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forced concrete lies in the building of structures which are especially subject to the fumes arising from the stacks of locomotives. This applies not only to engine houses and coaling stations, but also to over-head highway bridges which cross railroads. The concentrated gases of combustion have a corrosive action on steel which wears it away in the course of a few years. No matter how much the steel may be protected by paint, even the paint will be worn off by the mechanical action of the fine cinders which are blown out by the exhaust and which act as a very effective form of sand blast. Probably most kinds of paints are chemically affected more or less but the combination of chemical action and mechanical wear will destroy any protective covering in a comparatively short time. Reinforced concrete is absolutely unaffected chemically while the mechanical sand-blast action of the exhaust is so utterly insignificant that it need not be considered. Although a wooden structure is not seriously affected by the exhaust, its lack of durability, its danger from destruction by fire and the recent very great increase in the price of lumber, have combined to render wood an unsatisfactory and uneconomical material for such structures.

The advantages of reinforced concrete in the construction of coaling stations also is now being recognized. A frame work of structural steel, with steel plates for the floors and sides of the pockets, has been tried in order to obtain a noncombustible structure. But the sulphuric acid,.always present in the coal, corrodes the steel very rapidly and the life of such a structure is short. If the steel is adequately protected against corrosion by concrete, the cost is considerably in excess of a steel structure, but far greater permanence is secured.

In its application to the construction of masonry dams, reinforced concrete has entered another field. A solid masonry dam is usually constructed on the gravity principle, which means practically that the volume of its masonry is so great and so heavy that it is supposed to be safe against over-turning, but the cost of such a construction is so great that the cross section of the dam is usually reduced to the lowest limit which is considered permissible. The upper face of such a dam

usually makes an angle considerably greater than 45° with the horizontal and, under such conditions, a flood over the dam will raise the line of pressure and decrease the factor of safety. The higher the flood, the greater the danger. Under such conditions, a weakening of the foundation or an unsuspected washing out of the sub-soil may cause a settlement and a shifting of the line of pressure until the factor of safety, which for the sake of "economy" has been made very low, is wiped out and the result is a disaster which perhaps spreads destruction through a valley. # Another type of dam is illustrated in an old fashioned timber dam which is always constructed with a comparatively flat up-stream face, the angle of the upper face with the horizontal being less than 45°. Even the line of the resulting water pressure lies inside the base of the dam. There is never any tendency to over-turn and a flood only increases the pressure of the dam on its foundation.. As long as such a dam is kept tight, so that there is no flow of water through the dam to disintegrate the foundation, the dam is usually safe, but, being constructed of timber which is usually alternately wet or dry, the life of such a dam is exceedingly limited, and, considering the present price of lumber, is not even economical.

A reinforced concrete hollow dam combines all of the safe principles and advantages of a timber dam with the indefinite durability of first class masonry construction. The up-stream face- of a concrete dam is made with a comparatively flat slope, usually less than 45' with the horizontal. Hydraulic pressure being a perfectly definite quantity, it enables the engineer to design such a dam with a full knowledge of the stresses to which it will be subjected. These stresses are such that they may be easily provided for by the skeleton construction which is adopted for these dams. The dams consist essentially of an upstream "deck" whose chief duty is to withstand the direct and definite pressure of the water above it. This deck is supported at intervals by vertical walls which are parallel with the line of the stream and which transfer the pressure to the foundation of the dam. One great advantage in the method of construction is

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A WIDE-SPANNED, REINFORCED CONCRETE-FLOOR. FOR THE ROBRINS GARAGE, NEW

YORK CITY.

rapidly and at such a reduction of cost below that of ordinary masonry dams that such designs have rendered practicable the utilization of water powers which would not financially justify the construction of an ordinary stone masonry dam. The construction of these hollow concrete dams has even permitted the utilization of the space within them for gates and even for the location of water wheels and dynamos, thus permitting a very great reduction in the cost of the entire plant. Such a dam may even contain a passageway which will permit crossing the river in times of the highest floods, and thus save the construction of a bridge at that point. The

such as would be absolutely necessary with any other form of masonry construction. Many engineers are still skeptical on this point but the ultimate proof of such a theory lies in practice and it is indisputable that there are many examples of structures built of reinforced concrete which would unquestionably have shown temperature cracks if they had been built of ordinary masonry, but which, although built for several years —long enough for such cracks to have developed—have not shown any evidence of cracking.

The only apparent rational explanation of what appears now to be an undoubted fact is, practically, the same as that which forced concrete lies in the building of structures which are especially subject to the fumes arising from the stacks of locomotives. This applies not only to engine houses and coaling stations, but also to over-head highway bridges which cross railroads. The concentrated gases of combustion have a corrosive action on steel which wears it away in the course of a few years. No matter how much the steel may be protected by paint, even the paint will be worn off by the mechanical action of the fine cinders which are blown out by the exhaust and which act as a very effective form of sand blast. Probably most kinds of paints are chemically affected more or less but the combination of chemical action and mechanical wear will destroy any protective covering in a comparatively short time. Reinforced concrete is absolutely unaffected chemically while the mechanical sand-blast action of the exhaust is so utterly insignificant that it need not be considered. Although a wooden structure is not seriously affected by the exhaust, its lack of durability, its danger from destruction by fire and the recent very great increase in the price of lumber, have combined to render wood an unsatisfactory and uneconomical material for such structures.

The advantages of reinforced concrete in the construction of coaling stations also is now being recognized. A frame work of structural steel, with steel plates for the floors and sides of the pockets, has been tried in order to obtain a noncombustible structure. But the sulphuric acid,.always present in the coal, corrodes the steel very rapidly and the life of such a structure is short. If the steel is adequately protected against corrosion by concrete, the cost is considerably in excess of a steel structure, but far greater permanence is secured.

