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| Showing fancy foot work Impressive, precise maneuvers used to strengthen Golden Gate Brigde foundation Keeping the Golden Gate Bridge on solid footing is a significant undertaking. Preparing it to withstand the threats of potential earthquakes makes it all the more important. And accomplishing this task without disrupting day-to-day San Francisco commuter traffic talks it into the real of the remarkable. Avoiding delays has been the daily challenge faced and met by the contracting firm of Balfour Beatty Construction Inc., Atlanta. Ga. It has involved construction of temporary support towers and a series of intricate synchronized lifts transferring the loads from the existing bridge onto the temporary supports. The next step is demolition of the old tower and concrete footings, followed by new concrete footings on 2-ft-diam. CIDH shafts and the erection of the new structural steel tower. In terms of distance, the lifts are short. Massive bridge sections are raised and lowered just two tenths of an inch at a time. But given the importance and the intricacy of the operation, it is a task that is planned and carried out to the minutest detail. A complex electrohydraulic synchronous lift system was custom built for the application by engineers from Balfour Beatty and Enerpac. The lifts and tower restoration ar being accomplished as part of a three-stage upgrade of the main bridge and its northern and southern approaches. Estimated total costs for the project, to be completed in October 2005, exceed $297 million. The project has been carefully monitored and managed under self-imposed design rules that are stricter than those required at the start of the project. It has progressed steadily toward its scheduled completion, successfully fulfilling its dual mission of solid bridge reinforcement without traffic disruption. |
 Higher Failing marks With San Francisco's climate and transportation patterns toe contend with, and the city's civic charm and pride to uphold, the scope of the seismic restoration project is unusual, perhaps unique. It is regarded the most sizable and significant upgrade since the bridge opened in 1937. Selected in 1999 as one of the "Top 10 Construction Achievements of the 20th Century" in an international poll conducted for ConExpo-Con/Agg, the 63-year-old bridge is more than a civic treasure and landmark. It continues to serve as the community's northern rout into the city for emergency crews, in addition to being the chief transportation rout for more than 41 million motorists annually. More than 10 million visitors witness and tour the bridge annually, according to Golden Gate Highway & Transportation statistics. Located within a Zone Four seismic classification, the highest in the U.S., the bridge requires special care and provisions for earthquake protection. In spite of its excellent design, conscientious maintenance and a history of significant structural improvements, the bridge could fail during an earthquake of magnitude of 7.0 or greater occurring on the San Andreas of Hayward faults. A 1989 earthquake in nearby Lorna Pietra underscored the bridge's proximity and vulnerability to earthquake damage to a worldwide television audience watching the San Francisco Giants and Oakland A's in the World Series. This prompted a detailed study on which the seismic restoration upgrade is based. It called for the redesign and restoration of the bridge to withstand a maximum credible earthquake of 8.3. Towers just one story The seismic retrofitting of the Golden Gate bridge includes the addition of concrete and steel combined for a general strengthening of the structures to reduce violent actions and the aftermath of an earthquake. Also included in the retrofit is the addition of cast-in-drilled-hole piers, concrete, bearings, structural steel and restrainers of the main bridge and its approach viaducts. Much of the seismic restoration centers on the rebuilding of four towers and the construction and erection of the temporary supports on the outside of each of them. With the first synchronous lift, the bridge support transfers from the towers to the temporary supports. Once this slight and significant adjustment is made, the old towers are dismantled section by section to ground level. The temporary supports remain in position for approximately two months as crews build stronger footings and assemble the new steel tower. Upon completion of the new tower, another synchronous lift raises the bridge to transfer the load back to the new tower. While the overall project is described as a retrofit, it involves far more than tower and component replacement. Overall, the deck, towers and foundations are being considerably upgraded. Approximately 900 tons of steel is being added to replace or reinforce existing lattice members. That had ot be accomplished prior to tower removal. Seismic motion isolators and dampers are being added and the separate spans are being linked so they are moving together in an earthquake. As another protective measure, the bridge now rests on rubber seismic isolation bearings on top of the towers. Move packed with precision The enormity of the project becomes a matter of inches and precise engineering as the bridge moves from its original towers to the temporary pillars and back to its new permanent base. Enerpac's synchronous lifting system was selected as the prime mover after an evaluation of different lifting options and close collaboration between Balfour Beatty and Enerpac's Don Bishel. Numerous planning and progress meetings have been conducted with the contractor an Bishel, who has more than 16 years of hydraulics experience. The Massive lifts are carefully, centrally controlled. A microprocessor based control unit provides single station interface for lifting and lowering operation and position interface for lifting and lowering operation and position monitoring. The digital technology governs the automatic synchronous lifting and lowering of up to eight lifting points. High accuracy (.04 in.) sensors are attached to the load to ensure accurate positional control. Twenty-four Enerpac 200-ton (CLL Series) lock nut cylinders participate in each lift. The single-acting, spring return cylinders are grouped in clusters of four and positioned on each of the legs of the six pillars. The clusters act as one during the course of a lift. They are tied with a single manifold, so all four cylinders receive the same hydraulic and electronic signals from the controller. Preparatory steps for a lift include the loosening of the bolts that hold the bridge to the tower and the positioning and striking of the cylinders to bring them in contact with the bridge. Program controls conduct the lifts in two-tenths of an inch increments. Workers check pressure on each of the cylinders at this point before starting the next extension. Release from the tower generally occurs at approximately .6 in. Cylinder clusters are then turned off, and the exercise is repeated on each of the legs of the towers. The overall process takes approximately 30 minutes, the majority of which is spent checking and monitoring the lift status. Readings are frequent and verification of each step is necessary before the next step begins.  Enerpac's cylinder clusters hold a locked down position for a 24-hour period before the tower disassembly process starts. They remain in place until the new permanent tower is fully constructed. At this point, an equally meticulous lowering process is completed.To date one of the four towers has been replaced, while a second tower nears completion. All told, raising and lowering distances for each tower measure just 11/2 in. Given the weight and nature of the lift, its success is measured in precision and power, not distance. |