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The Tower of the Union Ferry Depot, San Francisco

By F.A. Koetitz

As is well known, the Ferry Tower received a severe shaking by the earthquake of April last; since then more or less had been written about it and much has been said which does not agree with the actual facts. The writer, having had special opportunity to learn the actual conditions, feels justified in making a statement of them especially as he is a member and the chief engineer of the company which originally built the greater part of the depot building, including this tower, and because the same company has just completed the tower's reconstruction.

Describing the tower briefly, it may be stated that it was originally built with a steel frame enclosed, up to the cornice, by brick walls with a sandstone facing, the walls for each story being practically self-supporting. The frame above the main cornice was covered with galvanized iron, painted to match the stone ... . The frame was constructed of steel columns, girders, beams and diagonal rods ... to carry, in addition to the dead load, a reasonable live load and to withstand wind strains of about 30 pounds per sq. ft. of exposed surface. The irregular distribution of the diagonals of the lower stories was made to suit various openings in the walls. The foundation of the tower, as well as that of the building, consists of concrete piers on piles, described in detail heretofore.

Although the tower was built several years ago, one of the first thoughts of the writer, after experiencing that never-to-be-forgotten shake was: What must have become of the Depot tower? and he was surprised to find, later, that it was still standing and apparently in fair condition. Immediately after the earthquake the military authorities inspected the Tower, declared it unsafe and ordered ropes stretched about the vicinity to keep the public from approaching within the danger zone in case the structure should fall from later shocks.

The Pacific Construction Company, of which the writer is chief engineer, being most familiar with the structure, was employed, within a few days after the shock by the Board of State Harbor Commissioners to investigate the exact damage and to immediately make sufficient temporary repairs to insure it against falling. Investigation showed that the stone and brick work of the fourth and fifth stories and of a part of the sixth story in the East and West walls were partly thrown down and that much more of the material in these stories was in immediate danger of following. The North and South walls were in better condition, being injured mostly in the fourth story. It was decided to encase these injured stories with wire cables, to prevent further damage from falling walls and to make the structure safe for further inspection. After this was done, it was found that the main damage was between the third and seventh floors and greatest between the fifth and sixth floors and that it consisted of the following detailed injuries:

All the diagonal rods in these stories were either buckled or injured in some way; one had the eye at one end broken off, in another the rivets connecting the gusset plates and angles sheered off; the stress finding, in each case, the weakest point.

The strain exceeded the equivalent of a 30 pound wind pressure many times as was shown by the elongation of these rods. In five cases, including three 2" sq. rods, it was necessary to cut off the screw ends to take up the slack. The elongation, in some cases, amounted to 3.125 inches.

On some gusset plates even 3/4" rivets were sheared off. The writer tried to locate the direction of the greatest motion and found that the tower, from the fourth to the eleventh floors was leaning 4 1/8" southward and 2" westward, but he was forced to conclude that there was little difference in direction. The rods were equally slack in both directions in all the four panels of the same story and it is difficult to imagine a movement that would produce such a result without allowing a considerable spreading between columns on the line of the struts or girders.

Five of the girders had the connecting bolts at the columns sheared off and had moved laterally away from the column as much as 2". At such ends these girds were resting just one inch on a 3" angle bracket and one felt walking lightly on the floors for fear that they might give way and drop floors, girders, walls and men to destruction.

These conditions show that the published reports, saying that the tower was good for many more shakes like the one of that memorable morning, were false, and writer felt inclined, when these reports were printed, to refute them, but did not do so at that time because he was chiefly interested to complete repaired which would make the tower safe.

Under the above conditions, it was necessary to provide temporary bracing for the steel frame until the broken walls could be removed and repairs made; wire cables were used, two being attached in place of each rod, and four to each girder that had parted from its column. These cables were twisted tight until they took a good strain. About 9000 feet of cable was used for this purpose and in protecting the walls.

All of the steel frame as well as the stone work above the seventh floor was practically without injury, the greatest damage being the broken eye of one of the rods. This made it evident that the upper portion of the structure was much less affected than the section noted heretofore, but the flagpole gave notice that some movement had taken place up there by developing sharp bends in two direction and by leaning at a pronounced angle. This pole was 42 feet long and was made of extra heavy steel pipe of 5", 4" and 3" inside diameter, the main bend coming near the bottom of the four inch section; this, however, was mainly caused by the weight of the large time-ball head casting which was attached to the pole near the top of the 4" pipe. It is evident that earthquake shock does not affect the tops of high structures in the same manner as the lower parts; this was proven about a week after the first main shock when the work of rescuing the tower was in progress; a rather severe shock came when the writer and several others were near the bottom of the tower and everyone rushed out of the building, being fearful that the tower would fall, but the men working near the twelfth floor did not know that anything had happened.

Although, as stated above, the upper portion of the tower was intact, it was decided to reduce its weight by taking down all the stone and brickwork, then to repair the steel frame, and to replace the covering with materials of lighter weight. The steel work was carefully repaired; all broken parts were removed and renewed; the separated columns and girders were riveted back to place; all injured rods were repaired, and the tower was brought to proper plumb ready for the covering.

It was, at first, decided to use galvanized iron covering on wooden sheathing, but the writer proposed that reinforced concrete be used, arguing with the Board that while, of course, concrete would be heavier than the galvanized iron construction, it would weigh about one-half as much as did the former brick and stone, and that it could be made to materially help to brace and stiffen the whole structure. He further argued that such a covering would be practically fire-proof, and could be readily designed to preserve the original outlines of the tower in keeping with the stone front of the main building. He also convinced the authorities that no great reliance should be should be placed on the capacity of rods which had been strained so much beyond the elastic limit, and pointed out that the reinforced concrete construction would, in itself, almost provide the necessary bracing.

A reinforced concrete design was ordered.. . The minimum thickness of panels was made only 4" in order to reduce the dead weight; pilasters, on the outside, were used to give the tower the same outlines as formerly; pilasters, on the inside, were designed to carry the weight of the walls and to transmit the wind strains to the steel girders. To assist in transmitting strains from the pilasters to the girders, these bars projected outward into the pilasters. The bars were also provided with vertical holes to space the vertical reinforcing rods and to anchor them to the girders. All other rods were securely connected to the steel framework, and the space between the beams forming the girders was filled with concrete to protect all fastenings against rusting. The columns, also, were entirely enclosed in concrete for the same reason after having been first wrapped with expanded metal. In the concrete corrugated steel bars were used as reinforcement and the mixture was high grade German cement, fine washed gravel and sand, with rock crushed to pass through a 1" ring and over a 1/4" screen; one part of the cement, three of sand and gravel and three of rock were the proportions, and the concrete was mixed by machine, quite wet, and was deposited into forms from each floor to the whole story below. The forms were carried up on the outside for the full height, so that the work of concreting could be rushed, when started; the inside forms were built up as the work progressed; this latter form work was difficult and required great care as the diagonal rods of the steel frame interfered in most cases. No attempt was made to finish the outside of the concrete, but it was given two coats of Asbestine paint colored to match the sandstone of the main building.

We are indebted to Mr. F.A. Koetitz, Vice-President and Chief Engineer of the Pacific Construction Company, and to Mr. F.M. Butler, Secretary-Treasurer of the same Company, for much kindly assistance in the preparation of this article.
American Builders Review
Vol. VI, No. 1
January, 1907

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