In its application to the construction of masonry dams, reinforced concrete has entered another field. A solid masonry dam is usually constructed on the gravity principle, which means practically that the volume of its masonry is so great and so heavy that it is supposed to be safe against over-turning, but the cost of such a construction is so great that the cross section of the dam is usually reduced to the lowest limit which is considered permissible. The upper face of such a dam

usually makes an angle considerably greater than 45° with the horizontal and, under such conditions, a flood over the dam will raise the line of pressure and decrease the factor of safety. The higher tbe flood, the greater the danger. Under such conditions, a weakening of the foundation or an unsuspected washing out of the sub-soil may cause a settlement and a shifting of the line of pressure until the factor of safety, which for the sake of "economy" has been made very low, is wiped out and the result is a disaster which perhaps spreads destruction through a valley. , Another type of dam is illustrated in an old fashioned timber dam which is always constructed with a comparatively flat up-stream face, the angle of the upper face with the horizontal being less than 45°. Even the line of the resulting water pressure lies inside the base of the dam. There is never any tendency to over-turn and a flood only increases the pressure of the dam on its foundation.. As long as such a dam is kept tight, so that there is no flow of water through the dam to disintegrate the foundation, the dam is usually safe, but, being constructed of timber which is usually alternately wet or dry, the life of such a dam is exceedingly limited, and, considering the present price of lumber, is not even economical.

A reinforced concrete hollow dam combines all of the safe principles and advantages of a timber dam with the indefinite durability of first class masonry construction. The up-stream face- of a concrete dam is made with a comparatively flat slope, usually less than 45' with the horizontal. Hydraulic pressure being a perfectly definite quantity, it enables the engineer to design such a dam with a full knowledge of the stresses to which it will be subjected. These stresses are such that they may be easily provided for by the skeleton construction which is adopted for these dams. The dams consist essentially of an upstream "deck" whose chief duty is to withstand the direct and definite pressure of the water above it. This deck is sup^ ported at intervals by vertical walls whicfl are parallel with the line of the strea^B and which transfer the pressure to foundation of the dam. One great^^H vantage in the method of constructi^

[graphic]
[merged small][merged small][graphic]

A WIDE-SPANNED, REINFORCED CONCRETE-FLOOR, FOR THE ROBMNS GARAGE NEW

YORK CITY.

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[graphic]

such as would be absolutely necessary with any other form of masonry construction. Many engineers are still skeptical on this point but the ultimate proof of such a theory lies in practice and it is indisputable that there are many examples of structures built of reinforced concrete which would unquestionably have shown temperature cracks if they had been built "f ordinary masonry, but which. ;'U; ugh built for several years jugh for such cracks to have ive not shown any evincing.

pparent rational explanation lars now to be an undmibW" ically, the same as that

permits a reinforced concrete beam to be deflected for a very considerable percentage of its span without showing any cracks on the stretched side. It is wellknown that plain concrete cannot be stretched more than a very minute fraction of its length without cracking. A very long monolith of plain concrete will nearly always develop cracks which are caused by a concentration of the stretching at the weakest points in the concrete and since the proportional amount at which concrete may be stretched without rupture is very small, a concentration of the extension at one place will cause rupture at that point. If the metal is properly imbedded in the concrete, so that the concrete and the metal will stretch together, then the deformity of the concrete by stretching will be distributed uniformly throughout its length instead of being confined to a few points.

Objection is sometimes . made to the policy of not using expanded joints on the ground that there have been several instances of monolithic reinforced concrete structures in which temperature cracks have developed. In such cases it is easily demonstrable that the metal was not well distributed through the body of the concrete. The effectual prevention of cracks is only accomplished by such an intimate union of the concrete and the steel that they must act together under all circumstances and conditions of temperature. It is not an easy matter to compute theoretically just what proportion of metal will be neeeded to insure a wall against cracking. It is probably true that the metal which will ordinarily be needed for reinforcement will also be able to take care of such stresses and it is certainty true that the uniform distribution of the metal is of far greater importance than its amount. The Harvard stadium has a length of fourteen hundred feet and was constructed without expansion joints. It has already experienced three northern winters. No cracks have developed in this structure

except at a point where the straight portion joins the semi-circular end and even here the cause of the crack is not considered due to changes of temperature.

Reinforced concrete has even invaded the realm in which stone masonry has been considered from ancient times the best building material and is now strongly competing with it in the construction of arch bridges both because it is cheaper and also better. Stone arch bridges have been built for many hundreds of years. Some of them have been built by men who probably had no knowledge of the theoretical mechanical principles now used in designing such arches. And yet these men constructed arches of long span which had comparatively little rise. But since the stone arch depends purely on compressive stresses the design has very definite limitations. It is almost invariably found that the dead weight of a stone arch is several times the maximum live load which may be safely placed on it and that even a portion of this load, if placed near one end of the arch, may test it more severely than the full load uniformly distributed. The ability of a reinforced concrete arch to withstand transverse stresses furnishes a large element of safety which is wholly unobtainable with plain stone masonry and actually permits dimensions and proportions which would be unsafe in a stone arch.

Although a reinforced concrete arch is usually designed so that the "line of pressure" for full loading will pass nearly through the center of the arch, which means that every portion of the arch is under compression, yet the arch wilf not necessarily fail if, for an eccentric loading, the line of pressures should pass entirely outside of the arch ring. In such a case, its stability would depend on the transverse strength of the arch section. A plain stone arch with the same dimensions and loaded in the same way would necessarily fail: Reinforced concrete is superior for such a purpose.

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