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)*

References

Fountain, Henry. “Building a Bridge of (and to) the Future.” The New York Times, The New York Times, 12 Oct. 2009, https://www.nytimes.com/2009/10/13/science/13bridge.html?_r=1&hpw

McLoud, Don. “Michigan Expands Use of Carbon Fiber as Alternative Bridge Material.” Equipment World, 21 Sept. 2020, https://www.equipmentworld.com/better-roads/article/14972567/michigan-expands-use-of-carbon-fiber-as-alternative-bridge-material

Van Den Einde et. al. “Use of FRP Composites in Civil Structural Applications.” Construction and Building Materials. 2003. https://www.sciencedirect.com/science/article/pii/S0950061803000400

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