Big Creek Bridge Restoration
By Luke Griffis | September 7, 2021
Big Creek Bridge is an open spandrel concrete arch bridge on California’s coast in Monterey County. The bridge was built with funding from the New Deal in the 1930’s, ultimately opening in 1938. Measuring 589 feet long and 24 feet wide, the bridge spans 65 feet above Big Creek near its mouth at the Pacific Ocean.
Though the bridge went through an extensive seismic retrofit in the 1990’s, additional maintenance was required to maintain the longevity of the bridge. Because of the bridge’s location along the Pacific Ocean, it is constantly subjected to the effects of the weather. Salt water can have a particularly detrimental effect on the bridge. Since the bridge was over 80 years old, replacing the bridge could have been an option except for the exceptionally high cost to replace a bridge of its size in its unique location. The bridge also has historic value to the Big Sur area of the California coastline. Furthermore, replacing the bridge would mean dealing with traffic interruptions and likely years of permitting issues and red tape due to the existence of Native American burial grounds near the base of the bridge as well as protected butterflies that inhabit the area.
Therefore, instead of replacing the bridge, Caltrans decided to extend the existing bridge’s lifespan through a process called electrochemical chloride extraction, or ECE for short. ECE works by applying electricity to the bridge to remove salt ions which can penetrate the concrete bridge and rust the steel rebar in the bridge. The ECE work was performed by subcontractor Vector Construction of West Fargo, ND. Vector was uniquely qualified, being the only company in North America that performs this type of ECE work.
To access the bridge to perform the ECE, Vector needed an access scaffold. Prime contractor Truesdell Corporation of Tempe, AZ hired Commercial Scaffolding, Inc. to provide and erect the scaffolding. In turn, DHC was contracted by CSI to design the access scaffolding to allow workers to access all faces of all concrete bridge members below the bridge deck.
The scaffold design had multiple unique requirements. First, because of the electricity used in the ECE process, none of the metal scaffold could be in physical contact with the bridge for risk of electrocution. DHC used rubber mats and timber cribbing to act as a barrier between the steel scaffolding and the bridge. Second, as previously mentioned, the bridge is located above Native American burial grounds and also home to a protected butterfly species. This meant that almost no scaffolding was allowed to touch the ground. Very specific locations were identified where scaffold could set on the ground. However, these were restricted to small areas adjacent to the existing bridge pier footings. The majority of the scaffold had to be supported off of the bridge’s lower arches. Third, because the ECE process had to be protected from the elements and contained for environmental purposes, the scaffold had to be designed to be fully enclosed with shrink wrap. With high winds coming off the ocean and knowing that the scaffold would be up for an extended period of time, this created a challenge to design the scaffold strong enough to resist the high wind forces without overloading the bridge structure and without restricting Vector’s accessibility to the bridge members and making their work too cumbersome.
DHC was tasked with designing a scaffold that was hung off of scaffold trusses bearing on sloped concrete bridge arches. The scaffold hung off the trusses to allow access to the underside of the concrete arches. Scaffold also was erected on top of the trusses to provide access to the concrete spandrel columns that extended up to the underside of the road deck. The design had to be light enough to not overload the scaffold trusses, other scaffolding components, or the bridge itself. It also had to be robust enough to support the large crew Vector planned to have on the scaffold and resist the winds coming off the ocean. A large crew was needed by Vector in order to complete the project in a specified window of time outlined by Caltrans.
As part of DHC’s design submittal, we provided the loads that the scaffold would impart on the bridge. This included the dead load of the scaffold, the live load of the workers and equipment on the scaffold, and the wind loads on the scaffold. To ensure the bridge would not be overstressed due to the combined loading from the scaffold and the vehicular traffic which would still be on the bridge during the ECE operations, Caltrans engineers used computer software to model the entire concrete bridge. They then applied the loads provided by DHC to their computer model to confirm the bridge had sufficient strength to support the scaffolding and traffic simultaneously.
The design phase lasted nearly a year. DHC began preparing the scaffold design plans in June 2019. It took 9 revisions of the scaffolding submittal to accommodate the needs of the contractor, subcontractor, and pass the Caltrans review process. Final construction plans were not issued and approved until May 2020. The project had minor delays due to high wind events that required the shrink wrap enclosure on the scaffolding to be repaired and the scaffolding inspected. The construction schedule and timeframe was also in part dictated by the mating season of the butterflies in the area. The project was accomplished through the detailed collaboration of all parties involved and having each company’s team understand the unique requirements and challenges that the project entailed.