A railway viaduct, one-half mile or more in length is another example of the Kahn methods. This structure belongs to the Richmond & Chesapeake Railway and is located at Richmond, Va. There is a span of 70 feet which has girders nearly 6 feet deep. At another span the girders, probably of about the same depth, sagged but ⅛ inch upon removal of the falsework.
A style of reinforcement much used consists of a net-like fabric of metal. This is employed largely in floors to bind the whole mass together. In the manufacture of this netting, a Canadian company has found it desirable to repair the inevitable breakages of strands in manufacture by the use of the Davis-Bournonville Company’s oxyacetylene torch. It is said that welds can be made on the average of one in two minutes in the case of an ordinary weight of the fabric. This netting is made by expanding sheets of perforated metal from a narrow to a considerable width. It is during this expansion that the strands sometimes break.
Another style of floor reinforcement is the fabric made from wire. That floors properly reinforced are quite substantial may be judged from the case of the United States Fidelity & Guarantee Company. Their building in Baltimore was exposed to intense heat in the great fire of 1904. In fact, a considerable part of the side walls and the front fell, leaving floors of concrete. A load of brick giving a pressure of 300 pounds per square foot was arranged on one of the floors to a distance of 5 feet to each side of one of the girders. The deflection amounted to ⅛ inch. This was about 1⁄20 of 1 per cent of the span. This is an example of Hennibique construction.
Reference has already been made to the Kahn truss reinforcement. With the same general object in view, the Hennibique truss has been designed. There are two horizontal bars, one above the other. The upper is, however, not perfectly horizontal except near the center. Towards
either side, this bar rises as it recedes from the center. These two bars are enveloped by loose stirrups arranged vertically and at intervals. These are open at the top and closed below.
There are two varieties of piles—the bearing pile and the sheet pile. Their duties are quite different. One sustains a compressive load, the other withstands a transverse thrust. But concrete has been used for both kinds. In the case of the bearing pile, its own intrinsic qualities are eminently suitable. It has good compressive resistance; it is teredo proof, and has the prospect of long life whether conditions are wet, dry or a mixture of the two. Wood makes an admirable bearing pile, if constantly submerged, but it is a prey to the teredo. The necessity for constant submergence limits the usefulness of the wood pile. It must be cut off below the hydraulic level, and this necessitates carrying the foundation footings to a lower level than would otherwise ordinarily be the case. With the concrete bearing pile, on the contrary, the footings may be constructed at any level desired as the pile itself may be partly submerged and partly in the dry. However, the concrete pile may be subjected to other than compressive stresses, especially during its placement. And so, some reinforce it. Some, no doubt, have in view a possible buckling when in the ground, particularly if the surrounding soil is yielding. Reinforcement both longitudinal and transverse is employed. Longitudinal bars are arranged at intervals around and within the periphery. These may be bound together by separate hoops disposed along the length or by wire wound about the longitudinals in spirals. In the case of concrete sheet piling, the concrete supplies a surface and forms a protective covering to the imbedded reinforcement which is here a vital matter and consequently indispensable. In the dock improvements at Baltimore, to which reference has already been made, reinforced concrete sheet piling was largely used. The steel sheet pile
could not well have been used here because of its susceptibility to corrosion. The concrete slabs, 12 × 18 inches in cross-section perform the duty of retaining masses of earth in place both above and below the water line. There were certain other concrete constructional elements of an auxiliary character. The total reinforcement amounted to about 1,200 tons.
With regard to its fireproof qualities, an eloquent testimonial arises from the fact that the immense Marlborough-Blenheim Hotel at Atlantic City, a concrete and tile structure, is said to enjoy a saving of $18,000 per year in fire insurance premiums. The insurance is based on $600,000. This structure is 560 feet in length and has a width varying from 60 to 200 feet.
Reference has been made to the close identity of the co-efficients of expansion for steel and concrete for moderate intervals of temperature. While this is so, if the thermal range is considerable, the concrete and the steel cannot be expected to expand and contract together. In most engineering construction, the range is small, say 150 degrees F., but there are exceptions. One of these relates to the material used in tall chimneys. The hot gases which pass up these give rise to rather high temperatures. In fact, it is well-recognized practice to build a large part of such chimneys double, one shell enveloping another, with an air space between. Some four or five years ago what is, perhaps, the very tallest concrete chimney in the United States was built for the Colusa Parrot Mining & Smelting Company, Butte, Montana. It is 352½ feet high and has a flue 18 feet in diameter. A solid wall 1½ feet thick constitutes the base of 21 feet in height. Above this level, an air space 4 inches wide radially is arranged between two shells of 5 and 9 inches thick. The inner one is the thinner. The steel reinforcements used were T-bars. The footing is of reinforced concrete and rests upon a fill 18 feet deep. A further important factor which has to be
considered is the serious effect of repeated stresses. Partly because of this, it is recommended that a large factor of safety be adopted. Further, the best practice would seem to be in the direction of a complete divorce between the inner and outer shells all the way up and of a uniformity in wall thickness from bottom to top. Vertical cracks have been noted in some chimneys. This would indicate the advisability of strong circular reinforcement. It is thought that a tone concrete following the formula 1:2:2 is better for the outer shell than a cement mortar. It is said to be stronger, denser and more impervious to water than a mortar following the formula 1:3. In order to secure adhesion between layers, the fresh concrete should be applied wet and the old should perhaps be resurfaced by tooling.