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.

Blizzards and Gondolas: The Squaw Valley Tram Car Failure

High wind has caused deadly consequences for gondolas. This #seismicsaturday we tell the story of the Squaw Valley Tram Car failure.

Figure 1: View of the squaw valley tram car after a cable sliced through the roof / Source: Moonshine Ink

On April 15, 1978, a blizzard with gale force winds caused the tram car (pictured above), which was heading down the mountain, to unlatch from the first of it’s support “track” cables. Track cables weight of the cable car (which rolls on wheels along them) while a propulsion cable pulls the car along. After detaching from the track cable, the gondola plummeted 75 feet, and then shot back up when the other track cable tightened. As the car bounced back up, the first track cable swung down, slicing through the roof of the cabin and pinning 12 people to the floor. Four of those people tragically died. The rescue operation of the other 40 people on the cable car would continue for 10 hours into the night amidst a blizzard of 60 mph winds (See Frohlich 2008). Figure 1 and 2 show the effects of the cable slicing through the car.

Why did the first cable derail? There is no clear answer – one passenger reported “swinging” and “twisting” of the car in the wind just before the accident, but the clamp should have been been able to handle this movement. Dr. Karl Bittner, a leading expert in cable cars, was called in to investigate the cause of one cable unlatching, but couldn’t figure out the specific cause. Following a State Investigation, the district attorney and Cal-OSHA officially declared the accident ‘an act of god’ meaning that it was due to unknown causes and could not have been prevented with “reasonable” foresight (Renda 2015).

It is likely that something went wrong in the clamping mechanism, causing one of the two cables to detach. The Squaw Valley Gondola was a multi-cable detachable grip system (see fig. 4). In this system, one cable is for propulsion, while the other cables are called “track” cables and are rolled on with wheels to provide stability and take vertical weight. The Gondola can detach from the cable mechanism so that, upon boarding of the gondola in the station, the car can moves slowly. These detachable systems are essential in maintaining a high capacity of modern gondola systems, as the propulsion cable doesn’t have to stop or slow down every time a gondola car enters the station to pick off or drop off passengers. The detachable clamp is designed to only detach in the station when subjected to a special un-clamping mechanism. A benefit of multi-cable systems is they have better stability than their single cable counterparts (where the single cable is the propulsion cable) due to their added stabilizing track cables, and therefore can reach higher speeds (See “Bicable Detachable Gondolas”).

Although the specific cause of the track cable derail was never determined, engineers tried to learn from what they think may have been the cause of failure to build the next cable car better. Dr. Bittner, in his inconclusive investigation, recommended that the new Squaw Valley cable cars system be built with improvements including deeper grooves on the cable hanger, a longer cable guard, and cable clamps on the support towers. These improvements were implemented when the new Squaw cable car system went into effect the next winter season in December of 1978 (Frohlich 2008). Squaw Valley also now conducts daily inspections of the brakes, the counterweights, and other machinery. A new Squaw Valley cable car is shown in fig. 4, with the glistening lake Tahoe in the background. Modern cable car hangers, like the Leitner 3S Carriage, feature innovative lateral damping system to improve wind resistance, a modern system of shock absorption, and an automatic shutdown which immediately notifies the operator when defects in the rolling mechanism are detected.

Figure 4: The new cable car at Squaw Valley, designed by based partly on knowledge gathered in the investigation following the 1978 failure

Note: in my research for this post, I was unable to find any technical report detailing the old clamping mechanism of the Squaw Valley Gondola, or any other technical report on the failure. If you find one, can you please share it below? I wonder if anyone has taken another look at the mystery as to what caused the initial cable derailment since Dr. Bitner’s inconclusive report in 1978 and OSHA’s “Act of God” determination.


Renda, Matthew. “Tram Car Trauma at Squaw Valley.” Tahoe Quarterly. Published winter 2014-2015.

Frohlich, Robert. “30 Years Later: The Squaw Valley Cable Car Tragedy.” Moonshine Ink. April 10 2008.

“Bicable Detachable Gondola.” The Gondola Project.

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.

Cruz del Sur: a Seismically-Resistant Coral-esque Structure

Figure 1: The Cruz del Sur Building / Photo Credit: Cristobal Palma para ArchDaily

How do you build an earthquake-resistant structure that looks like coral? This Seismic Saturday, we travel to Santiago, Chile, to feature the Cruz del Sur Building.

The first thing that the eye catches is the structure’s surprising form, starting thin and expanding outward up the building (figure 2). A thick central shaft, which takes both the base shear as well as the overturning moment, runs from the ground foundation all the way to the roof. Diagonal columns jut out from the central shaft to support the edges of the concrete slab, as well as the vertical columns which travel up the building.

Figure 2: Section cut of Cruz del Sur. Source: Lehman Izquierdo Arquitectos

This unconventional design causes an interesting load path under gravity load. The diagonal columns are in compression(shown in red), and the first floor slab is under tension (shown in blue) (figure 3). Concrete is very weak in tension; thus massive cables are routed through the first floor slab to carry the tension. The ends of the cables are capped with steel cylinders to allow for later access (figure 4). A structural detail of the slab is shown in figure 5, in which the the steel post-tension cable system can be seen with three slots for cables, as well as an angled cap to press inward (and slightly upward) against the concrete. Axes (or “eje” en Español) for the vertical column, the diagonal column, and the slab meet at a central point.

