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forced concrete lies in the building of usually makes an angle considerably structures which are especially subject greater than 45o with the horizontal and, to the fumes arising from the stacks of under such conditions, a flood over the locomotives. This applies not only to dam will raise the line of pressure and engine houses and coaling stations, but decrease the factor of safety. The higher also to over-head highway bridges which the flood, the greater the danger. Under cross railroads. The concentrated gases such conditions, a weakening of the of combustion have a corrosive action on foundation or an unsuspected washing steel which wears it away in the course out of the sub-soil may cause a settlement of a few years. No matter how much the and a shifting of the line of pressure until steel may be protected by paint, even the the factor of safety, which for the sake paint will be worn off by the mechanical of “economy” has been made very low, action of the fine cinders which are blown is wiped out and the result is a disaster out by the exhaust and which act as a which perhaps spreads destruction very effective form of sand blast. Prob- through a valley. ably most kinds of paints are chemically. Another type of dam is illustrated in affected more or less but the combination an old fashioned timber dam which is of chemical action and mechanical wear always constructed with a comparatively will destroy any protective covering in a flat up-stream face, the angle of the upper comparatively short time. Reinforced face with the horizontal being less than concrete is absolutely unaffected chem- 45°. Even the line of the resulting water ically while the mechanical sand-blast ac- pressure lies inside the base of the dam. tion of the exhaust is so utterly insignifi. There is never any tendency to over-turn cant that it need not be considered. Al- and a flood only increases the pressure though a wooden structure is not serious- of the dam on its foundation. As long ly affected by the exhaust, its lack of dur- as such a dam is kept tight, so that there ability, its danger from destruction by is no flow of water through the dam to fire and the recent very great increase in disintegrate the foundation, the dam is the price of lumber, have combined to usually safe, but, being constructed of render wood an unsatisfactory and un- timber which is usually alternately wet or economical material for such structures. dry, the life of such a dam is exceedingly

The advantages of reinforced concrete limited, and, considering the present in the construction of coaling stations price of lumber, is not even economical. also is now being recognized. A frame A reinforced concrete hollow dam work of structural steel, with steel plates combines all of the safe principles and for the floors and sides of the pockets, advantages of a timber dam with the has been tried in order to obtain a non- indefinite durability of first class masonry combustible structure. But the sulphuric construction. The up-stream face of a acid, always present in the coal, corrodes concrete dam is made with a comparathe steel very rapidly and the life of such tively flat slope, usually less than 45° a structure is short. If the steel is ade- with the horizontal. Hydraulic pressure quately protected against corrosion by being a perfectly definite quantity, it enconcrete, the cost is considerably in ex- ables the engineer to design such a dam cess of a steel structure, but far greater with a full knowledge of the stresses to permanence is secured.

which it will be subjected. These In its application to the construction of stresses are such that they may be easily masonry dams, reinforced concrete has provided for by the skeleton construcentered another field. A solid masonry tion which is adopted for these dams. dam is usually constructed on the gravityThe dams consist essentially of an upprinciple, which means practically that stream "deck” whose chief duty is to the volume of its masonry is so great and withstand the direct and definite pressure so heavy that it is supposed to be safe of the water above it. This deck is supagainst over-turning, but the cost of such ported at intervals by vertical walls which a construction is so great that the cross are parallel with the line of the stream section of the dam is usually reduced to and which transfer the pressure to the the lowest limit which is considered per- foundation of the dam. One great admissible. The upper face of such a dam vantage in the method of construction is

[merged small][merged small][graphic]

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

YORK CITY.

rapidly and at such a reduction of cost such as would be absolutely necessary. below that of ordinary masonry dams with any other form of masonry conthat such designs have rendered prac- struction. Many engineers are still skepticable the utilization of water powers tical on this point but the ultimate proof which would not financially justify the of such a theory lies in practice and it is construction of an ordinary stone ma indisputable that there are many examsonry dam. The construction of these ples of structures built of reinforced conhollow concrete dams has even permitted crete which would unquestionably have the utilization of the space within them shown temperature cracks if they had for gates and even for the location of been built of ordinary masonry, but water wheels and dynamos, thus permit- which, although built for several years ting a very great reduction in the cost —long enough for such cracks to have of the entire plant. Such a dam may developed—have not shown any evieven contain a passageway which will dence of cracking. permit crossing the river in times of the The only apparent rational explanation highest floods, and thus save the con- of what appears now to be an undoubted struction of a bridge at that point. The fact is, practically, the same as that which

forced concrete lies in the building of usually makes an angle considerably structures which are especially subject greater than 45o with the horizontal and, to the fumes arising from the stacks of under such conditions, a flood over the locomotives. This applies not only to dam will raise the line of pressure and engine houses and coaling stations, but decrease the factor of safety. The higher also to over-head highway bridges which the flood, the greater the danger. Under cross railroads. The concentrated gases such conditions, a weakening of the of combustion have a corrosive action on foundation or an unsuspected washing steel which wears it away in the course out of the sub-soil may cause a settlement of a few years. No matter how much the and a shifting of the line of pressure until steel may be protected by paint, even the the factor of safety, which for the sake paint will be worn off by the mechanical of "economy” has been made very low, action of the fine cinders which are blown is wiped out and the result is a disaster out by the exhaust and which act as a which perhaps spreads destruction very effective form of sand blast. Prob- through a valley. ably most kinds of paints are chemically. Another type of dam is illustrated in affected more or less but the combination an old fashioned timber dam which is of chemical action and mechanical wear always constructed with a comparatively will destroy any protective covering in a flat up-stream face, the angle of the upper comparatively short time. Reinforced face with the horizontal being less than concrete is absolutely unaffected chem- 45°. Even the line of the resulting water ically while the mechanical sand-blast ac- pressure lies inside the base of the dam. tion of the exhaust is so utterly insignifi. There is never any tendency to over-turn cant that it need not be considered. Al- and a flood only increases the pressure though a wooden structure is not serious- of the dam on its foundation. As long ly affected by the exhaust, its lack of dur- as such a dam is kept tight, so that there ability, its danger from destruction by is no flow of water through the dam to fire and the recent very great increase in disintegrate the foundation, the dam is the price of lumber, have combined to usually safe, but, being constructed of render wood an unsatisfactory and un- timber which is usually alternately wet or economical material for such structures. dry, the life of such a dam is exceedingly

