In many situations, steel would not alone supply the best material of construction. And the same remark applies to concrete. A striking instance is the case of the six new docks in Baltimore. Three of these had already been constructed of wood and stone, when it became evident that the building of the remaining three and of the long bulkhead which was part of the scheme along the same lines would entail a larger expense than the use of reinforced concrete construction. Steel by itself would have been impossible of consideration because of its susceptibility
to corrosion. Concrete alone could not be used because of the excessive cost of the increased amount of Portland cement. It is said, that a retaining wall of all concrete would have cost about $600 per linear foot. Reinforced concrete costs about $58 per foot.
Steel is used, not merely as a reinforcement, but as the material of forms. Used thus, it may, at times, not only retain the concrete in position but also prevent the action of the surrounding soil or water. The possibility of using steel for forms depends largely upon the fact that many applications of concrete are becoming standardized rapidly. Thus is permitted a re-use of the steel form that justifies the expense. But the employment of steel forms sometimes involves the use of steel in the handling of them. A further use, although perhaps more remote, is in connection with the arrangements for the handling of the mixed concrete and of the raw materials. Still more remote, but still a necessary application, is the use of steel and iron in the crushing mills and the like. When we look at the question and inform ourselves of the ramifications, it is not difficult to see that concrete and steel are materials whose engineering applications are mutually involved. Concrete is certainly replacing steel in some applications. But, notwithstanding this, these two are to be regarded as unopposed to each other on the whole.
When concrete is cast about steel, an adhesive bond ensues. But this is scarcely to be regarded as sufficient to enable the two to act as one under tensile stresses. A mechanical bond should be employed. This is the explanation of the somewhat complicated forms of standard reinforcement bars.
Concrete properly reinforced is an admirable material for factory construction. It permits of rapid erection, is fireproof, has a long life, is adapted to weather conditions, and is economical. The floors of concrete buildings are easily cleaned and do not develop splinters.
One of the large automobile factories—that of the Geo. N. Pierce Company at Buffalo, N. Y.—is a good instance of the rapidity with which reinforced concrete buildings may be erected. Within seven months of the date of signing the contract with the Trussed Concrete Steel Company, Detroit, Mich., which employs the Kahn system of reinforcement, certain large structures were ready for use. The floor space here is 325,000 square feet. It was necessary to provide a number of large areas unbroken by supports. It was found possible to use girders having spans of 55 and 61 feet. When subjected to a load, a girder develops compressive strains above and tensile ones below. The concrete is well adapted to withstand the one, but not the other. In an ordinary bridge truss, there may be diagonals that are also under tensile stress. In the Kahn system of reinforcement, a horizontal bar from which rigidly attached diagonals extend upward and outward is provided with a view of enabling the girder to withstand the tensile stress. In accordance with this design the long girders were constructed. Girders providing runways for 3-ton cranes were also constructed. A load of 14 tons placed upon one of the reinforced concrete girders having a span of 25 feet induced a deflection of only 1⁄16 inch. This girder is 12 inches wide and 22 inches deep and its reinforcement consists of three 1 x 3 in. Kahn bars. Hollow tile was largely employed here in connection with the concrete.
What is known in the trade as the corrugated bar, supplied by the Corrugated Steel Bar Company is a steel reinforcing rod which provides shoulders by means of which the concrete is mechanically engaged. This general type of reinforcement is, however, not confined to this concern. By means of this style of bar, the engineer is able to secure the desired mechanical interlock. As the concrete and steel expand and contract they do so together—unless the temperature change is excessive—and so the relation between the two is maintained. Such standard
types of reinforcing bars are applicable to multitudes of construction. An interesting example is the railroad bridge over the Vermilion River near Danville, Illinois. There are three arches, the central one of which has a span of 100 feet. About 130 tons of corrugated bars were employed in the construction of this beautiful bridge.
Another good example of bridge construction is the bridge over the Maumee River near Waterville, Ohio. This structure follows the designs of the National Bridge Company. It has a width of 16 feet between copings and crosses the river at a point where it is 1,000 feet wide. It is said that this reinforced bridge will carry a load of 5 tons per linear foot. The arches are 12 in number, the longest having a span of 90 feet, and the shortest, one of 75 feet. The loading of a bridge arch produces a lateral thrust upon the piers. If the next arch is not loaded, then this thrust is unbalanced and must be cared for. This was done in this case by employing part of the 100 tons of reinforcement in a vertical position. This bridge having a very long expectation of life was built at a cost of $77,000. The total amount of concrete was about 9,200 cubic yards.
The city of Philadelphia has gone into the construction of city bridges of concrete in rather an extensive way. Among a total of 30 or more is the reinforced bridge across Poquessing Creek, having a span of 71 feet. This bridge is rather flat, having a rise of but 9½ feet. The reinforcement employed here consisted in part of angle bars placed in pairs to form a kind of T-bar. The principal reinforcement here was the arch ribs. These were each composed of two of the T-bars arranged one above the other in such manner that their points of nearest approach were at the crown. These were latticed together. Such ribs were placed 4 feet apart. Transversely disposed steel rods held the whole together. The mechanical interlock here depended upon was due, no doubt, to the mutual disposition of the various rods, etc.