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.