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probably be calcined during a fire, the lower layer will of itself act as a fireproofing material which will prevent injury to the upper layers. Since the area of the lower layer is always regarded in computing the strength of the reinforced concrete, it is always possible after such a fire to scrape off the injured concrete and to replace it with a layer of other material which will again act as a fireproofing material. Structurally, the floor will be uninjured. A brief description of one of these tests will show the remarkable resistance of reinforced concrete to fire.

During November, 1905, a building was constructed near New Brunswick, New Jersey, for the special purpose of the test. The roof consisted of a four-inch slab of reinforced concrete supported on concrete beams. The side walls of the building were made of concrete. A grate of iron bars was built across the entire floor area and ample provision was made for draft. When the concrete had become suffi ciently hard, the roof was loaded with a dead load of pig iron to the amount of 150 pounds per square foot. On December twenty-sixth, the structure was tested. A fire was built and fed with cordwood until an electric pyrometer indicated a temperature of 1,700° F. This temperature, with small fluctuations above and below, was maintained for four hours. Then the firedoors were opened and a stream of water, having a pressure of ninety pounds per square inch at the pumps, was played on the under surface of the roof for ten minutes. As was expected, the lower layer of concrete, which had been calcined by the heat, was swept off by the mechanical

action of the powerful stream, but the roof still held its load of pig iron. On the following day, the concrete having cooled off and having recovered a large part of its deflection during the fire, still more pig iron was loaded on until the load amounted to 600 pounds per square

[graphic]

Hotel Dlemieim, Atlantic City, U. ]., A Type Of Reinforced Concrete Structure.

foot, and even at such a load, the fourinch slab, which had been subjected to such a severe alternation of intense heat and rapid cooling, was not broken down. The one fact that the structure was sufficiently elastic to recover, while cooling, a large proportion of its deflection during the intense heat shows a very remarkable quality of this material.

There was also a compensation in the San Francisco disaster when it was demonstrated that the few instances of reinforced concrete work which were located within the area of the disturbance were structurally uninjured by the earthquake. The monolithic character of these buildings prevented their disintegration when adjoining buildings, consisting of brick and stone joined by mortar joints having little cohesive strength,

[graphic]

View From Underneath The Playa Del Rey Bridge,

were rapidly disintegrated by the earthquake shocks. Owing to the limitations of the building laws there were no buildings in San Francisco itself which were constructed entirely of reinforced concrete, although there were many floors of this material. An official inspection of all injured buildings was made by an expert for the Hoard of Underwriters. His report on the injury to reinforced concrete floors was almost monotonously "no structural damage." The very few cases of reported injury were invariably accompanied by the statement that the supports of the flooring had given way.

Perhaps the most remarkable characteristics of reinforced concrete construction is the fact that girders, beams and

floor slabs, having a very considerable span and comparatively little vertical depth, may be built so as to carry the heaviest working loads desired by modern conditions. This characteristic only becomes possible on account of its power of resistance to transverse bending. Such resistance depends on the ability of the material to resist tensile stresses. This tensile strength is furnished by the steel which is so proportioned and placed that it will furnish the desired resistance. It is not very many year^since an engineer would have been considered foolish to have predicted that two such dissimilai materials as concrete and steel could be combined into a composite structure and that they would mutually reinforce each other and each supply the qualities the other lacked. The tensile strength of concrete is usually very small. Although some specimens have required a pull of 300 or 400 pounds per square inch and even more to break them, the breaking strength is usually not more than >ounds per square inch, which is so small that it becomes practically useless to depend on such strength for transverse stresses of any magnitude. It may be easily demonstrated by practice as well as by theory that a concrete beam, whose span compared with its depth is comparatively large, will not even support its own weight, to say nothing of carrying a live load. It is not considered safe practice to depend on a working tensile stress of more than 50 pounds per square inch in concrete. On the other hand, even a low-carbon steel will usually have an ultimate tensile strength of 55,000 to 60,000 pounds per square inch and a high-carbon steel, such as is frequently used in reinforced concrete, has an ultimate ten

200

sile strength of about 100,000 pounds per square inch. Even if we only allow a working stress of 16,000 pounds per square inch in the steel, we are using a working stress which is 320 times as great as that which is permissible in the concrete. A cubic foot of steel weighs about 490 pounds. At three cents per pound this is worth $14.70. On the other hand, a cubic foot of concrete is worth perhaps 20 cents or, let us say, l-75th of the cost of steel. But if the steel is 320 times as strong as the concrete we can afford to pay 75 times as much for the unit area of steel as for the unit area of concrete and even then the steel is more than four times as cheap as the concrete, considering what it will accomplish. On the other hand, with a good grade of concrete we may safely use a working stress of 500 pounds per square inch in compression. We cannot safely use more than 16.000 pounds per square inch as the working compression for steel. This is only 32 times the allowable working

material which furnishes it. Although the above unit values of concrete and steel may be varied, both actually and relatively, they are substantially correct and will never be modified so greatly as to alter the general conclusion that by constructing our beams and slabs by such a method that the tension is furnished by steel and the compression by concrete, we have the most economical combination of materials.

Of course, there is far more to the theory of reinforced concrete than the mere placing of steel in the tension side of a beam or slab. Every ounce of tension in the steel is only effective as it is transferred to the concrete. In the case of a plain beam with free ends, there is no stress in the steel at the ends while the maximum tension is usually at or near the center of the beam. The entire amount of this tension must be gradually transferred from the steel to the concrete. In the earlier designs the adhesion of the concrete to the steel was relied

[graphic]

BRIDGE OF REINFORCED CONCRETE AT PLAYA DEI. REY, NEAR LOS ANGELES, CALIFORNIA.

