The Green Bridge is an ideal candidate for a retrofit. Yet for more than two years, Caltrans has not responded to several comment letters that included retrofit methods that would meet the same standards as the replacement bridge alternatives. My suggested methods are commonly used by Caltrans and would be much cheaper, faster and less disruptive to the creek and the community than the preferred replacement options.
Caltrans decided to retrofit the bridge in 2007, but changed its mind later. In their public meetings in Point Reyes Station, Caltrans representatives described a phony retrofit alternative that would entail replacing every component of the bridge and lamented how expensive that would be. They never described to the public the retrofit design proposed by their own technical staff. This history fits the definition of a “make work” project—a project that is expanded to fit the budgeted funds.
Caltrans’s own 2010 statewide seismic retrofit guidelines state that “a retrofit must not be done unless it can be demonstrated that the bridge will collapse” in an earthquake predicted for that location. A guideline update in 2016 says that “the retrofit strategy for Ordinary Bridges is to do no more than is necessary to prevent collapse…” Since Caltrans describes our Green Bridge as an ordinary bridge, it seems clear that the local agency office violated this guidance by not evaluating retrofit alternatives in its draft environmental impact report.
Nor do I think any of the Caltrans presenters at the workshop held at the Dance Palace were familiar with the condition of the Green Bridge, which was tented, spray-washed and re-painted in the early 1980s, according to a local resident. The speakers ignored their own technical staff’s recommended retrofit alternative, summarized in a March 8, 2017 seismic evaluation report.
The main concern over the bridge is the connection between the concrete piers at the four corners of the bridge deck and the piles below, which carry the loads into the supporting soils. These are probably inadequate, and the piles could separate from the piers during seismic shaking. According to Caltrans engineers, one solution is to drive additional piles deep down to the bedrock around the piers and then connect them to the piers themselves. This can be done without going into the creek bed—a big advantage. Caltrans design engineers even provided a drawing of the bridge with the retrofit pieces in place.
The connections between the pier tops and the steel bridge deck also need to be strengthened. Methods include the use of shock absorbers or connections that allow some bridge movement, such as shear keys or catcher blocks. These systems are in common use on small bridges such as ours. In 2015, the Point Reyes Station Village Association sent a letter to Caltrans that opposed the new bridge alternatives as too disruptive to the town and asked for a retrofit. All available evidence shows that several retrofit alternatives are possible. Caltrans should retrofit our bridge and use the rest of the funds on other projects.
Bob Johnston is a retired U.C. Davis professor living in Inverness.