Susquehanna River Bridge: A Wonderfully Redundant and Riveted Historic Railroad Bridge

Figure 1: The 1904 Susquehanna River Bridge

This #seismicsaturday is riveting! We feature a 1904 railroad bridge across the Susquehanna River near Cooperstown, NY, which your Seismic Saturday correspondent spotted while canoeing.

In late 19th century and the early 20th century, when Cooperstown and surrounding areas were manufacturing hubs, the bridge was part of a bustling train route that brought raw goods up to the factories of Cooperstown, and manufactured goods back down to New York City.

The bridge looks like a modified warren truss (fig. 2). It looks to have several Warren Trusses overlain on top of one another. In contrast to the warren truss in figure 2, the Susquehanna Bridge’s design is redundant, because if one of the diagonal elements fails, the load can travel through the members of other adjacent diagonals to the base at either side. The corner joint of the bridge, shown in figure 3, is crucial in that it connects on the left with two diagonal elements and one vertical element, each of which travels downward to support a diagonal coming in from the left side.

Figure 2: Geometries of different types of truss bridges. The Susquehanna River Bridge is a modified warren truss, with two warren Trusses overlain on top of one another. / Source: Teach Engineering
Figure 3: Corner joint of the bridge (on the top right) connect with two diagonal elements and one vertical element, which in turn each support a diagonal element coming in from the left. It is interesting to note the difference in the slimmer tensile members, sloping from top right to lower left, with the compressive members, sloping from top left to bottom right,

The foundation of the bridge is made out of flat stones (fig. 4). This may be a foundation from a pre-1904 bridge, when flat river stones were commonly used as foundations (for example underneath the original white house). Stones were popular because they could be gathered from nearby. In this case, the stones were probably gathered from the adjacent riverbed. In modern bridges, river stones have been replaced with steel reinforced concrete.

Figure 4: Flat stones make up the abutment foundation

See all those bumps (fig. 5, 6, 7)? Those are rivets. Riveted connections are fascinating. They 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 process figure 8). This type of riveting was used up till the mid 1900s, and was used to build the Golden Gate and Brooklyn bridges.

Figure 8: Riveting process of joints. / Source: Industrial Studio YouTube Channel

In civil structures like bridges, rivets have now been largely replaced by bolts, which are easier to install, don’t require an on-site furnace, and do better in earthquakes due to their ductility. 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. However, riveting is still commonly used in aircraft (pic 9), and even is used on SpaceX’s 2021 Starship Rocket (pic 10). It is riveting to see technology of the past being used in structures of the future!

Note: The UC San Diego Steel Bridge Team did a 2-day Bridge Design-athon Challenge based on designing a replacement for this bridge. It includes all sorts of fun constraints, including the presence of an endangered fish species in an inlet stream. Here is a link: Bridge Designathon: Susquehanna River Bridge


Ramirez, Miguel. Doing the Math: Analysis of Forces in a Truss Bridge. Teach Engineering.

Smolen-Gulf Bridge: 613 ft-long and Made of Wood!

Figure 1: The Smolen-Gulf Bridge in Ashtabula County, Ohio / Photo Credit: Visit Ashtabula County

What did bridges look like before steel and reinforced concrete? This #seismicsatueday we feature the Smolen-Gulf Bridge in Ashtabula, Ohio. Measuring 613 ft, the bridge is the longest wooden truss covered bridge in the US.

Popular in the 1800s, a “covered bridge” is a wooden truss bridge with a truss roof. The roof protects the bridge deck from snow, rain, and sun, extending the life of the structure. The Smolen-Gulf bridge was built in 2008 and due to its roof covering, shown in fig. 2, engineers estimate it to have a design life of over 100 years.

Figure 2: Roof covering of the bridge from inside. The roof is supported by a interesting wooden truss structure with bolted steel gusset plates at the connection. The trusses are stiffened in the lateral (out of plane) direction by tie rods in the shape of X’s

The bridge was built in homage to Ashtabula County’s many old covered bridges. The county is known for its covered bridges, and hosts the anual Covered Bridge Festival. It is a dream of this bridge blogger to attend some day!

The Smolen-Gulf bridge is designed as a Pratt Truss (see fig. 3). The top and bottom chords are made by sistering together planks of wood. For examble the bottom, shown in figure 4, uses four 2X planks of southern yellow pine connected together with a steel plate. The diagonals of the bridge are made with three steel rods that stretch diagonally across the frame (fig. 4). These rods are in tension, and slot in between the pieces of wood in the bottom chord, as shown in figure 4. In the 1800s, wood planks would have been used in place of these rods.

