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History and Background:
Construction of the Old Bridge |
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Dan Gallagher, Ph.D.
Dr. Dan Gallagher: served as Executive Editor for the Monroe County Environmental Story in 1991 while working with the Newfound Harbor Marine Institute at Seacamp. Since that time he became a founding member of the Pigeon Key Foundation, and currently serves as Education/ Museum Director for the Pigeon Key Foundation. Publications include Pigeon Key and the Seven-Mile Bridge (1996), and editorship of the Teacher Activity Guide for the Monroe County Environmental Story (1996). The Overseas Railroad (officially known as the Florida East Coast Railway Key West Extension) lay atop many miles of fill and bridges built by the F.E.C. Ry. Construction Division between 1905 through 1912. This long right-of-way passed down through the Keys, much of it forming the current corridor now called U.S. Highway One. The F.E.C. Ry. bridges served first as a highway for trains, then later for motor vehicles. Today they are used for fishing piers, scenic overlooks, and exercise areas for visitors and residents. The bridges themselves are significant contributions to the Florida Keys built environment. In total distance the 42 F.E.C. Ry. bridges covered 17.98 miles. Estimates vary on the total cost of construction, ranging between $20,000,000 to $33,000,000, all privately financed by the F.E.C. Ry. Bridges are made of two parts: piers and spans. Piers are upright fabrications that anchor to the bottom of the sea and rise to support the upper portions. The upper sections (or spans) cross between the piers to form the roadbed. The piers for all of the F.E.C. Ry. bridges were built in the same fashion, but the upper spans varied depending on the conditions. Water depth dictated how high the bridges must stand above the water. In shallow waters, the F.E.C. Ry. engineers designed low bridges made of concrete arches. Deeper waters (which generate larger waves and storm surge in hurricane conditions) required taller bridges. Three of the bridges (the Long Key Bridge, the Seven-Mile Bridge and the Bahia Honda Bridge) passed over deep water, and each bridge was more than 20 feet above mean high tide. The Long Key bridge was built with 180 spandrel arches; the two other tall bridges were made with steel spans. Spandrel Bridge Construction Most of the East Coast Extension bridges were built with spandrel arches. These graceful arches do not show us much about the supporting piers; these are submerged. The spandrel arch bridges, while simple in form and graceful to the eye, required several complex steps and concrete forms in their construction process. Before any spandrel pier forms were set in place, construction workers and divers cleaned off the layer of sand and sediments built up on top of the coral rock that underlies the Florida Keys. This coral limestone bedrock provided an excellent substrate for concrete construction -- it is a solid mass of rock that affords a good anchor for the piers supporting the bridge. At this time the workers also drove many wooden pilings in the footprint where the pier was to be built. They then brought in pre-fabricated cofferdams -- boxes of wood much larger than the pier -- and set these in place on the bottom. These cofferdams had no bottom, and, when in place, were full of water at this time. After the cofferdam was anchored in position, workers gently pumped concrete into the bottom of the form. The sides of the cofferdam contained the concrete, and, when it reached a thickness of two feet, the first pour was complete. The concrete hardened underwater in a few days, firmly holding the wooden pilings driven into the bedrock, and making a "seal" to the bottom of the bay. When the concrete cured, workers pumped out the water captured in the cofferdam, and began to build the next part of the pier on this seal. The second stage molded the lower section, the rising part of the pier. The lower section forms were partially built on land and assembled inside the now-dry cofferdam. These forms shaped the concrete with a spreading set of "ears" at the upper part. Steel reinforcing rods were used extensively in this construction, and protruded from the upper part of the lower section pier as may be seen in the photo of the completed lower section with all forms removed. This photo also shows gangs of pilings to be used to support the next forms. The span portion of these spandrel arches was built upon these piers. The first form used for the spandrel arch was termed an arch ring mold, and was also prefabricated on land. These arch ring molds were transported by barge to the job site, jockeyed in place, and positioned between the protruding "ears" of the lower section. As the tide rose, the arch ring mold was gradually forced up to meet the "ears", and was then wedged into place on supporting pilings as shown in the photo. After the arch ring mold was braced, workers began building wall forms for the spandrel arches. These were built to be re-used, as were all of the forms for the bridges. Again, they placed tons of reinforcing steel within each arch, tying the spandrel spans to the lower section piers. At this time they also put a center form inside the growing spandrel form. This form kept concrete