Lagunitas Creek Bridge: Retrofit options


I have spent significant amounts of time chasing fish with a spear while holding my breath, giving me insight into what a fish might need to survive in its environment. I also have considerable experience in the analysis and design of bridges—mostly Caltrans projects I worked on with private engineering firms in San Francisco before the turn of the millenium.

My concern with the Lagunitas Creek bridge project is making sure that any work neither adversely affects the fragile ecosystem of the creek or its runs of salmon and steelhead trout. I worry about a heavy construction project in this fragile creek, and am adamant that creek work should only be allowed for a retrofit option and not for a replacement option. 

When the Green Bridge was built, the creek critters were not yet imperiled, at least compared to today, when we are staring at the demise of most of the animals with which we share this planet. We are lucky to have salmon and steelhead runs in the creek, and should do everything possible to make sure each and every run is protected during our watch.


A new bridge versus a retrofit 

A replacement bridge will be much more expensive and disruptive to the stream and the community than a retrofit. You have to dismantle and remove the existing bridge and then build the new one, all with heavy machinery, instead of keeping but fixing the existing bridge in place with smaller machinery and less construction effort. Yet a new bridge would make more money and be easier for a large construction firm used to doing this (most are). A retrofit could cost up to 50 percent less, but would make less money for the large construction firm and take more time. It would be more difficult to construct (overhead clearance, old bridge in the way, difficult existing conditions, have to erect falsework for temporary access, less volume of materials on which money is made, smaller machinery, etc.)

Both a retrofit and the new bridges Caltrans is proposing would be designed for a “no-collapse” criterion instead of a much higher “performance based” criterion. The former requires a lot less analysis and design effort than the latter. This means that the four new designs would not get us any more than a suitable retrofit when it comes to seismic safety or convenience. Both, after a large earthquake, would need to be removed completely and replaced with a new bridge. This is a loosing proposition for the commuity and the creek.

Instead, a retrofit designed for performance-based criteria—or, alternatively, a new bridge built to those criteria—would allow us to use the same bridge into the 22nd century. This is not a pie-in-the-sky, but it does require significantly more analytical and design effort from Caltrans engineers than the agency has been willing to provide up to now. 

In summary, the structural analysis and design is cookie-cutter for the proposed replacement options, and can be done very quickly and cheaply. These options maximize profitability for the contractor. A retrofitted bridge built to the higher standards does almost exactly the opposite: it is much harder to analyze and design, and the contractor’s work is harder.


Suitability of a retrofit 

In my professional opinion—one developed from experience with numerous Caltrans retrofit projects—this bridge is an ideal candidate for a number of retrofit options, but these need to be investigated carefully by qualified professionals. To that effect, I provided Caltrans numerous feasible retrofit options in May that could be investigated by Caltrans engineers to significantly reduce the project scope, cost and effort. I did not receive any acknowledgement of receipt of my proposal, or of any other letter I sent them with regard to this project. (The Point Reyes Library has a copy of most of the documents and letters I provided to Caltrans for this project, along with a handy glossary of terms. These materials are available for borrowing or copying.) The suitability of any retrofit can only be determined by careful consideration of existing conditions and permitted and available repair options. So far, this process has not yet begun for this bridge, to any appreciable extent.


Increase of bridge rating

If there is a legitimate reason why Caltrans needs to increase the rating for this bridge— such as the heavier vehicles than what has been seen up to now in the community that Caltrans has consistently cited in their meetings—the entire green portion of the bridge (the supported steel truss frame) could be replaced with a bigger, stronger and more ductile steel truss frame and the existing piles and foundation could be retrofitted instead of removed and replaced. In other words, to increase the rating for this bridge, a retrofit option would work as well as a replacement.

This new bridge truss—let’s say, painted blue with polka dots—could be built nearby, such as in the field where Davey tree service parks their trucks and equipment or on a corner of Love Field, fairly quickly. Then the old Green Bridge truss could be removed, raised, rolled out and parked near the new Blue Bridge, while the new Blue Bridge could be rolled to its new location on the retrofitted substructure. Once in place, the new Blue Bridge would have a roadway slab poured in place, and a day later could be open to traffic. In the meantime, the efforts to dismantle and recycle the existing green truss could be well underway. And why not hire small local firms and local workers to do this, so the community benefits directly?

I hope Caltrans eventally makes the right decision and considers a suitable retrofit, shelving their current replacement options for the foreseeable future. Our creek and its irreplaceable residents deserve better, and I hope our community stands together to protect this fragile environment against any and all threats. The salmon and steelhead were here way before any of us, and it would be a terrible thing if we allowed them to be sacrificed or compromised for what just seems to be more business as usual. 


Alistair Lizaranzu, P.E., is the sole proprietor of North Bay Seismic Design. He lives in Inverness. An unedited and unabridged version of this letter is available at the Point Reyes Library.