First. About 29 per cent of the total weight of the trusses and laterals of this anchor arm is included in its outer half, and the average weight per foot of this portion is about 71 per cent of that for the entire structure. Applying this to the already computed weights of a double-track-railway cantilever-bridge having a 1,500-foot opening, and using the unit costs of materials in place as stated, makes the average value per linear foot of the outer half of the anchor arm $640. The weight of metal per lineal foot for a double-track steel-trestle one hundred and forty feet high is 4,200 pounds and its value is $210, to which should be added not to exceed $5 per lineal foot for the cheap concrete pedestals required to raise the column feet a short distance above the rock foundation. This shows that the trestle costs only one-third as much as does the outer half of the anchor arm.

Second. While it is conceded that the remaining portion of the anchor arm may weigh somewhat less per foot than it would as an independent arm, the difference will be small for the following reasons:

(a) As the moment over the pier is the same for all lengths of the anchor arm (because it comes entirely from the loadings on the cantilever arm and the suspended span), the weights of metal in the truss members lying near the pier will not differ greatly in the two cases.

(b) While the negative stresses due to the uplift will be increased by the halving of the resisting lever arm, on the other hand the direct live load stresses will be greatly diminished because of the halving of the span length, these two effects tending to offset each other.

(c) With the short anchor arm, the stresses in the outer diagonals (as well as in all the other main diagonals) and in the top chord members will always be tensile, hence eye-bars can be used for these members, thus effecting a great saving; because, owing to the increase in sectional area (to allow for rivet holes) and to the weight of the details, it takes nearly fifty per cent more metal to build a riveted tension member than is required for the corresponding eye-bars and their pins.

(d) While it is true that the short arm produces a greater uplift and, consequently, necessitates a heavier anchorage, it must be remembered that the value of an economically-designed anchor-pier is very small in comparison with the cost of the rest of the structure. Again, it must not be forgotten that with the long arm there is positive as well as negative loading on the anchor pier, and that, in consequence, it is possible that there would be no difference worth mentioning in the costs of the two anchor piers.

It seems to the author that, in view of the preceding, it ought to be evident without further calculation that a length for the anchor arm equal to two-tenths of the opening ought to be decidedly more economic than a length twice as great.

In respect to the substructure, Dr. Steinman in his design for his 1,500-foot-span, four-track, steam-railway-and-highway, cantilever bridge found the cost of two  main  piers  to  be  $1,262,000,  and  that  of two anchor piers