Retrofitting San Diego’s “People’s Bridge” to be Seismically Safe

Figure 1: The 1st Avenue Bridge over Maple Canyon

Shrouded by eucalyptus in Maple Canyon is one of San Diego’s most impressive bridges. This #seismicsaturday we feature the 1st Avenue Bridge.

The 463-foot bridge was built in 1931, and is nicknamed “The People’s Bridge” as it was funded by San Diego’s first public infrastructure tax. The bridge was actually assembled first in Ohio, before pieces were loaded on trains and brought to San Diego. The main span is a massive steel arch (fig. 1). The arch is connected to concrete abutments on either side with slanted pin connections (fig. 2). The pin allows the beams and columns to rotate slightly, allowing the bridge to flex in earthquakes and when heavy trucks drive across without putting repetitive stress on the support.

Figure 2: Massive pin connection at foundation. The pin allows the columns to rotate slightly and flex.

Seismic Retrofit

A 13-million dollar seismic retrofit was completed in 2010. If you look closely, you can spot the differences between old and new:

Connection Sleeves

Above the pinned connections, sleeves have been added around the bridge columns (fig. 3). The sleeves help secure the main columns of the bridge to the steel embedment into the foundation. One can spot these sleeves by noticing that they are secured by bolts with hexagonal nuts, as opposed to the original components which are are all connected by rivets with circular heads.

Figure 3: New retrofit sleeves assed to tbe left and right of the columns. The new sleeves are identifiable by their use of hez bolts rather than rivets (with circular heads).

Tie Rods

To tie the bridge to the side abutments, large threaded rods have been added (fig. 4). The rods are anchored into the concrete abutments on one side and bolted to the deck on the other, keeping the bridge tied to it’s abutment in an earthquake, and preventing the deck from being thrown-off and separating from the concrete abutment. These rods are are analogous to tension ties (fig. 5) used when building wooden decks beside houses.

Figure 4: Threaded rods, embedded on one side to the concrete abutments and bolt in the other side to tbe bridge deck, keep the deck from sliding off the abutments during an earthquake.

Figure 5: Tension tie connecting a wooden deck joist to a house floor joist. This connection would keep the deck tied to the house in tbe case of an earthquake. In this instance, two Simpson Strong Tie DTT2Z are used, connected by a tie rod.

Old and New Nuts Reveal Confining Sleeve

Now is where things get super cool. Look closely at the nuts in fig. 6 and 7. Do you notice a difference?

The first is slightly conical on top while the second is flat. The difference likely means that the bolts were installed at different times, with the second bolt probably installed during the 2010 retrofit. The second bolt looks to be part of a layered metallic sleeve (fig. 8), that encases the the concrete of the original foundation to prevent shearing during an earthquake and deterioration over time.

Figure 8: Metallic sleeve encasing the original foundation, added during the 2010 retrofit, to strengthen the foundation during an earthquake

The People’s Bridge is a wonderful example of how old, corroding structures can be retrofitted to be safe and beautiful once again. It must have taken a lot of creativity on the part of the retrofit bridge engineers, T.Y. Lin International, to add modern reinforcement while retaining the bridge’s rustic appearance.

Quince Street Bridge: The Oldest Functioning Bridge in San Diego

Figure 1: The 1905 Quince Street Bridge

What is the oldest functioning bridge in San Diego? This #seismicsaturday we feature the Quince Street Bridge.

The Quince St. Bridge was built in 1905 to connect residents of the then fast-growing Bankers Hill neighborhood to the trolley line in 4th avenue. The bridge is 263 ft long and 60 ft tall. It cost just 850 dollars in 1905 to build. The bridge has a wooden truss structure.

Figure 2: View from above of the lovely white deck, which has been the site of many romantic strolls and at least one proposal!

The bridge was originally made out of Redwood. Redwood contains a natural chemical called tannin, which makes the wood rot, bug, and fire resistant and gives the wood it’s beautiful red color. The tannin in the redwood allowed the bridge to last 82 years outdoor under the elements untill 1987.

In 1987, the bridge was suddenly closed after a city inspection found it to be “infested with termites, full of rotting wood, and generally unsafe” (McDonell 1987). A consultant hired by the city determined that the bridge should be torn down. Local residents were were up in arms about the possibility of their romantic redwood bridge being torn down and they mobilized to have the bridge saves, convincing the City to designate it as a historic landmark. In 1990, the bridge underwent a major refurbishment, in which much of the redwood was replaced by pressure-treated pine.

