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.

**Figure 5 (left):** Steel pin supporting the bridge
**Figure 6 (right):** Riveted bridge joint

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 On modern steel bridges, rivets have been replaced by bolts, which are easier to install, don’t require a furnace onsite, and do better in earthquakes because of their ductility.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!