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probably be calcined during a fire, the action of the powerful stream, but the lower layer will of itself act as a fire- roof still held its load of pig iron. On proofing material which will prevent in the following day, the concrete having jury to the upper layers. Since the area cooled off and having recovered a large of the lower layer is always regarded in part of its deflection during the fire, still computing the strength of the reinforced more pig iron was loaded on until the concrete, it is always possible after such load amounted to 600 pounds per square 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
HOTEL BLENHEIM, ATLANTIC City, N. J., A TYPE 5F REINFORCED CONCRETE per square foot. On December twenty-sixth, the structure was tested. A fire was built and foot, and even at such a load, the fourfed with cordwood until an electric py- inch slab, which had been subjected to rometer indicated a temperature of 1,700° such a severe alternation of intense heat F. This temperature, with small Auctu- and rapid cooling, was not broken down. ations above and below, was maintained The one fact that the structure was suffifor four hours. Then the firedoors were ciently elastic to recover, while cooling, opened and a stream of water, having a a large proportion of its deflection durpressure of ninety pounds per square inching the intense heat shows a very remarkat the pumps, was played on the under able quality of this material. surface of the roof for ten minutes. As There was also a compensation in the was expected, the lower layer of con- San Francisco disaster when it was crete, which had been calcined by the demonstrated that the few instances of heat, was swept off by the mechanical reinforced concrete work which were
located within the area of the disturbance floor slabs, having a very considerable were structurally uninjured by the earth- span and comparatively little vertical quake. The monolithic character of depth, may be built so as to carry the these buildings prevented their disinte- heaviest working loads desired by modgration when adjoining buildings, con- ern conditions. This characteristic only sisting of brick and stone joined by mor- becomes possible on account of its power tar joints having little cohesive strength, 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 years 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 VIEW FROM UNDERNEATH THE PLAYA DEL Rey BRIDGE.
breaking strength is
usually not more than were rapidly disintegrated by the earth- 200 pounds per square inch, which is so quake shocks. Owing to the limitations small that it becomes practically useless to of the building laws there were no build- depend on such strength for transverse ings in San Francisco itself which were stresses of any magnitude. It may be constructed entirely of reinforced con- easily demonstrated by practice as well crete, although there were many floors as by theory that a concrete beam, whose of this material. An official inspection of span compared with its depth is comall injured buildings was made by an ex- paratively large, will not even support its pert for the Board of Underwriters. His own weight, to say nothing of carrying a report on the injury to reinforced con- live load. It is not considered safe praccrete floors was almost monotonously tice to depend on a working tensile stress "no structural damage.” The very few of more than 50 pounds per square inch cases of reported injury were invariably in concrete. On the other hand, even a accompanied by the statement that the low-carbon steel will usually have an ultisupports of the flooring had given way. mate tensile strength of 55,000 to 60,000
Perhaps the most remarkable charac- pounds per square inch and a high-carteristics of reinforced concrete construc- bon steel, such as is frequently used in tion is the fact that girders, beams and reinforced concrete, has an ultimate ten
sile strength of about 100,000 pounds per material which furnishes it. Although square inch. Even if we only allow a the above unit values of concrete and working stress of 16,000 pounds per steel may be varied, both actually and square inch in the steel, we are using a relatively, they are substantially correct working stress which is 320 times as and will never be modified so greatly as great as that which is permissible in the to alter the general conclusion that by concrete. A cubic foot of steel weighs constructing our beams and slabs by such about 490 pounds. At three cents per a method that the tension is furnished by pound this is worth $14.70. On the other steel and the compression by concrete, hand, a cubic foot of concrete is worth we have the most economical combination perhaps 20 cents or, let us say, 1-75th of of materials. the cost of steel. But if the steel is 320 O f course, there is far more to the times as strong as the concrete we can af- theory of reinforced concrete than the ford to pay 75 times as much for the unit mere placing of steel in the tension side area of steel as for the unit area of con- of a beam or slab. Every ounce of tencrete and even then the steel is more than sion in the steel is only effective as it is four times as cheap as the concrete, con- transferred to the concrete. In the case sidering what it will accomplish. On the of a plain beam with free ends, there is other hand, with a good grade of con- no stress in the steel at the ends while crete we may safely use a working stress the maximum tension is usually at or of 500 pounds per square inch in com- near the center of the beam. The entire pression. We cannot safely use more amount of this tension must be gradually than 16,000 pounds per square inch as transferred from the steel to the conthe working compression for steel. This crete. In the earlier designs the adhesion is only 32 times the allowable working of the concrete to the steel was relied
BRIDGE OF REINFORCED CONCRETE AT PLAYA DEL REY, NEAR LOS ANGELES, CALI
FORNIA. Extreme length, 205 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 on to permit the transfer of this stress costs about 75 times as much as the con- from one material to the other. Elabocrete, the concrete is far cheaper as a rate tests have been made to determine material with which to withstand com- the amount of this adhesion. Although pression. It should be realized that the the experimental values vary, as was to real test is the actual cost of obtaining be expected, there was sufficient uniformso many pounds of tension or compres- ity apparently to indicate a fairly constant sion, almost regardless of the kind of safe working value. A great deal of reinforced concrete work has been done ment and sub-basement. The photograph