Figure 3: Transfer of gravity load into the ground
Figure 4: Cables ends at the end of the slabs
Figure 5: Structural detail of slab edge. Axes (“Eje” in Spanish) for the columns and slabs are shown as meeting at a center point. / Credit: Izquierdo Lehman Arquitectos

For any structure in Santiago, Chile, the million dollar question is, of course, what happens in an earthquake? Most of Chile borders the the Nazca Plate, which is being thrust underneath the south-American plate at a rate of ~80 cm/year, creating a subduction zone. Huge earthquakes happen every decade up and down the coast of Chile. For instance, in the last 12 years, Chile has been jolted by the 2015 Coquimbo Earthquake (8.3 Mw), the 2014 Iquique Earthquake (8.2 Mw), and the 2010 Maule Earthquake (known also as the Concepción Earthquake) (8.8 Mw). Santiago regularly experiences earthquakes which cause peak ground accelerations in excess of .6 g (Hussain et. al. 2020). Imagine falling sideways 60% of the speed you fall downwards; that is how fast the ground moves. Since acceleration and motion is amplified for higher floors, the top floor of a structure, under these ground accelerations, can easily be jerked back and forth with an acceleration in excess of 1 g.

For structures with thin bases, like the Cruz del Sur Building, withstanding these extreme earthquake forces is challenging. To understand why, think of a person on a metro. When the train accelerates away from a station (and no handhold is in reach), people naturally stand with their legs wide apart, in line with the train’s acceleration. Anyone with their feet together would fall over. The same problem exists for the Cruz Del Sur structure with its slim base – the base has to resist the urge to overturn with “its feet close together” and thus a small lever arm. This overturning scenario, as well as tensile and compressive stresses induced by the earthquake forces, are shown in figure 6. The solution found by the structural engineers to prevent the overturning is to extend the shaft deep into the ground and design a mat slab beneath. When the building tries to turn, the soil pushes back on the mat slab (green arrows) and on the shaft (pink arrows) to prevent it from doing turning. To deal with the stresses induced by the overturning, which can cause tensile failure or compressive failure as shown in fig. 6, the walls on either side the central shaft incredibly thick (~2 m deep). These walls are loaded with both vertical rebar to take tension, and confining rebar to prevent spalling compressive failure. Essentially, if the building were a person on the metro, it would have its feet close together, but its feet would be latched to the ground and its legs would be bigger than Dwayne “The Rock” Johnson’s legs.

Figure 6 :Potential stability (overturning) failure of the Cruz Del Sur Building and how such a failure is prevented.

One of my favorite aspects of the building is that it reminds me of a coral reef (figure 7). The base starts out thin, and then the building expands as it goes upward. It is interesting to think that coral reefs face a similar constraint to buildings in a city environment – there is a high demand for ground-level space, but upon building upward, there is more space available to expand. For coral, only a small anchor point is needed to flourish upward and then outward (and to be able to host the largest number of photosynthetic algae). For the Cruz del Sur building, a smaller anchor point allows for a public Plaza underneath, while expansion as the building travels upwards allows for more rentable office space at higher (and more desirable) elevations.

A fascinating effect, whereby light reflects off of the surface of a small pool onto the column of the structure, reminded me of the moving light patterns on the ocean floor caused by reflection through the surface (figure 8).

Figure 8: Light reflects off of the surface of the water to create an interactive effect.

Also notable in the structure is the use of circular windows, both in the floor slab above the lobby, and in the 2 meter-thick shaft wall. For me, the circles add to the oceanic aesthetic, as they resemble the windows in a ship’s hull. It is interesting to add that a circular window does a better job at preventing stress concentrations that occur in the corners of square windows. (This is why airplanes have oval windows: /

Also of note in the structure is the strategic hiding of circular vents behind the diagonal column groups, to keep the the visible surface of the building cleaner. Additionally, the ~60 cm overhang of the slab above each window which shades the window while the sun is at its highest angle during the Summer. The structure maintains the modern window-facade look from afar, while being more energy efficient than glass-walled office buildings.

The Cruz del Sur’s unconventional shape, seismically resistant technology, and bio-inspired form, make it fascinating from the perspective of both engineers and architects. As I walk down Apoquindo (the Wall Street banking hub of Chile nicknamed San-hattan), I’m drawn towards the structure by its overhanging inverted form, and its audacity to be supported by a thin shaft in one of the Earthquake capitals of the world.


Hussain, Ekbal et. al. “Contrasting seismic risk for Santiago, Chile, from near-field and distant earthquake sources.” Natural Hazards and Earth System Sciences 20 (2020): 1533–1555. Published 29 May 2020.

Izquierdo, Luis. Lehmann, Antonia. Edificio Cruz del Sur, Santiago. Revista de la Escuela de Arquitectura de la Pontificia Universidad Católica. ARQ, n. 73 Valparaíso, Santiago. Publicado diciembre 2009, p. 12-19.

“Edificio Cruz del Sur / Izquierdo Lehmann” 20 jul 2012. ArchDaily en Español. Accedido el 16 Ago 2022. Fotos por Cristobal Palma.

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

Create a website or blog at

Up ↑