from the middle of the spandrel arches; they were hollow inside. This hollow area was later filled with coarse gravel. Workers layed ties and track directly on this ballast rock bed; the gravel cushioned the train weight from the concrete arches. The summary graphic on the following page shows these steps in bridge construction. Tall piers and steel spans About five miles of the Seven-Mile Bridge were built with tall piers and steel spans. These spans, termed steel deck plates, were each 80 feet long, except for a small section at the curve over Pigeon Key, where they are 60 feet in length. The tall piers are graceful, tapered, solid concrete fabrications with steel reinforcing. Initial steps for building these tall piers were much the same as for the spandrel piers below the water line. Again, they used cofferdams, poured a seal, then poured a lower section below the water level. When this was completed, workmen built upper forms on top of them. These piers were then ready for the steel deck plates to be set in place on large pads that distributed the 19-ton weight of each deck plate. The deck plates were then tied together with a network of angle iron crossmembers. While the deck plates came prefabricated from the American Bridge Company, all of the assembly was done "in the field" by ironworkers who riveted the parts together. The Bahia Honda Bridge was also built on tall piers -- some of them more than 40 feet below the surface of the water. While steel deck plates were used for the approaches at each end, most of the Bahia Honda Bridge is built from trusses. The train passed through the trusses on track laid inside the bridge. A few notes on the bridges of the F.E.C. Ry. Key West Extension... Building the Florida East Coast Key West Extension was a massive undertaking, financed through a private corporation, not with government funds. It was also massive in the scope -- eighteen miles of bridges, more than a hundred miles of track laid on fill in an environment hostile with insects, heat and hurricanes. All the supplies had to be brought in from outside...rock, sand, steel, cement and water. More than 20,000 workers contributed to the project during the construction years. It was an incredibly labor-intensive task, completed with strong backs and relatively unsophisticated tools. Most of the vessels and all of the machinery (cranes, cement mixers) were driven by steam engines. The bridges of the great Key West Extension were built to last. It may be noted that none of the F.E.C. Ry. bridges have ever been damaged by hurricanes -- those that are no longer in place have been removed by man in order to build our modern highway bridges. The three great bridges...the Long Key Bridge, portions of the Seven-Mile Bridge, and the Bahia Honda Bridge...are listed on the National Register of Historic Places. It is reasonable to assume that these great bridges will long be a part of the Keys’ built environment. As they stand today, they afford opportunities for fishermen, recreation, and offer a rich niche for marine life as structure and habitat on the bottom of the inlets that connect the Atlantic with Florida Bay. FIGURE CAPTIONS Fig. 1. Cofferdams. Three are in place in the foreground; a fourth is lowered by a crane to seat on the bottom of Florida Bay.
Fig. 2. Pumping sea water from the cofferdam before beginning construction of the lower section form.
Fig. 3. Lower section form under construction inside the cofferdam.
Fig. 4. Lower section completed. Note the reinforcing steel rods protruding to tie into the next stage.
Fig. 5. Three spandrel arches of the Long Key Viaduct under varying stages of completion. At the left, a barge is bringing an arch ring form to place between the completed lower sections. The central part shows an arch ring form braced in place between the piers. The rightmost part of the picture shows the side forms resting on the arch ring form.
Fig. 6. Tall pier forms. This photo is from a F.E.C. Ry. postcard series.
Fig. 7. Workmen assembling the crossbraces for the steel deck plates as bridge work progresses on the Pigeon Key Trestle.
Building the Bridge Piers... Steps for building a spandrel arched pier... Step 1. Cofferdam forms are placed in a square on the bottom. The central pilings go through the bedrock to hold the completed block in place. Workers drop sandbags around the cofferdam to stabilize the form. (the right side is not shown in these diagrams.)
Step 2. Concrete is piped into the cofferdam form to make a "seal." When it hardens, the pilings will tie the seal to the next level to be poured.
Step 3. Workers use large pumps on barges to pump water from the cofferdam. They then build the lower section form inside the dry cofferdam. This form is for a spandrel bridge pier.
Step 4. Cofferdam and form are removed. The steel reinforcing rods protrude to tie into the upper form (see below).
Assembling the arched spans into a bridge... Arch ring forms for the underside of the upper sections of the spandrels are brought by barge to the completed piers. When located properly, the wooden forms are braced in place on a scaffold of pilings (not shown). Side forms are added to the arch ring forms to shape the outside walls of the spandrel. These forms were re-used if they could be taken apart without damage.
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