The advantages of reinforced concrete limited, and, considering the present in the construction of coaling stations price of lumber, is not even economical. also is now being recognized. A frame reinforced concrete hollow dam work of structural steel, with steel plates combines all of the safe principles and for the floors and sides of the pockets, advantages of a timber dam with the has been tried in order to obtain a non- indefinite durability of first class masonry combustible structure. But the sulphuric construction. The up-stream face: of a acid, always present in the coal, corrodes concrete dam is made with a comparathe steel very rapidly and the life of such tively flat slope, usually less than 45o a structure is short. If the steel is ade- with the horizontal. Hydraulic pressure quately protected against corrosion by being a perfectly definite quantity, it enconcrete, the cost is considerably in ex ables the engineer to design such a dam cess of a steel structure, but far greater with a full knowledge of the stresses to permanence is secured.

which it will be subjected. These In its application to the construction of stresses are such that they may be easily masonry dams, reinforced concrete has provided for by the skeleton construcentered another field. A solid masonry tion which is adopted for these dams. dam is usually constructed on the gravity The dams consist essentially of an upprinciple, which means practically that stream "deck” whose chief duty is to the volume of its masonry is so great and withstand the direct and definite pressure so heavy that it is supposed to be safe of the water above it. This deck is supagainst over-turning, but the cost of such ported at intervals by vertical walls whic a construction is so great that the cross are parallel with the line of the strea section of the dam is usually reduced to and which transfer the pressure to the lowest limit which is considered per- foundation of the dam. One great missible. The upper face of such a dam vantage in the method of construct

[merged small][merged small][graphic]

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

YORK CITY.

rapidly and at such a reduction of cost such as would be absolutely necessary. below that of ordinary masonry dams with any other form of masonry conthat such desigr have rendered prac- struction. Many engineers are still skepticable the ut of water powers tical on this point but the ultimate proof which would

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permits a reinforced concrete beam to except at a point where the straight porbe deflected for a very considerable per- tion joins the semi-circular end and even centage of its span without showing any here the cause of the crack is not concracks on the stretched side. It is well-, sidered due to changes of temperature. known that plain concrete cannot be Reinforced concrete has even invaded stretched more than a very minute frac- the realm in which stone masonry has tion of its length without cracking. A been considered from ancient times the very long monolith of plain concrete will best building material and is now strongnearly always develop cracks which are ly competing with it in the construction caused by a concentration of the stretch- of arch bridges both because it is cheaper ing at the weakest points in the concrete and also better. Stone arch bridges and since the proportional amount at have been built for many hundreds of which concrete may be stretched without years. Some of them have been built by rupture is very small, a concentration men who probably had no knowledge of of the extension at one place will cause the theoretical mechanical principles now rupture at that point. If the metal is used in designing such arches. And yet properly imbedded in the concrete, so these men constructed arches of long that the concrete and the metal will span which had comparatively little rise. stretch together, then the deformity of But since the stone arch depends purely the concrete by stretching will be dis- on compressive stresses the design has tributed uniformly throughout its length very definite limitations. It is almost ininstead of being confined to a few points. variably found that the dead weight of a

Objection is sometimes made to the stone arch is several times the maximum policy of not using expanded joints on live load which may be safely placed on the ground that there have been several it and that even a portion of this load, if instances of monolithic reinforced con- placed near one end of the arch, may crete structures in which temperature test it more severely than the full load cracks have developed. In such cases it uniformly distributed. The ability of a is easily demonstrable that the metal was reinforced concrete arch to withstand not well distributed through the body of transverse stresses furnishes a large elethe concrete. The effectual prevention of ment of safety which is wholly unobtaincracks is only accomplished by such an able with plain stone masonry and acintimate union of the concrete and the tually permits dimensions and proporsteel that they must act together under all tions which would be unsafe in a stone circumstances and conditions of tem- arch. perature. It is not an easy matter to Although a reinforced concrete arch is compute theoretically just what propor- usually designed so that the “line of tion of metal will be neeeded to insure a pressure” for full loading will pass wall against cracking. It is probably nearly through the center of the arch, true that the metal which will ordinarily which means that every portion of the be needed for reinforcement will also be arch is under compression, yet the arch able to take care of such stresses and it will not necessarily fail if, for an eccenis certaindy true that the uniform dis- tric loading, the line of pressures should tribution of the metal is of far greater pass entirely outside of the arch ring. importance than its amount. The Har- In such a case, its stability would depend vard stadium has a length of fourteen on the transverse strength of the arch hundred feet and was constructed with section. A plain stone arch with the out expansion joints. It has already ex- same dimensions and loaded in the same perienced three northern winters. No way would necessarily fail: Reinforced cracks have developed in this structure concrete is superior for such a purpose.

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