One can still see some of the original redwood columns on the bridge along with the original bolts from 1905 (fig. 3). The fact that these columns are still standing is a testament to the incredible durability of redwood.

Figure 3: Original redwood column with original bolts dating back to 1905

The pressure-treated pine from the 1990 rebuild can be identified by the staple-sized indents (fig. 4). These indents are made during the treatment process so that chemicals, including chromium (bactericide), copper (fungicide), and arsenic (insecticide), can penetrate into the wood. These chemicals make pressure treated pine last much longer outdoors than untreated pine.

Figure 4: Pressure-treated pine, identifiable by the staple-sized perforations

The bridge is not in pristine condition. The dirt around one of the foundations is quite eroded (fig. 5), and some of the steel strips that sister beams together are coming off (fig. 6).

The Quince Street Bridge is a beautiful and historic redwood structure. Thanks to the efforts of dedicated community citizens in the 1980s, it has been saved for generations to come.


McDonell, Patrick. “Despite Emotional Attachments, Future Bleak for Quince Street Bridge.” Los Angeles Times. 28 November 1987.

Rebuilding the Historic Georgia Street Bridge

Figure 1: The 1914 Georgia Street Bridge

Would you fight CalTrans to save historic arches? This #seismicsaturday we feature the Georgia St Bridge in the Hillcrest neighborhood of San Diego.

In the early 1900s, city planners in a rapidly-expanding San Diego wanted a streetcar line down University Ave to connect the new suburb of North Park with the city. The biggest roadblock was a steep hill between the Hillcrest and North Park neighborhoods. As a solution, a 40 ft cut was made through the hill (fig. 2). Retaining walls were built on either side of the cut and in 1914, the Georgia St. Bridge was built above the cut.

Figure 2: Looking east through the excavated cut towards what would become North Park. Tracks have been laid for the new trolley line.
Figure 3: An electric streetcar running underneath the Georgia Street Bridge likely sometime in the 1920s-30s (Note the Ford model T)

The bridge was designed by engineer James Comley. 3 parabolic arches span the cut, each one anchored into either side of the hill (fig. 4). The main arches support secondary arches, rising at regular intervals to support the deck (fig. 5). A beautiful reinforced-concrete cantilever extends outward to support the bridge crosswalks. It is curious that the top of the cantilever seems to be the only major structural component of the whole bridge in tension.

There have been two major threats to the bridge: earthquakes and CalTrans. By the 1990s, after over 85 years of heavy use, the bridge was deemed unsafe in an earthquake. CalTrans decided to tear the bridge down and replace it with a modern earthquake-resistant overpass. However, community members fought hard to stop this demolition. Included in the effort to save the bridge was Toni G. Atkins, then a small-time assistant to a City council member. She recalled a meeting with a city engineer, where the engineer said that he would be around long after she was out of politics and the bridge was torn down. Atkins said that the city engineer is now gone, and Atkins is now our San Diego representative in the California Senate, and Senate Pro-Temp.

As a result of the community campaign to save the bridge, the bridge underwent a 14 million dollar seismic upgrade from 2015 to 2017 (one can see the point of view of Caltrans, who would have saved taxpayer money by just tearing the bridge down and building a new one). Everything but the 3 initial parabolic arches was demolished and rebuilt (fig. 6). According to a report by Kleinfelder, the contractor on the project, fiber-reinforced concrete was used (fig. 7). This type of concrete is strengthened with fiberglass and carbon fiber in the concrete mix to give the concrete more strength in tension and resistance to cracking/fracture.

Structures shouldn’t just be practical, they should also be beautiful. The Georgia Street Bridge is proof that we can save our beautiful historic structures while at the same time making them earthquake safe.

The Carbon Fiber Cable-Stayed Bridge that Could Have Been…

The bridge that could have been… For #seisimicsaturday we feature the proposed (but never built) carbon fiber/fiberglass Gilman Street Bridge.

With UC San Diego expanding east in the 90s, there was need for a bridge across highway 5 at Gilman Drive. Several UCSD professors, among them prof. Van Den Einde and Frieder Seible, designed an innovative cable-stayed bridge to be made with carbon fiber and fiberglass (fig. 1, 2, 3, 4).

