This beacon stands just at the entrance to the Bay of San Francisco—at the Golden Gate. It is located one mile from the southern mainland, and nearly three miles from the North Heads. Mile Rock light station was completed only a very short time before the great disaster that befell San Francisco on April 18, 1906. Though only a "third-order" light station, yet its total cost to the government reached $100,000.
For a great many years, Mile Rock was a menace to every vessel that entered and passed out of San Fran cisco Bay. A station would have been built upon this little point of rocks long ago, but for the many and great engineering obstacles that beset such an undertaking. Originally Mile Rock was a mere ragged pinnacle of rock—just jutting above the level of the sea. At extreme low tide, the highest point of this rock stood only 14 feet above th8 ocean's level.
Before a foundation could be secured, it was neces sary to chip off a broad base, or bench, extending clear around the point of rock some distance below the highest pinnacle. The chiseling of this "rim" proved a very slow, difficult, and dangerous undertaking. During high tides and stormy weather, no work could be done at all, as the rock was practically under water or wave-swept.
Finally, with great labor and danger, a mammoth steel cylinder was built up around the rock, the base resting on the rim or bench. Owing to the shape of the rock, this huge cylinder could not be made per fectly round, but had to be elliptical in form. It was 40 feet the long way, and 25 feet the shorter way, and was slowly built up to a height of 42 feet from its base.
The plates of this cylinder were steel boiler metal % of an inch in thickness, and were riveted together very strongly with heavy bolts. As the cylinder was being built upward, the cavity was gradually filled with strongly reinforced concrete, which was tamped down very firmly. The immense cavity in the cylinder was 25 by 40 by 42 feet, and over 1,200 barrels of cement alone were required to make the concrete for this mas sive foundation.
One of the heaviest gales ever known in the history of California swept for several hours over San Francisco, doing great damage to the shipping and buildings.
Again it was feared that the Mile Rock station would go; but it stood—Gibraltar-like—and defied the old storm king. A subsequent examination of the concrete founda tion by government engineers showed everything firm and intact—just as when completed.
Concrete Vats. Reinforced concrete is being used to a large extent for tanks or vats to con tain liquids. They require careful design to see that there is sufficient steel to resist the pressure, and also very careful proportioning and placing of the concrete. A system of square tanks or vats in the basement of the American Oak Leather Company, Cincinnati, is illustrated in Plate 28. These are 6 feet by 8 feet and 6 feet deep, with reinforced walls 4 inches thick. They were built in groups of six, with specially pre pared aggregates. These vats, after over a year's service, have given entire satisfaction and show no signs of leakage.
Penstocks. A comparatively new and rela tively untried opportunity for the use of rein forced concrete is presented in the construction of penstocks for the conveying of water, fre quently under considerable head. A recent in vestigation conducted by Mr. Leonard C. Wason, of Boston, throws interesting light upon the sub ject. He was particularly anxious to learn whether expansion joints would be necessary with this form of construction. Literature upon concrete construction was carefully scanned as regards authoritative statements. A typical competent authority for the theory stated that reinforced concrete can be built of indefinite length if properly reinforced, and be free from shrinkage due to temperature and settling stresses.
Not content, however, with such a general statement, Mr. Wason sought the personal opin ions and experiences of various engineers. Their replies are interesting as they are varied. One engineer experienced in concrete and hydraulic work showed a drawing of a penstock of curved section with flat base, laid out for continuous reinforcement from end to end without expan sion joints. He emphasized the necessity of standpipes for taking care of water-hammer, air pressure, and vacuum. After careful investiga tion he stated his belief that a penstock without joints, but of proper shape, with sufficient longi tudinal reinforcement, is practicable, and would prove satisfactory.