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This bridge is an early and rare surviving example of a Strauss vertical overhead counterweight type bascule bridge. Designed by Joseph Strauss's company, this bridge represents the design and appearance of this type well, despite alteration. The vertical overhead counterweight type bascule is a lot more compact that the heel-trunnion bascule design that Strauss also designed and can be seen at the nearby 3rd Street Bridge. The vertical overhead counterweight type bascule appears to have been better suited to smaller bascule spans, while the heel-trunnion was better suited for larger bascule spans.
The bridge's mechanical and operational systems have been drastically altered as part of an unusual improvement for seismic safety. However, these alterations were made with the intent of maintaining the bridge's original visual appearance, and the attempt appears to have been largely successful. In particular, this bridge's overhead counterweight was found to be vulnerable to damage from earthquakes but it was found that if its weight could be reduced it could be protected from earthquake damage. As a result, the bridge has actually been converted into a bascule bridge with a counterweight underneath the roadway. A new counterweight was constructed under the roadway and connected to the bridge. This allowed the original counterweight to be reconstructed so that it only weighs 100,000 pounds and provides only seven percent of the counterweight action. Also, the post that supports the main trunnion was also replaced. Other repairs were made to the truss. The altered metal portions of the bridge are readily identifiable by the presence of bolts instead of rivets. The overhead counterweight however does a great job as masking its substantial weight reduction as it appears the same as it always has to the casual observer. Even the impressions of the wooden planks used as forms to pour the concrete remain visible.
The contract for building this bridge was awarded on June 25, 1915. The bridge was built for $89,672.09 and formally accepted as complete on May 23, 1917. When originally built, the overhead counterweight weighed 700 tons.
The 1915-1916 San Francisco Municipal Report offered the following details of the ongoing construction of this bridge.
The construction of a Strauss Bascule Bridge across Channel Street Waterway at Fourth Street, has been in progress since August. 1915. Six weeks' delay in starting was occasioned by repair work on the Third Street
Bridge, which prevented closing of operation of the old drawbridge at Fourth Street.
The Thomson Bridge Company has the general contract to furnish all material and install the bridge and approaches. Sub-contracts for the fabrication of the structural steel and making and installation of the machinery have been let to the Ralston Iron Works and the Joshua Henry Machine Company, respectively, by the Thomson Bridge Company.
The first work started at the bridge site was the wrecking and removal of the old drawbridge superstructure and the precasting of the concrete piles for the deck approach foundations.
Pile foundations and concrete for the seawalls at the extreme shore ends of the approaches were then placed.
Progress of the construction depended on the placing of the cylinders that form the bridge seats which are on the north side of the channel. Before this work could be started the channel had to be cleared by removing the pile foundation of the old bridge, the piles of the protection work of the drawbridge, and innumerable old snags of piles of old construction that had broken off and rotted. A clamshell dredger was used to clear the channel up and down stream from the center cluster forming the old drawbridge foundation. In order to remove the piles of the cluster to n point below the official channel bottom level, Masting was resorted to and, to be effective, a steel cage guide was used to keep the powder near enough to the pile. This work occasioned considerable delay, as the caissons for the cylinders are placed by a floating driver which had to operate at the obstructed points.
Work of placing the caissons for the cylinders started in January. Two caissons forty feet in length were driven and excavated to a level clone to the point proposed for starting the pile foundations. The foundation material of sand and clay was unable to resist the water pressure from outside of the caissons and they "Mew in", filling with water. Both caissons were then lengthened, reset and driven, but the same trouble was experienced as before, with the addition that one caisson partially collapsed.
The other caisson was further lengthened and heavily reinforced and successively used in placing the ten cylinders on the north side. These cylinders have a ten foot diameter concrete base ten feet high, resting on twelve piles driven to a firm bearing, the cylinder then extending up to the bridge seat as a 4 foot diameter reinforced concrete shaft. Four cylinders are tied together with a five foot reinforced concrete slab to form the seat of each trunnion bearing.
Unusual rainy weather during January and February was the cause of great delay in placing the cylinder.
At the present time the work completed for the north side has been the placing of foundations for trunnion towers, deck approaches and seawall; the placing of most of the forms and reinforcing steel for the deck approaches and the trunnion towers and portals for the superstructure.
For the south side the seawall and concrete pile foundations for the deck approach are in place and the pile foundation for one cylinder driven. Only three cylinders are required on the south side, as the bridge, a single leaf bascule, operates from the north side.
In the near future the concrete deck for the north side will be completed, the machinery will be installed and the structural steel superstructure started. Work on the south approach will be carried on simultaneously with that of the superstructure and both should be finished shortly.
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