Extreme length, 305 feet 8 inches; span, 146 feet (15 feet longer than any other cement bridge span in the world); width, 19 feet; spring, 18 feet; height above water, 20 feet.

stress in the concrete, and, since the steel costs about 75 times as much as the concrete, the concrete is far cheaper as a material with which to withstand compression. It should be realized that the real test is the actual cost of obtaining so many pounds of tension or compression, almost regardless of the kind of

on to permit the transfer of this stress from one material to the other. Elaborate tests have been made to determine the amount of this adhesion. Although the experimental values vary, as was to be expected, there was sufficient uniformity apparently to indicate a fairly constant safe working value. A great deal of rein forced concrete work has been done —and is still being done—on the basis of the permanency of this adhesion. But it is now being realized that this adhesion is not permanent and that, regardless of its value in comparatively new and fresh test specimens, the adhesion is very greatly reduced with age and under certain unfavorable conditions, such as continued soaking of the concrete in water, long continued vibration, etc. Failures of floors have already occurred, due to loss of the adhesion after they have successfully supported heavy loads for many years. On this account "deformed" bars, which have an irregular surface and which furnish a "mechanical bond" are now being extensively and even exclusively employed by many engineers. Some of these bars require to pull them out of concrete more than twice the force that is required by plain bars of the same cross-section. This shows that even if the adhesion were entirely destroyed, the mechanical bond will still furnish as much resistance to slipping as will be furnished by adhesion alone under the most favorable circumstances. Such a union between the concrete and the steel at all points along its length is an absolute es sential to the stability of such structure

An unusual case of long span is illustrated in the Robbins garage recently built in New York City by the Reinforced Cement Construction Company. The span of the longest girders is fifty feet. It was designed for a live load of 150 pounds per square foot. The main girders have a total depth—to the top of the slab—of about three feet, and a width of about two feet. The smaller beams have a width of twelve inches, a total depth of eighteen inches and are spaced seven feet between centers. The slab itself is five inches thick. The illustration is a typical example of this method of floor construction.

A view, taken during construction, of one of the first large business blocks to be built structurally of reinforced concrete is also printed here. The skeleton of the building, the main columns and girders, as well as the floors, are made of reinforced concrete. The cut shows the Ingalls Building in Cincinnati. This building was constructed in 1903 and has sixteen office floors besides an attic, base

ment and sub-basement. The photograph of the Hotel Blenheim at Atlantic City illustrates another type of building which is also structurally of reinforced concrete.

It is said that a florist first conceived the idea of combining metal and cement, in making flower pots. He found that they could be made more tough and less liable to break by imbedding wire netting in the concrete. The success of these flower pots encouraged the extension of the principle of combining steel and concrete.

One of the most economical applications of reinforced concrete lies in the construction of retaining walls. Although there is some variability and uncertainty as to the amount of the actual lateral pressure of earthwork, the proper design of a solid masonry retaining wall becomes an exact problem when we have once assumed the direction, point of application and amount of the earth pressure. This usually requires a very large cross section of masonry, which is correspondingly expensive. The reinforced concrete method employs a comparatively thin vertical curtain wall and a large base plate which is as wide and perhaps a little wider than the ordinary plain retaining wall, the base plate being tied to the thin face wall by buttresses spaced at frequent intervals. The face wall and base plate are both capable of withstanding transverse stresses, while the stress in the buttresses is usually that of. tension. Since reinforced concrete is the one form of masonry which can withstand any considerable amount of transverse and tensile stresses, the above form of construction can only be made in reinforced concrete. Of course, the same form could be adopted if we used steel or wood, but the durability of either material would be so little that it would not pay to construct a retaining wall of such materials.

Another remarkable application of reinforced concrete is the possibility of making columns which are much stronger than plain concrete columns, and yet which do not employ a core of st:el to take the most of the compression. A column whose length is 20 or 25 times its diameter will probably fail by buckling, in which case the steel on the convex side of the column would be subject to tension rather than compression. But a "short" column must fail by compression, if subjected to sufficient stress. Even in this case, steel may be employed to furnish strength on account of its resistence to tension. Although the explanation is not theoretically exact, the principle might be explained by an illustration of filling a stove pipe with sand and subjecting it to compression. The sand alone, especially if dried, would not sustain its own weight as a column. When confined by the stove pipe the compression of the sand will cause a bursting pressure on the pipe. If the pipe were filled with a liquid instead of sand and if a piston, which fitted the pipe tightly, were placed on top of the liquid so that a load could be placed on the piston, the resulting bursting pressure on the pipe would be a perfectly definite mathematical quantity depending on the load which was placed on the piston and also on the weight of the liquid. When we use sand instead of the liquid, the grains of sand will tend to lock themselves together and the load on the sand would need to be proportionately far greater to produce any given tension in the pipe. Using concrete instead of sand the resistance to the "flow" of the material will be still greater, which practically means that a comparatively small amount of tensile strength in the pipe will produce a very much added resistance to compression. In practice, instead of using an actual pipe of metal, a series of rings made of light bars and spaced a few inches apart are bent around a few longitudinal bars whose chief function is to form a framework on which to fasten

the horizontal rings and prevent them from becoming displaced during the laying and tamping of the concrete. Such compression members are used not only for vertical columns, but also as the compression members of truss bridges,

[graphic]

Ingalls Building,

Cincinnati. O., An All-concrete Structure, In

Process or Erection.

of which several have been constructed. Tests of such columns have required a compression of over 6,000 pounds per square inch to cause failure. Although the construction of trussed forms in reinforced concrete is not common, the reinforcement of vertical columns in such a manner that they may be safely subjected to greater loads than should be placed on plain concrete columns of equal size, is now recognized as safe engineering practice.

Another useful application of rein

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