Figure 2: Architectural Rendering of the underside of the proposed cable-stayed bridge (Source: Van Den Einde et. al. 2003)
Figure 3: Elevation view of proposed bridge (Source: Van Den Einde et. al. 2003)
Figure 4: Cross-section view of proposed bridge (Source: Van Den Einde et. al. 2003)

The bridge tower (aka pylon) is designed with a carbon shell system (fig. 5). In this system, a carbon fiber sleeve is filled with concrete on site. Concrete provides strength in compression and carbon fiber provides strength in tension. A cool aspect is that you need no temporary formwork as you do with most concrete structures, since the carbon fiber sleeve IS the form.

Figure 5: Carbon shell system willed with concrete proposed for the diagonal columns of the main tower (Source: Van Den Einde et. al. 2003)

The cables from the bridge tower attach to two longitudinal beams, called girders (fig. 6). These cylindrical girders also use the carbon shell system. The girders are connected to the deck with steel rebar shear connectors.

Figure 6: Proposed lightweight deck system for the bridge. (Source: Van Den Einde et. al. 2003)

The deck is designed with hollow fiberglass channels (fig. 7) that are filled with concrete on site. Such a composite deck weighs only 1/4 of a typical reinforced concrete slab deck (Van Den Einde 2003). This is important in earthquake country – a lighter deck means less load on the cables and tower when an earthquake jerks the deck back and forth.

Figure 7: Prototype of the proposed lightweight fiberglass decking system built for testing

Unfortunately, the carbon fiber/fiberglass bridge was never built. Instead, a concrete arch bridge was completed in 2019 (fig. 8). While the current bridge is pretty, the carbon fiber/fiberglass bridge will always have this Seismic Outreach Correspondent’s heart. But the research on the potential bridge, conducted in the early 2000s at UCSD, helped spur other construction and research. For instance, the Neal Bridge in Maine uses carbon fiber arched tubes (pic 9). The Michigan Department of Transportation is researching replacing steel rebar and steel cables with carbon fiber (pic 10), which doesn’t corrode. Look for more carbon fiber bridges in the near future!

Figure 8: The Gilman Drive Arched Bridge built as a more cost-effective to the carbon fiber cable-stayed bridge.
Figure 9: Arched bridge constructed using a carbon fiber tubular system with a concrete core (source: Fountain 2009)
Figure 10: Carbon fiber rebar zip-tied in place before a concrete pour (source: McLoud 2020)


Fountain, Henry. “Building a Bridge of (and to) the Future.” The New York Times, The New York Times, 12 Oct. 2009,

McLoud, Don. “Michigan Expands Use of Carbon Fiber as Alternative Bridge Material.” Equipment World, 21 Sept. 2020,

Van Den Einde et. al. “Use of FRP Composites in Civil Structural Applications.” Construction and Building Materials. 2003.

Sweetwater Bridge: a Beautiful and Historic Steel Bridge in Eastern San Diego

Figure 1: The Sweetwater Steel Truss Bridge in Eastern SD County

Nestled in the Sweetwater River Valley along Highway 94 is the 1929 Sweetwater River Bridge. For #seismicsaturday , we feature this wonderful example of an old steel truss bridge.

The design of the bridge is a Pratt Truss (fig. 2). This means that the diagonal members slope downward toward the center of the bridge, and thus are all in tension. Look closely at the diagonal member and you can see that it is designed to be in tension (fig. 3). The member is so slim that in any compression, it would buckle.

Figure 2: Common Types of bridge trusses
Figure 3: Diagonal Member in Tension

The top chord on the bridge will carry a strong compressive force. Its design is fascinating: you can see two C-shaped members, that are connected by diagonal strips of steel riveted to one another. This design helps get the cross sectional area away from the midpoint of the member, increasing its resistance to buckling. Imagine the difference between smashing a thin 250 mL coke can, and and a wide-diameter 12 oz one (fig. 4). The wider one will be take much more force before it buckles. This same principle – of getting material further away from the center point – is what gives the top chord is resistance to buckling.

Figure 4: Comparison of two coke cans to show that the slimmer member will buckle under less load

Both sides of the bridge rest on massive pins (pic 5). When a heavy load, say 3 trucks, crosses over the bridge, the pin allows the bridge to flex downward and take the load. Without the pin, a rigid connection could crack and rupture when it is rotated, or even put stress on the concrete foundations.

See all the bumps on the connections (fig. 6)? Those are rivets! Riveted connections are done with a cylinder with a smooth head, which is inserted into a punched hole. The cylindrical side is then smashed down to create a pin connection (see fig. 7). On modern steel bridges, rivets have been replaced by bolts, which are easier to install, don’t require a furnace onsite, and do better in earthquakes because of their ductility.

