Wednesday, April 12, 2017

Sunshine Skyway Bridges across the Tampa Bay

(Bridge Hunter, Old Bridge Hunter, Satellite, 201 photos)
The title uses the plural for "bridges" because originally there was a cantilever bridge built in 1954. A second cantilever bridge was built in 1969 to increase the number of lanes from two to four to meet Interstate standards for I-275. But it was not opened until 1971 "for reinforcing of the south main pier, which had cracked due to insufficient supporting pile depth."


By ←Baseball Bugs What's up, Doc?carrots→ - I (←Baseball Bugs What's up, Doc?carrots→) created this work entirely by myself., Public Domain, Link
The width of the shipping lane was 800 feet. There had been a history of allisions with the bridge. But the one that destroyed the 1200 feet of span you see missing in the far bridge abpve happened on May 9, 1980 during a fierce thunderstorm. "The south main pier (the one that required reinforcement before completion) withstood the ship strike without significant damage. It was the second pier to the south of it that was destroyed, a secondary pier that was not designed to withstand a large ship strike." [Wikipedia]

MiamiSprings Memories posted a video of  the demolition of the old bridges with the following comment. Note that some of the scenes in the beginning of the video are in slow motion.
The Sunshine Skyway Bridge Demolition - 1993. Removal of the parallel spans and causeways of the Florida Department of Transportation's Sunshine Skyway Bridge over Tampa Bay between Bradenton and St. Petersburg was the largest, most complex bridge demolition contract ever issued by the State. The collisions of oceangoing vessels with the old channel piers prompted construction of a new cable suspension bridge with a much wider channel. The southbound span (opened in 1971) of the original bridge was destroyed at 7:33 a.m. on May 9, 1980, when the freighter MV Summit Venture collided with a pier (support column) during a blinding thunderstorm, sending over 1200 feet (366m) of the bridge plummeting into Tampa Bay. The collision caused six cars, a truck, and a Greyhound bus to fall 150 feet (46 m) into the water, killing 35 people. 
The picture below catches the transition from old to new. The approach trestle for the old bridge was retrained as a fishing pier. And the debris from the demolition of the truss spans was put along the fishing pier to create artificial reefs. The pier must be very popular because I read objections to a proposal to remove a couple of the piers spans because they were too old. The objection said people are practically standing on each other now to get a fishing spot. What will it be like if there is even less room?

State Archives of Florida/Florida Memory from OnlyInYourState
The replacement cable-stayed bridge has a shipping channel width of 1200 feet. "The piers of the main span and the approaches for 1⁄4 mile (400 m) in either direction are surrounded by large concrete barriers, called "dolphins", that can protect the bridge piers from collisions by ships larger than the Summit Venture like tankers, container ships, and cruise ships.[Wikipedia] The bridge needs to be closed to traffic during high winds.
A major problem with the Sunshine Skyway Bridge is corrosion of the steel in the precast concrete segmental columns on the high level approaches. Because the segments are hollow, workers were able to enter the bridge superstructure in 2003 and 2004 to reinforce the corroded sections of the bridge, ensuring its future safety. Another problem arose around 2005–06 when several news bureaus reported paint discolorations on the bridge's cables. These paint splotches and patches were a result of touch-ups that were performed over the years but began to show through over recent years. In 2008, the Florida Department of Transportation (FDOT) began an overhaul including repainting the cables in their entirety (instead of touching up) and rehabilitating the lighting system at the summit of the bridge. [Wikipedia]
The clearance is 190 feet above the water, 50% higher than the old bridges. [PBS, Study] But cruise ships have grown higher than that. [Wikipedia] Most specifications of Post Panamax ships are for what will fit in a lock, not under a bridge. However, the clearance of the Verrazano–Narrows Bridge is 225 feet, and I believe it is the bridge that is too low to allow the Post Panamax ships to access New York and New Jersey port facilities. Another data point is the St. Lawrence Seaway, which was built in 1959, has a clearance of 116.5 feet.

The low level trestles are made with 256 spans built with pre-stressed girders. The high level approach spans are made with 584 post-tensioned segmental box girder segments totaling 180 spans for a total length of 5,000ft on either side of the man spans. [RoadTraffic-Technology]

I have found several references that indicate the 94-foot wide deck is suspended by 42 continuous stay cables sheathed in 9in diameter steel pipes. And most references point out that it is the longest cable-stayed bridge in the world. I also came across a lot of information about suicides, but I choose to ignore that topic. However, I can't find any statistics on the towers. Since the towers are the most important feature of a cable stay bridge, I find the lack of information on them to be quite frustrating. Finally, I found: "The twin cable towers are 432 feet tall." [FloridaAlmanac]

Finding information on the piers was even harder. The figure below does not specify the depth of the steel casing, but if the drawing is to scale, it is about 50-feet tall with about two-thirds under the sea bed.

GAO RCED-85-32

According to Florida's contract specifications, each pile was to be poured in 2 hours so that there was not a "cold joint" --- concrete poured on top of concrete that had already set. But 18 pours took more than 2 hours and some took as much as 4 hours. Further study indicated the specification was rather silly and 4-hours was fine because it took at least 6 hours for the concrete to set. And they took a core sample of a long-pour pile and determined the strength was as good as the 2-hour pour piles. [GAO RCED-85-32 p. 17 (p. 29 in the .pdf)] The piles do not go to bedrock. I quote from the report:
FDOT'S DECISION TO PLACE THE PILES DEEPER
During construction of the south main pier, the contractor h d difficulty obtaining core borings from the bottom of the shafts. The shafts were drilled to specified depths in the sea bed, core borings taken, and then the concrete piles were poured into the shafts. (See fig. 3 on p. 18 showing the foundation shafts.) The core borings were used to determine the composition of the soil at the tip of the shafts and verify prior geotechnical work. The contractor was unsuccessful in obtaining core borings krorn the first two shafts of the south main pier. FDOT officials directed the contractor to modify the core boring equipment. The shaft drilling and concrete pouring continued while the equipment was being modified. On the 15th shaft, the contractor, with modified equipment, obtained a boring that indicated the subsoil was not as expected. However, the boring had been infiltrated withwater and broken up when removed from the core boring equipment. FDOT requested the assistance of its geotechnical consultant who observed two more drillings and then recommended drilling the remaining 27 shafts 6 to 7 feet deeper than originally planned. FDOT adopted the recommendation as a conservative measure based on safety.
Core samples were taken on the remaining 27 shafts which confirmed the qeotechnical survey results. According to the consultant, drilling the shafts deeper, which places the piles deeper, actual.ly increased the safety factor of the piers. FHWA regional officials agreed and explained that much of the piles' support capability comes from the side friction of the shaft walls adhering to the concrete pile. Placing the piles deeper exposed more of the concrete pile surface to the shaft walls, thus obtaining greater side friction. FHWA concurred with FDOT's decision and has approved the main support piers as meeting construction standards. [GAO RCED-85-32 pp. 17,19 (pp. 29,31 in the .pdf)] 

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