Bay Bridge Rebar Cage
By Jasper Calcara | November 1, 2022
Such is the evolving world of construction that each time we pick up the phone at DHC, we expect there’s a chance we’ll be asked to do something we’ve never done before, or to support a construction practice or method that is pushing the boundaries of what has been done in the past. It is in our core and culture to support our customers with innovative and creative designs which ensure safety and stability, even when the challenges are overwhelming.
When we began designing rebar cage support systems in the late 1990’s, we faced many of these situations as the design process, methods and codes were not well established to provide guidance for the industry. One extreme example was the design of a rebar cage support system on the rocky cliffs of Yerba Buena Island, in the middle of the San Francisco Bay.
Excavations for the foundation and pier footing concrete had been complete, and various bridge support towers began to make their way skyward, when we were asked to focus our attention on a particularly challenging situation. The contractor needed to lift and place tall and slender, circular rebar cages into position, temporarily stabilize them with guy wires, install formwork, transition the stability support, and finally pour concrete in lifts with staged formwork removal. This would have been standard if the work was not being performed on the water’s edge, where regular high winds funneled through the Golden Gate, and if it was not stepping up an unstable rocky slope, giving workers very few safe access points to install supports.
Lastly, to reduce exposure and complications, the cages would be of exceptional height instead of multipole shorter lifts as were implemented on other piers.
After much consideration of the site geometry and evaluation of options, a concept of guy wires to rock anchors was proposed and penciled out. Unfortunately, due to the site restrictions and cage heights, the cables were to be installed at extremely steep angles, and in an unbalanced fashion since the water’s edge cut off the west end of the site.
The most significant challenge facing the engineering team was the massive downward forces that would be applied to the rebar cage when subjected to high winter storm winds, and the risk of buckling the web of rebar, laced together with wire ties and limited internal bracing.
When presented to the CA DOT for approval, they asked the design team to prove the cage would be structurally stable during all phases as well as exposure to high wind events. When asked for approach or specifications they would accept, they could not provide anything, and we were left to develop our approach from scratch.
After review of technical papers, discussions with the contractor, test configurations and running structural analysis with various assumptions of cage frame action, we began to develop a process we could stand behind. We prepared and presented a complete analysis to accompany the PE stamped design plans and waited while the submittal was reviewed by the field office, bumped up the chain of command and eventually landed on the desk of the senior structural department in Sacramento.
We were presented with question after question before ultimately satisfying the reviewing engineers and gained approval for construction. Although our work was done, the challenges in the field were just beginning as unstable rock and changes in layout and sequence piled up one after the other. After multiple site inspections and adjustments to the design, the cages were finally lifted into place, and the stability system put to the test. Even after being exposed to extreme winds and middle of the night engineering support, the cages and support system held true, and the towers completed. DHC has gone on to design hundreds of support systems since, but the challenges faced on this lone pier still prove to be the greatest of rebar cage support to date.