backing. In the peak the surface will be of marble instead of granite. The frame consists of twelve outside or main columns and four interior columns, which enclose the central elevator shaft; all securely braced by beams and wall trussing. The elevator runs to the level just below the belfry floor. At elevation 180 feet there is a huge clock with four faces. The chimes are supported from a circular ring in the belfry ceiling at elevation 220 feet. At the ground line the finished granite tower is thirtysix feet square, tapering to thirty feet square at the belfry. The steel frame extends into the ground ten feet where it rests upon a monolith foundation to which every column is securely anchored by four 12-inch bolts. The monolith slab is eight feet thick, forty-eight feet square, contains two levels of reinforced concrete bars meshed at right angles, and two layers of 24-inch I-beams, forming a grillage structurally bolted to the steel columns. The bed of foundation is eighteen feet below finished ground and rests on an excellent hardpan. The structural steel in this obelisk weighs 501 tons. The total weight of monument on foundation bed is 13,750,000 pounds, giving a uniform dead load pressure on the bed of foundation of three tons per square foot. A wind pressure of thirty pounds per square foot vertical projection decreases the pressure on foundation at windward toe to two tons and increases it to four tons per square foot at the leeward edge. The structure is immensely strong so far as wind, dead and live load stresses are concerned. The pressure on hardpan foundation bed is conservative; there can be no unequal settlement. The engineers paid particular attention to certain qualities of framing because of the possible effects to such a slender structure coming from severe earthquakes. The California earthquake of April 18, 1906, had a maximum acceleration of six feet per second. This would throw a horizontal force upon the tower of about one-fifth its own weight, the force acting against the structure in a manner similar to wind pressure, but much more severely. An earthquake with the acceleration named by tending to tilt the tower would produce a pressure of about nine tons per square foot at the extreme edge of foundation, provided the structure acted as a rigid monolith. This extreme pressure, however, can never be reached because the tower is elastic and is designed to yield. The hardpan can take ten tons per square foot, particularly when one considers that the extreme pressure could exist only for an instant due to the oscillation of the earthquake, throwing pressure first on one edge and then on the opposite edge of the foundation. The sub-structure and anchorage details have been designed to resist any possible uprooting of the mass or any sliding or hearing at the base, such as might take place in a rigid stone monument through the action of an impulsive sudden earthquake shock. The designers have been concerned, however, more particularly with the phenomena of vibration. The natural period of the tower will be two seconds or possibly a little more. That is to say, due to the action of wind or any other horizontal pressure the tower will sway, taking two seconds to move from one extreme position to the other lateral extreme and back again. We picture here a swaying motion such as one would observe in a flag pole, a slender tree, a chandelier or a pendulum. Earthquakes usually have a period vibration of about one second. They tend to produce a forced oscillation in a body of the same period as that of the earthquake. Now, whenever the natural period of the body and that of the earthquake differ, the two effects tend to balance or neutralize. That is why a regiment is ordered to break step when crossing a suspension bridge. On the other hand, if a steel tower had the same period as the earthquake, both one second, the effect of the earthquake push would be accumulative. The tower would vibrate in unison with the earthquake. A condition which is described as resonance would occur. The structure would be whipped like a slender tree or flagpole in a storm, and if the earthquake only lasted long enough, the structure, no matter how well designed, must ultimately fail under the great amplitudes of the cumulative vibration. An analogy would be that of a little child pushing its father in a rope swing. If the child pushes at the right time it ca ntually cause the grown person to swing high. For these reasons the designers have been anxious to make the natural period of the tower at least two times that of the likely period of a future earthquake. Moreover, it is desirable to have the structure yielding as well as strong. Remember in this connection the fable of the stiff oak tree and the slender willow. The oak is broken by the storm, but the willow, though less in strength, being greater in yielding qualities, rides the tempest. An examination of the Campanile shows X-bracing in every other story. These stories in which lateral or wind bracing is omitted give a yielding quality. In earthquake countries this is a necessary feature of high steel towers. Where towers are X-braced throughout they must fail in earthquakes, as witness the Ferry Tower in San Francisco in 1906. Earthquakes are nature's force. They may be unlimited in their strength. When they crack the ground they may break a steel pipe as easily as a wooden fence. A great redwood tree standing on the fault line is sheared in twain; but just as easily, the crust of the earth, the very rocks, are rent for a length of three hundred miles. Such a crustal fault happened in 1906 and ran from Point Arena in the north to Hollister in the south. In other words, an earthquake will produce amounts of stress directly proportional to the resistance offered by such structures as pipes, fences, buildings, towers, redwood trees, and the rock of the earth itself. That is why the particular form of framing described was used in the Campanile. The tower will yield and not resist vibration. With its large period of oscillation and these willow-like qualities of the frame we predict that the monument will withstand severe shocks. President Wheeler remarked in his speech that, from listening to the description given by the engineer, he understood that lightning rods carried the thunderbolt from the clouds to the earth, while campaniles would take the rumbling out of the crust and shiver it up into the sky. MONTHLY AND SEASONAL METEOROLOGICAL SUMMARY FOR 1913-1914, July 30.00 60.2 59.6 73.6 53.7 63.6 95.0 11 48.9 14 54 54 91.8 % inch inch tenths tenths 76 77 .409 .403 5 95.4 1 45.3 27 47 48 75 72 .328 .334 4 January.. 30.06 46.8 48.8 56.6 43.7 50.2 Observations made daily at 8. a. m. and 8 p. m. Pacific (120th meridian) time. |