752. Expansion joints were provided in the spandrel arches at the ends, two at each pier and one at each abutment, to allow some movement due to changes in temperature. The expansion joints were made by placing in the joint several thicknesses of corrugated asbestos board protected by a 1/8-inch lead plate folded into the joint, forming a trough at the top. The lead plate lies flat on top of the concrete for five inches from the joint, and about two inches at each end of the plate is bent down at right angles and set into the concrete. An asphaltic composition is then laid over the lead plate, entirely covering it and filling the trough.

The centering was erected on pile bents spaced about 14 feet centers, the calculated pressure on each pile being about eighteen to twenty tons. For the center span, five 60 foot deck plate girders resting on pile piers were used over the deepest portion of the channel to provide for possible floods bringing large amounts of drift.

753. The arch ring was laid in voussoir courses as described in § 746. Face joints were made by securing triangular shaped pieces to inner face of the molds in lines approximately at right angles to the line of pressure. All exposed work was faced with a layer of about 1 1/2 inches of Portland cement mortar placed and rammed with the concrete. The surfaces were not given, in general, any further finish, no attempt being made to remove or conceal the usual marks left by the mold boards.

Portland cement was used throughout, the quality of the concrete being varied by the amount of cement used to given quantities of the aggregates. In the centers of large masses the poorer mixtures were employed, while the richer concretes were used in those places subjected to the most trying conditions.

In making the concrete the principle followed seems to have been to keep the mixer as near the work as practicable, moving the mixer and carrying materials to it, rather than to transport the mixed concrete from a certain fixed location of the mixing plant. Much of the concrete was handled in barrows, but derricks were also used in portions of the work. As traffic on the old bridge had to be maintained during the erection of the new structure, considerable extra handling of concrete was necessary and additional work was involved in ramming the concrete in places difficult of access. The concrete was mixed rather wet, so that but little tamping was required to make it quake.

754. Cost

The total amount of concrete was over 12,000 cubic yards, which was placed at an average Cost of $5.43 per cubic yard. In cofferdams and centers 400,000 feet B. M. of timber was used, and about 300,000 pounds of steel was employed in the skeleton structure of the spandrels. This steel Cost 1.2 cents per pound, the punching, fitting and erecting costing but about 0.61 cent per pound. The total cost of the bridge is estimated to have been $125,000.00, or about the same as the estimated Cost of a steel structure designed for the same duty.

755. The Melan Arch Bridge At Topeka

The Melan Arch Bridge At Topeka, Kan., is one of the most important concrete-steel structures yet erected in the United States. It consists of one span of 125 feet, two of 110 feet each, and two of 97.5 feet each. The foundations for piers and abutments are piles in soft sand. The steel reinforcement is in the form of a latticed member. The bridge is fully described in Engineering News of April 2, 1896, and Engineering Record, April 16, 1898.

756. Concrete-Steel Viaduct

A viaduct of ten concrete-steel arches, of about 65 foot span, carries a double track electric line across West Canada Creek near Herkimer, N. Y.1 The piers rest on piles driven into hard blue clay, the surface of which is 6 to 12 feet below the creek bed. The segmental arches have a rise of 12 to 14 feet, with thickness of 21 inches at the crown and 4 1/2 feet at the springing; the radius of intrados is about 46 feet, and of extrados about 57 feet. The stresses were computed for full load and for live load on half span, Prof. Cain's graphical method being employed. The maximum stresses allowed were six hundred pounds per square inch compression in concrete and ten thousand pounds per square inch tension in steel. The stresses caused by a variation of fifty degrees in temperature were allowed for. The tensile strength of the concrete was disregarded. Thacher bars, 1 1/4 inches diameter, were used for the reinforcement, being placed eleven inch centers near both intrados and extrados.

757. Expansion joints were provided in spandrel walls by nailing to the sides of the forms for arch pilasters a narrow strip of timber, thus forming a groove into which the spandrel wall is tongued. These joints show some motion and allow some water to leak through.

The concrete was mixed three parts sand and seven parts gravel to one volume packed cement for foundations and piers, and two and one-half parts sand and five of gravel to one cement for the arch rings and spandrel walls. All concrete was mixed wet and by hand. The work was faced with mortar composed of one part cement to two and one-half parts sand, and after the removal of forms the face was brushed with thin mortar wash and rubbed with sandstone blocks, giving a uniform color to the surface.