(b) An I-beam span concreted.
(c) A reinforced concrete deek-girder slab.
The different types were drawn up and an estimate made, with the following result, this estimate being per square foot of bridge : Reinforced deck-girder spans 100 per cent.
I-beam span 171 per cert.
Trough girders 208 per cent.
This shows conclusively that a great saving could be made by the use of the first type.
Abutments. The soil underlying this location is com posed chiefly of soft, blue clay and quicksand, and, taken as a whole, is not very desirable material to place a foun dation on; but it was decided that with the bearing of the footing, in addition to the piles, it was amply safe to carry the abutment load.
The footings for the several streets were put in dur ing 1905 in advance of the other work, and were built up to a point about three feet above top of footing proper. This was found to be quite a help as a line and lock for the forms.
The abutments are of the solid type, as it was found that a buttressed abutment of this height would cost as much per linear foot, if not more; and as the forms were built and concreted after the temporary bridge was up, the work was done in cramped space.
The forms were continuous, composed of studding and sheeting, the former being 3 inches by 6 inches, and the latter 2-inch tongued-and-grooved. The sides were held together by rods, spaced about 4 ft. C. to C. both vertically and horizontally. This scheme worked far better than wire, which was continually stretching and breaking. With wire it was not wise to fill to a depth of over four feet ; while with the rods, the form could be tompletely filled without danger of collapsing.
The rods are encased in 1l/2-in. pipe of such lengths that the ends were about 3 inches from the faces of the forms, waste being packed around the rods. This worked very nicely, although there was considerable trouble in pulling the rods. A lever and chain were used to remove rods from wall.
In concreting, the abutments were divided into a number of sections, each containing about a day's work. The mixing was done by machine, driven by power from the hoisting engine, the outfit being all mounted upon a flat car.
The mixer was set directly over the section where possible, and the material delivered by chute; but in cases where the section was too far away to use chutes, a locomotive derrick car having a yard bucket was used. Facing was obtained by spading, giving a smooth, uniform surface. The mixture was 1 :5, com posed of 11/4 barrels of cement to a cubic yard of pit-run gravel, this amount of cement being increased where the gravel was very fine. The gravel as a general thing was very fair quality, containing very little clay or other for eign matter, and varied in size from 3-inch ring down to fine sand, the sand being in about the right proportion to fill the voids nicely.
The sections were, as a general thing, completed in one day; but where this could not be done, the surface was thoroughly cleaned the following morning, being scrubbed with wire brushes to remove all laitance, clay, or other foreign substances, and then well rinsed off, before placing the fresh concrete. Frequently heavy horizontal lines are seen upon concrete work, marking the successive days' work. This can, to a great extent, be taken care of by drawing up the forms so as to take up the shrinkage due to the setting of the previous day's work.
Column Footings. The loads upon the columns are as follows: Center column, 204 tons; and sidewalk column, 138 tons. The question as to kind of foundation lay between the use of an ordinary pile and concrete footing and cylinders, which, after investigation, was estimated to be about a stand-off as regards price. Cylin ders were adopted, as they would rest upon a known substance; while the bearing power of a pile footing would be rather indefinite.
These cylinders are 3-ft. 8-in. wells, lagged with 2-in. lumber in about 5-ft. 6-in. lengths, and held from collapse by two 2-in. by steel rings. These rings are made in two pieces, and, after being driven into place, are bolted together. The depth of the cylinders varied from 15 to 65 feet. In all there were about 600 wells dug, averaging about 40 feet deep, and costing approximately $250.00 each.