Sunday, July 31, 2016

Monterey County, California Bridges: Cholame Creek Bridges

July 2016 (35.8863,-120.4357) Cholame Creek Bridge
There are several other bridges across Cholame Creek and it's tributaries near last week's Parkfield Bridge. The Little Cholame Creek Bridge (44C0036) is a single span truss with rods for tension members (and with a 35 degree skew). It was built in 1910 but still looks brand new (see photo above).
Further south is the Cholame Creek Bridge (44C0146), an eight span steel stringer bridge very similar to last week's Parkfield Bridge. It has the same substructure consisting of three steel 'H' piles with cross bracing and it was also built in 1960. It's surprising that such a flimsy bridge survives the moderate earthquakes that occur every 21 years.
A little further to the south (in San Luis Obispo County) is another Cholame Creek Bridge (49 0036).  It is a five-span, 130 ft long, slab bridge on pile extensions. This state owned and instrumented bridge was only 500 ft from the San Andreas Fault, it recorded accelerations greater than 1.0g during the 2004 Parkfield earthquake, and it sustained some minor damage. A report on the seismic performance of the bridge is available online.
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Sunday, July 24, 2016

Monterey County, California Bridges: Parkfield-Coalinga Road Bridge across Little Cholame Creek (and the San Andreas Fault)

We've traveled to the east side of Monterey County after spending two months along the coast. This week's bridge is in the town of Parkfield, which is famous for experiencing a moderate earthquake on the average of every 21 years (1857, 1881, 1901, 1922, 1934, 1966, 2004).

 The Parkfield-Coalinga Road Bridge (44C0141) crosses over Little Cholame Creek, which is also the surface trace of the San Andreas Fault (the source of these recurring earthquakes).  The fault is right lateral, which means when you face it, the plate across from you is moving to the right (see figure below). Therefore, a bridge crossing a right lateral fault would eventually have a flattened 'S' shape.
The photo below was taken by the US Geological Survey on September 29, 2004, one day after the 2004 earthquake. They reported that the bridge has about 5 ft of offset (due to the 2004 earthquake, the 1966 earthquake, and due to aseismic creep).  However, the photo shows the bridge rail with a reverse 'S' shape. However, the abutments may have allowed the ends of the bridge to slide instead of restraining them.
In the photo below, we can see the girders are sitting on a seat-type abutment with shear keys a few feet to each side. Professor Goel reported that after the 2004 earthquake, "Angle iron apparently installed in an effort to restrain transverse motion of the superstructure at the bearings was knocked free."
I took photos of the abutments and piers to try to understand what happens to a bridge over a fault. 
The first bridge at this site was built in 1936 (according to the University's Space Research Association) but it was replaced in 1960 with the current steel stringer bridge. However the 1960 bridge was recently retrofit with new concrete bent caps cast under the existing superstructure and with new CIDH piles cast on each side of the 'H' pile supports. The retrofitted structure obviously hadn't been around for the 1964 temblor, but the bridge may still have the displaced shape from the earlier earthquake.
During an earthquake, the bents on each side of the fault move in parallel but opposite directions. The superstructure is restrained to follow this offset by the bearings, the angle irons, and eventually by the concrete shear keys and to take on the noticeable 'S' or 'Z' shaped configuration.
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Sunday, July 17, 2016

Monterey County, California Bridges: Pitkins Curve Bridge (and Rainrocks Rock Shed)

July 2016 (36.0107, -121.5246) Pitkins Curve Bridge (and Rainrocks Rock Shed)
A few hundred yards north of last week's Limekiln Creek Bridge is the Pitkins Curve Bridge and adjacent Rainrocks Rock Shed. As I've previously discussed, addressing all the slides in Big Sur is slowly turning the Pacific Coast Highway into one long viaduct. We saw how the Rocky Creek Viaduct was built to protect the highway from erosion of the surrounding soil. Similarly, Pitkins Curve Bridge and Rainrocks Rock Fall were built to prevent the rockslides from causing accidents, closing the highway, and from requiring continual maintenance of the road.
The photo above shows Pitkins Curve and Rain Rocks before the new construction. A cost-benefit analysis showed that Caltrans could save $115 million by building a bridge away from the slides and a rock shed that pushed the slides away from the road (see photo below).
Pitkins Curve is a 620 ft long cast-in-place, post-tensioned, variable depth, box girder bridge. The 155 ft long side spans were cast with falsework and the 311 ft long main span was segmentally constructed. The two piers are supported on pile caps with four CIDH piles socketed into rock (along with tie downs). Construction was completed in 2014 and I believe that Caltrans engineer Mike Van de Pol was the designer (however, engineering is a collaborative effort and so I might be mistaken). More information on the design and construction of this bridge is available online.
I drove out on Friday (on July 15th) to photograph the bridge (top photo) but the fog coming in from the Pacific was so heavy I could barely see it from 20 yards away. Also, this was the only location (standing on the slide) where I could see the bridge. The Caltrans photographer who took the photo above (during construction) must have had a helicopter at her disposal. I took a photo of the rock shed (below) but the simulated rock makes it look like a prop for a Hollywood movie.
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Sunday, July 10, 2016

Monterey County, California Bridges: Limekiln Creek Bridge

December 2003 (36.0090-121.5193) Limekiln Creek Bridge
We've headed to the southwest corner of Monterey County to study a few more coastal bridges. The Limekiln Creek Bridge (44 0058) is a six span prestressed precast I girder bridge that was built in 1957. The girders sit on hammerhead bent caps and single column bents. The bents at the ends of the bridge are round while the bents on each side of the creek are rectangular (to provide less resistance to high stream flows).  This bridge was retrofitted similarly to the previously studied Dolan Creek Bridge. The rectangular columns were retrofit with flying buttress-type shear walls while the round columns were retrofit with steel casings.
I took these photos in December of 2003 while looking for bridge damage following the San Luis Obispo County Earthquake (to the south). However, the only significant bridge damage was to the nearby Villa Creek Bridge that was damaged by a falling boulder during the earthquake. We'll continue exploring the theme of rockfalls and safety in the next few blogs.
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Sunday, July 3, 2016

Monterey County, California Bridges: Malpaso Creek Bridge

April 2013 (36.48171, -121.93712) Malpaso Creek Bridge
Arch bridges were once quite popular in California. However, precast concrete allowed engineers to design longer spans and arch bridges became uneconomical in most situations.
The Malpaso Creek Bridge is 210 ft long with a single open spandrel arch span that was built in 1935. Like the other arch bridges along the coast, the Malpaso Creek Bridge was recently repaired due to corroded reinforcement and spalling concrete (see photo above).