheels.
The third line of figures indicates the distance from the first wheel to the wheel to the right. For instance, 37 is the distance from wheel 1 to wheel 7.
The values in the fourth line indicate the sum total of all the loads to the left of the value given. For example, 245 signifies that the loads 1 to 15 inclusive weigh 245 000 pounds.
The values in lines 5 and 6 are similar to those of lines 3 and 4, except that the starting point is at the head of the uniform load. For example, 40 in line 5, and 112 in line 6, indicate that it is 40 feet from the head of the uniform load to the wheel 12, and that wheels 18 to 13 inclusive weigh 112 000 pounds.
The values in lines 7 to 16 indicate the value of the moment of all the wheels from the zigzag line up to and including the one to the left or the right, according as the value is to the left or the right of the zigzag line. For example, 2 745, line 11, indicates that the moments of wheels 8 to 14 inclusive about the zigzag line just under wheel 15, is 2 745 000 pound-feet; or the value 1 704, line 14, shows that the moments of wheels 13 to 18 about the zigzag line just under wheel 12 is 1 704 000 pound-feet.
When line 4 of figures is under the uniform load, the values refer to the vertical line to the right;'thus 324 is the value of all loads to the left of line 3 about that line.