Movable Bridges - Isleton Bridge

Since the Rio Vista Bridge is near the mouth of the Sacramento River, I thought we could continue upstream. The only other river we've explored with so many movable bridges is the Harlem River in New York City.

So far we've looked at a swing bridge (The Spokane Street Bridge) and a vertical lift bridge (The Rio Vista Bridge). The next bridge across the Sacramento River is a double leaf bascule bridge. These are probably the most popular movable bridges because they can open quickly, they provide a wide channel, and they look like a bridge.

State Route 160 switches from side to side as it follows the Sacramento River upstream. The Isleton Bridge takes SR160 back to the west side as we move upstream. The bridge is 600 ft long and includes two tied arch spans as well as the movable bridge. In the open position it provides 200 ft of channel clearance.

I believe this is a heel trunnion bridge. That means the two leaves pivot on the supports (the heel trunnions) that are under the trapezoids. The counterweights are allowed to rotate downward and as the weights fall, the trapezoids become pinched, which pulls the two leafs into the vertical position. Note the little spans on each side of bascule bridge with triangular trusses to support the counterweight trunnions.

We'll take another look at this interesting bridge tomorrow.

Movable Bridges - Isleton Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Movable Bridges - Rio Vista Bridge (2)

A view of the Rio Vista Bridge in the closed position. The bridge consists of two long truss approaches and a vertical lift span (for tall ships traveling on the Sacramento River). Despite the fenders surrounding the towers, the bridge is occasionally hit by ships, especially in the winter when the Tule Fog blankets the valley. An Italian freighter once knocked out an approach span on it's way to the Port of Sacramento.

I find it irritating that the San Francisco Chronicle uses Caltrans as a scapegoat whenever a ship collides with one of our bridges. They claim that the resulting oil spill is due to Caltrans not employing enough collision protection  on our bridges. I don't think there is any protection if a large vessel is traveling over 10 knots.

This bridge got a seismic retrofit in the 1990s. I think that most bridges were retrofitted in the closed position since they are only opened a few times a day (the opposite for railroad bridges). This retrofit employed lead rubber bearings for the approaches and viscous dampers at the towers (in case the anchor bolts broke). I believe there was also something done to keep the counterweights from swinging into the towers during an earthquake.

Movable Bridges - Rio Vista Bridge (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Movable Bridges - Rio Vista Bridge

The Rio Vista Bridge is actually called the Sacramento River Bridge, but so is every other bridge over the Sacramento River (it's also called the Helene Madere Memorial Bridge). It's a vertical lift bridge built in 1960 that provides 270 ft of channel width and 146 ft of vertical clearance between the towers.

Vertical lift bridges have pulleys at the top of the towers. Cables are connected to the lift span and go up and over the pulleys where they are connected to counterweights hanging from the towers. The machinery  for lifting and lowering the span are mounted at the top of each tower. There is also equipment to keep the span level while it is lifted. The operator's house can be seen at the bottom of the far tower.

Although this bridge looks to be in a rural location, traffic can get backed up for miles on State Route 12 whenever the bridge is opened. We'll take a closer look at this bridge tomorrow.

Movable Bridges - Rio Vista Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.