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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