-and is still being done—on the basis of of the Hotel Blenheim at Atlantic City the permanency of this adhesion. But it illustrates another type of building which is now being realized that this adhesion is is also structurally of reinforced concrete. not permanent and that, regardless of its It is said that a florist first conceived value in comparatively new and fresh test the idea of combining metal and cement, specimens, the adhesion is very greatly in making flower pots. He found that reduced with age and under certain un- they could be made more tough and less favorable conditions, such as continued liable to break by imbedding wire netsoaking of the concrete in water, long ting in the concrete. The success of these continued vibration, etc. Failures of flower pots encouraged the extension of floors have already occurred, due to loss the principle of combining steel and conof the adhesion after they have success- crete. fully supported heavy loads for many One of the most economical applicayears. On this account “deformed” bars, tions of reinforced concrete lies in the which have an irregular surface and construction of retaining walls. Alwhich furnish a "mechanical bond" are though there is some variability and unnow being extensively and even exclu- certainty as to the amount of the actual sively employed by many engineers lateral pressure of earthwork, the proper Some of these bars require to pull them design of a solid masonry retaining wall out of concrete more than twice the force becomes an exact problem when we have that is required by plain bars of the same once assumed the direction, point of apcross-section. This shows that even if plication and amount of the earth presthe adhesion were entirely destroyed, the sure. This usually requires a very large mechanical bond will still furnish as cross section of masonry, which is corremuch resistance to slipping as will be fur- spondingly expensive. The reinforced nished by adhesion alone under the most concrete method employs a comparatively favorable circumstances. Such a union thin vertical curtain wall and a large base between the concrete and the steel at all plate which is as wide and perhaps a points along its length is an absolute es. little wider than the ordinary plain resential to the stability of such structure taining wall, the base plate being tied to
An unusual case of long span is illus- the thin face wall by buttresses spaced at trated in the Robbins garage recently frequent intervals. The face wall and built in New York City by the Rein- base plate are both capable of withstandforced Cement Construction Company. ing transverse stresses, while the stress The span of the longest girders is fifty in the buttresses is usually that of. tenfeet. It was designed for a live load of sion. Since reinforced concrete is the 150 pounds per square foot. The main one form of masonry which can withgirders have a total depth—to the top of stand any considerable amount of transthe slab—of about three feet, and a width verse and tensile stresses, the above form of about two feet. The smaller beams of construction can only be made in rehave a width of twelve inches, a total inforced concrete. Of course, the same depth of eighteen inches and are spaced form could be adopted if we used steel seven feet between centers. The slab or wood, but the durability of either maitself is five inches thick. The illustra- terial would be so little that it would not tion is a typical example of this method pay to construct a retaining wall of such of floor construction.
materials. A view, taken during construction, of Another remarkable application of reone of the first large business blocks to inforced concrete is the possibility of be built structurally of reinforced con- making columns which are much crete is also printed here. The skeleton stronger than plain concrete columns, and of the building, the main columns and yet which do not employ a core of st -el girders, as well as the floors, are made of to take the most of the compression. A reinforced concrete. The cut shows the column whose length is 20 or 25 times its Ingalls Building in Cincinnati. This diameter will probably fail by buckling, building was constructed in 1903 and has in which case the steel on the convex sixteen office floors besides an attic, base- side of the column would be subject to
tension rather than compression. But the horizontal rings and prevent them a "short" column must fail by compres- from becoming displaced during the lay
sion, if subjected to sufficient stress. Even ing and tamping of the concrete. Such . in this case, steel may be employed to compression members are used not only furnish strength on account of its re for vertical columns, but also as the sistence to tension. Although the ex- compression members of truss bridges, planation 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 Ingalls BUILDING, CINCINNATI, O., AN ALL-CONCRETE STRUCTURE, IN would need to be proportionately far greater to produce any given tension in the pipe. of which several have been constructed. Using concrete instead of sand the re- Tests of such columns have required sistance to the "flow" of the material a compression of over 6,000 pounds will be still greater, which practically per square inch to cause failure. Almeans that a comparatively small amount though the construction of trussed of tensile strength in the pipe will pro forms in reinforced concrete is not comduce a very much added resistance to mon, the reinforcement of vertical colcompression. In practice, instead of umns in such a manner that they may be .using an actual pipe of metal, a series of safely subjected to greater loads than rings made of light bars and spaced a should be placed on plain concrete colfew inches apart are bent around a few umns of equal size, is now recognized longitudinal bars whose chief function is as safe engineering practice. to form a framework on which to fasten Another useful application of rein
PROCESS OF ERECTION.