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.

“El Mirador” Observation Platform in Baja California, Mexico

Figure 1: “El Mirador” lookout platform cantilevers out over a precipitous drop

We travel south of the border this #seismicsaturday to Baja California, Mexico. Featured is an observation platform at “El Mirador” (The Lookout) in San Pedro Mártir Nacional Park, at 9100 ft.

⛰️The platform is made with a cantilever steel truss structure. The main truss extends outward, reducing its depth as it goes (Fig. 2). Secondary truss structures go across, to tie the main trusses together (Fig. 3). Every single steel-to-steel connection is welded (Fig. 4). It is notable that not a single bolt was used on any of the steel-steel connections. Given the extreme temperature variations at 9100 ft, with sub-freezing temperatures in the winter and hot weather in Summer, it is interesting to think about thermal stresses that would develop in this stiff structure.

Figure 2: Outward extension of truss, reducing depth as it goes

⛰️One thing that makes this structure unique is the way it is kept in place on the mountainside. A huge pile of rocks, kept in place by steel tubes and mesh (pic 5), serves as a counterweight, keeping the structure on the hillside. Furthermore, a bunch of oddly (and seemingly arbitrarily) angled steel plates are bolted to rocks on the mountain (pic 6). Your Seismic Outreach correspondent has never seen anything like this before.

⛰️In an area battered by rain in the Summer, freeze-thaw cycles in the Winter, and high humidity, painting the structure is important to prevent corrosion. Except for a little section to the right of the joint in pic 4, this structure has been well painted, and as such, doesn’t seem to show signs of corrosion.

From the cantilevered steel tubing, to the rock boulder counterweight, to the wacky steel plates anchored to the ground, engineers building the lookout came up with a creative solution to create a platform over a precipitous drop. The platform gives hikers a gorgeous (and nail-biting) view.

Can You Make a Bridge Out of Telephone Poles?

Figure 1: The 150 ft telephone pole bridge across the Ausable River

This #seismicsaturday we feature a 150ft long bridge over the Ausable River in the Adirondacks Mountains of New York.

The bridge foundation is built from stones piled on top of one another, with cement in between (pic 2). The foundation is built in a hydrodynamic shape with a pointed front, making it look similar to a boat hull. This shape reduces the force on the foundation from the river, and makes it less likely that the foundation would be washed away downstream during heavy flows.

Figure 2: Hydrodynamic and rustic bridge pier

The beams of the bridge are simply two telephone poles, laid from one foundation pier to the next (pic 3). The beams are strapped down to the foundations using strips of steel and anchor bolts (pic 4). You can learn a lot about this pole from its marking (pic 5). “WT” is the sawmill. “97” means it was milled in 1997. SP SK means it is Southern Pine, treated with Vascol 500 SK, an insecticide. And 65 is the length in feet. Learning to read these telephone pole acronyms is a great way to impress you friends!

Guardrail posts are simply installed. Tops of the posts are covered with fake leather, which keep water from seeping into the top and rotting the wood (fig. 4a). The posts are bolted to 2×4 wood joists, and leaned against the telephone poles (fig. 4b)

The use of telephone poles was a resourceful choice by the builders. These poles are cheap and readily available, are treated to withstand the elements, and help the bridge blend into to the forested landscape.

Richmond San-Rafael Retrofit: “The most complex single retrofit program ever attempted by Caltrans”

Figure 1: The Richmond San Rafael Bridge, know as the ‘Roller Coaster’ Bridge for its undulations / Photo Source: San Francisco Chronicle (link)

“Mr./Ms. Engineer, your mission, should you choose to accept it, is to retrofit the Richmond-San Rafael ‘roller coaster’ Bridge. The bridge, measuring 22,000 ft, has 4 different steel structural systems along its length. Most spectacular are the two 1070 ft spans over active shipping channels – each made by two 535 ft-long cantilever arms that meet in the middle (figure 2). In the current state of the bridge, the foundation piers can shear in an earthquake, causing collapse, the steel diagonal members on the rigid steel tower can fail, causing collapse, and the brittle old riveted connections can fail, causing collapse. The bridge must be able withstand an 8.3 mag quake on the San Andreas fault. You get 700 million dollars. GO”

Figure 2: Dimensioned span of the spectacular cantilever section of the Richmond-San Rafael Bridge

Such was the challenge faced by engineers in 1999, tasked by CalTrans with designing a retrofit of the Richmond-San Rafael Bridge. They came up with interesting, creative solutions to make the bridge earthquake safe:

1️⃣ NEW TOWER FRAMES – Compare the new and the old towers in figure 3. New steel frames were installed, called eccentrically braced frames (fig. 4). The frames are eccentric rather than co-centric because the diagonals do not frame directly into one another, but instead are separated by an offset structural fuse. This offset fuse did not exist on the old towers. As well as the fuse, the frames feature a network of diagonal bracing, shock-absorbing viscous dampers, and rotating pins. The frame can rock back and forth while the offset fuses and dampers absorb energy in earthquakes, saving the more fragile cantilever deck structure above. Testing of the frame components was performed at UC San Diego’s CalTrans Lab on the hydraulic shake table called the Seismic Response Modification Device (link).

Figure 3: Comparison of old, concentrically braced frames, and new, eccentrically braced frame bridge towers
Figure 4: Modern eccentrically-braced bridge tower with fuses to dissipate energy / Source of original photo: Norcal Structural Inc.

2️⃣ CONCRETE JACKET – Compare the old piers with the new piers in figure 5. Engineers designed pre-cast concrete jackets, which surround the older concrete piers, to prevent the piers from shearing in strong earthquakes. Most of the jackets were installed underwater by cranes on barges and teams of divers, one of whom is shown in figure 6. The installation of jackets over the piers is similar to the strengthening of foundations for land-based bridges with jackets in seismic retrofits (see post: Retrofitting San Diego’s “People’s Bridge” to be Seismically Safe – Seismic Saturday)

Figure 5: Concrete jacket is secured with cables around the old piers to increase shear strength
Figure 6: Diver/Construction worker entering the water from a barge to help install underwater jacket / Source: Tutor Perini

3️⃣ BOLTS – 250,000 old rivets were replaced with new, high-strength steel bolts. 500,000 new bolt holes were drilled and new retrofit plates were added. Bolts are easier to install, don’t require an on-site furnace, and do better in earthquakes due to their toughness. A concern of rivets is that because they cool down so fast after taken out of the furnace, they are essentially quenched, and thus very brittle, whereas bolts can be cooled down slowly and subjected to other forms of heat treatment during manufacturing and thus obtained greater toughness (See Heat Treatment of Bolts…). Going across the bridge, one can see the old and new connections (fig. 7). The new bolts have a hexagonal nut, while the old rivets have a cylindrical dome-like head.

Figure 7: Addition of a new retrofit plate complete with hex bolts

Upon completion in 2004, the project was called “the most complex single retrofit program ever attempted by Caltrans” by the Metropolitan Transportation Commission. That the bridge can now withstand an 8.3 magnitude quake on the San Andreas Fault is a testament to the creativity and ingenuity of the engineers.



Richmond-San Rafael Bridge Retrofit Completed. Metropolitan Transportation Commission. September 22 2005.

Richmond – San Rafael Bridge Seismic Retrofit. Tutor Perini.

Richmond-San Rafael Bridge Seismic Retrofit. Norcal Structural, Inc.

Heat Treatment of Bolts and Fasteners. Bayou City Bolt.,of%20the%20ASTM%20A320%20specification.

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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