Sunday, March 11, 2018

Mendocino County Bridges: Confusion Hill Bridges Carrying Highway 101 across the South Fork of the Eel River

August 2010 (39.92088-123.76359) Confusion Hill Bridges
The Confusion Hill Bridges are two structures that carry Highway 101 across the South Fork of the Eel River in Mendocino County. The bridges were designed and constructed to avoid a massive landslide on the south bank of the river.  The North Bridge (10 0299) was completed in July of 2009 and the South Bridge (10 0300) was completed in September 2009 and the bypass was opened in October of 2009.

The North Bridge is a 581 ft long continuous prestressed box girder bridge on sloping single column bents (see top photo). This shorter bridge was built using the traditional method of supporting the wet concrete and rebar on falsework.

The South Bridge is a long three span continuous prestressed box girder bridge on vertical single column bents. It has a total length of 1239 ft (348 ft, 571 ft, 436 ft). It was built segmentally using cantilever construction (where the weight of the cured concrete supports the cantilevered spans).

Both bridges were designed by Caltrans engineers and built by Finley Engineering Group out of Tallahassee, Florida and by MCM Construction of California.

Sunday, March 4, 2018

Mendocino County Bridges: Philo Greenwood Road Bridge across the Navarro River

August 2017 (39.08580-123.48450) Philo Greenwood Road Bridge
Continuing northwest from last week's Russian River Bridge, we arrived at the Philo Greenwood Road Bridge across the Navarro River. This is an open spandrel arch bridge (with timber approaches) that was built in 1951. The arch span is 130 ft long and the entire structure is 352 ft long.
The twin arches are founded in rock on each river bank and support a thin deck on spandrel columns. This is a local agency bridge (10C0032) supporting a county road.
The Navarro River is 28 miles long and flows northwest through the Coastal Range to the Pacific. Philo is the name of a town and Greenwood is the name of a ridge that are connected by the road.
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Sunday, February 25, 2018

Mendocino County, California Bridges: Highway 101 Bridge across the Russian River

August 2017 (38.95389-123.10167) Russian River Bridge
We've gone back to Mendocino County to look at bridges we missed during our last trip in August. California built a realignment of Highway 101 in 1933 that included this bridge over the Russian River. We can see in the photo above that the bridge has long (steel girder) approaches on both ends. The main span is a steel through truss bridge but with such a high skew that the portal and sway braces are attached to the previous panels. In the plan view below we can see more clearly how the end panels are cut to accommodate the skewed bracing.
The main span is 245 ft long and the total bridge is 1125 ft long over the Russian River flood plain. The Russian River is the second longest waterway in California with a length of 110 miles. It was originally called the Ashokawna by the First Nation people and then Slavyanka by Russian fur traders in 1817. It's a surprisingly wild river, especially during the winter. Beavers were once hunted along the river and even today 8 ft long sturgeons have been caught in its deeper regions.
The river flows south nearly parallel to Highway 101 causing the bridge to have its high skew. Past our bridge it eventually turns west for about 40 miles before emptying into the Pacific near Jenner. Despite the river being somewhat isolated I counted about 20 major bridges crossing the river including it's forks (the East Russian River) and tributaries (Austin Creek). You can look at some of these interesting bridges on my website or on the Bridgehunter website.
The vertical clearance of the through truss is limited by the cross bracing at the top. The minimum vertical clearance = 15' - 4", which makes the bridge structurally deficient (see photo below). We can also see in the photo below that the road is climbing out of the Alexander Valley and over the Mayacamas Mountains to the north (shown in the background).
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