Figure 3: Geometries of different types of truss bridges. The Ashtabula bridge is a Pratt truss, with steel rods on the diagonals

The bridge is built in 4 sections, with small gaps in between. These gaps of give the sections space to expand and contract in the extreme temperature variations of Ohio (fig. 6), which can reach 100 degrees F in the Summer and plunge below 0 degrees F in the winter.

Figure 6: Expansion gap between two sections of the bridge truss

The wooden deck is directly paved with asphalt. After 15 years of wear, some asphalt appears to be worn down (fig. 6). It looks to be overdue for a repaving. The pavement may have been initially applied in a thin layer to reduce the weight added onto the structure, meaning that it wore down faster.

Figure 7: Wear and tear of the thin asphalt layer has exposed the underlying wood structure

The Smolen-Gulf bridge structure hybridizes 20th-century bridge technology, like steel rods and modern reinforced concrete abutments, with 19th-century covered wooden bridge building techniques. The bridge is a wonderful break from conventional modern highway bridges, and is a fantastic homage to the many historic covered bridges of Ashtabula County.


“Covered Bridge Festival.” Ashtabula County.

“Smolen Gulf Covered Bridge.” Ashtabula County.

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 until 1987. Other notable San Diego structures built with old growth redwood include the Hotel del Coronado, old Alpine City Hall, the Del Mar Library, and the incredible Goat Canyon Trestle Bridge, as well as many of the city’s old victorians-style homes.

In 1987, the Quince Street 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.

UC Santa Cruz Pedestrian Bridge: A Bridge Strong in 3 Directions

Figure 1: UC Santa Cruz Pedestrian Bridge

How do you build a 200 ft long wooden bridge over a ravine just 10 miles from the San Andreas fault? This #seismicsaturday we feature a beautiful glu-laminated wooden pedestrian bridge at UC Santa Cruz.

The bridge deck is supported by three thick glu-laminated beams (fig. 2). These “glu-lam” beams are made by gluing many strips of wood together, and can be used to create continuous beams that are to large to be cut from one tree.

The bridge is supported by two large columns which are braced diagonally to provide lateral strength in side to side shaking from an earthquake. One mystery strucuture is circled in pink. Anyone know what this is?

UC Santa Cruz is less than 10 miles from major fault lines (fig. 3, USGS).

A structure that is this close to a fault experiences not just sideways shaking, but also violent up-and-down acceleration. Research conducted following the 1994 Northridge Earthquake showed that vertical acceleration is typically around 2/3 of horizontal acceleration for structures near an earthquake epicenter (Borzogonia etc. al. 1996). We can think of the effect of this vertical acceleration with two scenarios. First, when a vertical acceleration of “1 g” works with gravity, the weight of the structure can double. Alternatively, in tbe second scenario, when the “1 g”works against gravity, a structure can, for that instant, essentially become weightless. The momentary weightlessness reduces the friction between a bridge (or a house) and its foundation and can, when combined with side to side shaking, throw beams off columns and columns off foundations. The UC Santa Cruz Bridge is designed ingeniously to resist these upward forces, as well as normal downward, and lateral (sideways) loads. Check out how the different components in tbe bridge work together to resist these three distinct loads in figure 4.

Figure 4: Load paths in tbe downward, upward and sideways loading scenarios

Diagonal pieces of steel are hidden underneath the deck (fig. 5). Called “bridging,” these diagonals tie the deck beams (or stringers) to the beams, preventing them from toppling over sideways in an earthquake.

Figure 5: Lateral bracing, called “bridging” is skillfully hidden under the bridge to prevent the decks beams from toppling over in a quake.

Sometimes, earthquake-resting mechanisms (like huge “in your face” steel diagonal bracing) can take away from a structure’s aesthetic. However, the primarily wooden and minimally steel components blend in with the UC Santa Cruz redwood forest, making the bridge both aesthetically pleasing and earthquake safe.


US Geological Survey. “Earthquake Probabilities in the San Francisco Bay Region: 2000 to 2030. A Summary of Findings By the Working Group on California Earthquake Probabilities.” USGS Open-File Report 99-517. 1999.

Bozorgnia, Y. Mansour, N. Campbell, K. “Relation Between Vertical and Horizontal Response Spectra for the Northridge Earthquake.” Eleventh World Conference on Earthquake Engineering. 1996.

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

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.

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