Bridge removal in record time13 July 2018
Burkhalter Rigging has used an Enerpac JS-Series jack-up system to remove parts of San Francisco–Oakland Bay Bridge's old east span
The dismantling and removal of the old Bay Bridge east span started in 2015, with the demolition of the cantilever section and S-curve to Yerba Buena Island. Phase II involved the removal of 1.7 million-pound truss sections, and included five 504ft segments and 14 288ft segments.
Burkhalter Rigging was chosen to remove the 288ft segments. The project posed significant challenges. At its highest point the bridge is 36m (135ft) above water level, with a slope to the shore to 40ft above the water.
This meant that each span was at a decreasing angle as the segments reached the shoreline. Lifting would need to take account of tidal changes and weather on the bay. All the segments had to be removed between November 2016 and April 2017 ahead of migratory birds returning.
Earlier removal of the 504ft truss sections was accomplished using strand jacks mounted within a supporting frame. The same lifting method was considered for the 288ft sections; however, it would have required more time to set up and lift each truss than using the jack-up technique. Burkhalter decided to use the Enerpac hydraulic jack up system for this job, as it had the experience of using it on a barge for installing the Fore River bridge in Boston.
The Enerpac JS-Series jack-up system is a multi-point lifting system comprising four jack-up towers positioned under each corner of a load. Burkhalter used a JS-750 jackup system for the Oakland bridge lift with barrels measuring 2.3m x 2.3m x 1m (7.5ft x 7.5ft x 39in).
In reaching the highest truss section at 36m above the water, side load on the jack-up towers was a major factor in planning a lift.
Following extensive analysis and modelling by Enerpac, the concept of a box type bracing system was developed for lifts above 20m. This is based on using a special fabricated intermediate barrel with connection points for crane booms and steel wires attached to strand jacks to provide horizontal bracing: creating a hydraulic tensioning device used to stabilise and monitor the side load.
As the towers are raised, the crane booms—resting on steel supports above ground—are automatically latched on. The bracing wires and strand jacks resting on the boom sections were pinned to the special barrels. Each tower was then raised to the height required to lift the bridge section. For lifts below 20m, the bracing system was dispensed with.
Burkhalter minimised side load during movement of the jack-up on the barge by performing additional lift and trim calculations and maintaining the spans’ centres of gravity at a precise point to keep the barge level, with constant monitoring of tide levels, during truss removal.
Given the length of each truss section, the four jack-up towers were deployed on a twin barge assembly to form a single floating platform, with two towers per barge. Ahead of the arrival of the barge, the bridge sections were cut from the adjoining segments ready to be lifted.
The barge was floated in position under the segment and the jack-up towers raised. The west and east facing towers were each connected by a beam on which the truss would rest when lifted. Because of the angle of the segment, the west side towers were approximately 1m higher than the east side towers. The East Span removal began in the middle section, at the intermediate fixed pier, and removed progressively towards the shoreline in the direction of the lowest section. Then, the spans were removed from the middle working towards the highest truss section.
Height flexibility of the jack-up system meant that the same process could be used for the highest span and lowest near the shore, where only a single barrel was needed. Once the bridge section was removed, the barge was then moved away from the bridge, the towers lowered, and the span transported to the Port of Oakland for disposal and recycling.
“Using the Enerpac jack-up system, we were able to cut each span removal from six weeks to about one week per span,” concluded Mike Cassibry.