Bridges.
At first, properly enough, reinforced concrete was adopted with much caution in bridge-building. To-day hundreds of bridges in this material are doing service throughout the world. A good example of a small bridge is that in Forest Park, St. Louis, spanning the River des Pêres. A noteworthy design on a large scale, by Professor William H. Burr, of Columbia University, New York, has been accepted for the Memorial Bridge to cross the Potomac River at Washington. A centre-draw span of 159 feet in steel is to be flanked on each side by three spans of reinforced concrete, each of 192 feet. These spans are ribbed arches, having a rise of twenty-nine feet, with their exteriors in granite masonry. In arguing for bridges in reinforced concrete, Mr. Edwin Thacher points out that under normal circumstances their steel is not strained to much more than one quarter of its elastic limit, so that a large reserved strength is available for emergencies, while the structure is more durable than a steel bridge and ultimately more economical, comparatively free from vibration and noise, proof against tornadoes and fire, and against floods also if the foundations are protected from scour.
CHAPTER XXX
MOTIVE POWERS PRODUCED WITH NEW ECONOMY
Improvements in steam practice . . . Mechanical draft . . . Automatic stokers . . . Better boilers . . . Superheaters . . . Economical condensers . . . Steam turbines on land and sea.
Francis vertical turbine wheel. Allis-Chalmers Co., Milwaukee.
In every industry a threshold question is how motive power may be had at the lowest cost. In this field within twenty years wholly new methods have been introduced, while old processes have been greatly amended. Thanks to economical water-wheels and generators, efficient transmission, and motors all but perfect, water-powers, as at Niagara Falls, now send electricity to thousands of distant workshops, to serve not only as an ideal means of actuation, but as a source of light, heat and chemical impulse. While electrical art has thus been marching forward, all the heat engines have been improved in every detail of construction. New valve-gears, economizers and superheaters, united with triple-expansion cylinders of the boldest dimensions, worked at pressures and speeds greater than ever before, combine to make the best steam engines to-day vastly more effective than those of a generation ago. And these engines are withal facing the aggressive rivalry of the steam turbines devised by De Laval, Parsons and Curtis, all much less heavy and bulky than engines, simpler to build and operate, while their motion is continuous instead of interrupted at every piston stroke.
Competing with steam motors are the new gas engines, twice as efficient in converting heat into motive power. For this reason and because much improvement seems to be feasible in their designs, and in systems for supplying them with cheap gas, their adoption on a large scale in the near future appears to be certain. Especially will this be the event should the turbine principle be as successfully applied to gas as to steam motors. Already gases from coke ovens and blast furnaces, formerly thrown away or used only in part, are being employed in gas engines with success.
To-day the production of motive power largely centres in stations so huge that they adopt with gain appliances too elaborate for use in small installations. At the power-house of the Interborough Rapid Transit Company, New York, for example, automatic machinery conveys coal from barges to vast bunkers under the roof, an even distribution being effected by self-reversing trippers. Twelve of the furnaces have automatic stokers. Ashes are removed by conveyors. Lubricating oil is pumped to high reservoirs whence it descends to flush all the bearings; it is then carried to filters from which it passes to another round of duty. It is plain that the huge scale of such a plant opens new doors to ingenuity, especially in the dovetailing of one service with another.
In some central stations, as at Findlay, Ada, and Springfield, Ohio, the exhaust steam is utilized for district heating, so that the generation of motive power is merged into the larger field of fuel economy treated as a whole. Where there is a profitable market for exhaust steam it pays to use a group of engines or turbines which are either non-condensing, or only some of which are condensing, for the aim is not simply to use the motor which asks least fuel, but to install such motors and heaters as together will earn most for the capital invested.