Figure 7: Riveting process. Source: Industrial Studio YouTube Channel

The historic Sweetwater Bridge was closed to cars after it was replaced by a modern post-tension reinforced concrete bridge (fig. 8). It is now a wonderful place to nerd out on the bridge construction techniques of the past, and enjoy some wild grapes!

Figure 8: Modern post-tensioned reinforced concrete bridge that has replaced the Sweetwater Bridge
Figure 9: Your Seismic Saturday correspondent enjoying wild grapes that abound on the bridge

Floating Airport in San Diego?

Figure 1: Artist Rendering of floating airport off the coast of Point Loma

Where to build a new airport? This question has dogged San Diego for years. San Diego International Airport (AKA Lindbergh Field) has only 1 runway and is overloaded. One proposed solution is a floating airport. We feature this idea for #seismicsaturday.

✈️In the early 2000s, several SD companies began advocating for a floating airport to be built off the coast of Point Loma (fig. 2). The airport would be built on a pneumatically stabilized platform, created with concrete cylinders with trapped air inside that are connected together (fig. 3a-b). Source: Float Inc.

Figure 2: Proposed location of floating airport and flight & water corridors

✈️Proponents for the floating airport point to a successful test of a prototype in Tokyo Bay (fig. 4). This 1000 meter scale model was built in 1999 and had successful airplane takeoffs and landings. Proponents also say that a airport in the ocean wouldn’t create so much noise over residential areas. Currently, airplanes landing in SD rattle the Banker’s Hill and Little Italy neighborhoods during final descent.

Figure 4: 1000-meter floating airport built in Tokyo Bay

✈️Opponents of the floating airport idea point to practical problems, like transportation too and from the airport, motion and stability of the airport during tides and storms, corrosion from salt water and marine air, poor visibility in fog, and ridiculously high construction costs. Other opponenents raise ethical concerns: “”if we build floating airports, can floating strip malls be far behind?” (Aviation Pros 2007).

It is not likely that the floating airport will be built any time soon in SD. But with air travel at record highs, and the SD Int’l Airport full to overflowing, the floating airport idea may soon resurface.

How do you use bedrock and octagons to build a strong bridge?

Figure 1: The historic Black Canyon Arch Bridge in eastern San Diego County

This #seismicsaturday, we feature the Black Canyon Arch Bridge, built in 1913 Northeast of Ramona in Eastern SD County.

An arch relies on its foundations to push both upward and inward. The shallower the arch, the more sideways force is needed (fig. 2). For the Black Canyon Bridge, the arch is built directly into the bedrock on the side of the canyon (fig. 3). As the bridge pushes outward and down on the rock, the rock provides an equal and opposite reaction. It’s likely that this site was selected because of the presence of exposed bedrock.

Figure 2: Shallower arches need more horizontal supporting forces. Source: “Why did medieval architects…”
Figure 3: Bedrock supports bridge arch with equal and opposite reaction

The concrete in the bridge looks continuous at first glance. But look again! One can spot many joints in the concrete structure (fig. 4, 5). In fact, the two segments of the main arch are not continuous – they lean against one another and are joined in the center (fig. 6). The numerous visible joints show us that this bridge was probably built using pre-cast pieces. Pre-casting is when concrete elements are manufactured off site, trucked to the site, and then fit together like pieces of a puzzle (fig. 7).

Figure 6: The two sides of the arch lean on one another at the center
Figure 7: Precast Beam and Column assembly for a modern parking garage.

How would this 109 year old bridge do in an earthquake? The interior of the bridge is designed to resist some sideways load with the shape of an irregular octagon (fig. 8). The octagonal shape is more stable than a rectangular, where the joints could fail.

Figure 8: Irregular octagon increases lateral strength from a rectangle

The quality of the 109 year-old concrete is questionable. Several exposed parts look to be quite porous (fig. 9) and possibly degraded. Someone seems to have been hard at work filling in these areas of degraded concrete, as many patches are visible (fig 10). Modern concrete construction techniques, where vibrators are inserted into concrete as it cures to densify it, help mitigate this issue.

The Black Canyon Bridge shows how builders can harness characteristics of the natural landscape and geometric shapes. By anchoring the bridge into the bedrock and strengthening the interior with an octagon, the builders created an elegant arch over the San Ysabel Creek that stands strong 109 years later.


“Why did medieval architects use a pointed arch instead of a round one?”. Quora Post. Accessed February 2022.

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