The usual railway steel trestle consists of alternate towers and intermediate spans. In determining the economic span-lengths for such a structure, there are two separate factors to be studied:

1. The ratio of length of intermediate span to that of tower-span.

2. The distance from center to center of towers.

Investigation of the first of these factors for a single-track-railway trestle shows that, for Class 40 loading, the length of tower-span should vary from 44 per cent of the distance between centers of towers for a trestle 60 feet high, to 35 per cent thereof for one 240 feet high, the length of intermediate span for these limits being from 1.27 to 1.86 times that of the tower-span. For a Class 70 loading the corresponding percentages for tower-spans vary from 46 to 38, and the corresponding span-ratios from 1.18 to 1.63. After the first factor has been investigated the second one can be determined.

Fig. 27a gives the economic distances from center to center of towers and the economic lengths of intermediate and tower-spans for single-track railway trestles, varying in height from 60 feet to 240 feet, and for Classes 40 to 70. Considerable variation from these economic relations will affect the total weights but slightly. It will be noted that the economic lengths are considerably greater for light loadings than for heavy ones.

Fig. 55oo on page 1259 of "Bridge Engineering" covers the same data as Fig. 27a, but the lengths there given are incorrect. In the investigation for Fig. 55oo no attempt was made to ascertain the economic ratio of intermediate span-length to tower span-length, it being assumed that it would be sufficiently accurate, in determining the economic distance from center to center of towers, to consider the length of the tower-span fixed, and to vary the length of the intermediate span only.

The weights of longitudinal bracing given in Fig. 55pp on page 1260 of "Bridge Engineering" are also incorrect. The weights should vary from 450 pounds per vertical foot for 30-foot tower-spans to 580 pounds for 60-foot tower-spans, being independent of the height.

The total weights of metal given by Fig. 55rr on page 1262 of "Bridge Engineering" are but slightly in error, being about 2 per cent too great for a 60-foot height, and about 6 per cent too small for a 240-foot height.

In Fig. 27b are given the economic span-lengths for various heights of low, single-track-railway  trestles  consisting  of towers with two rocker

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