California Bridges - Route 105/110 Separation (2)

We've spent the last week looking at unusual supports for freeway interchanges in California. We've studied the use of outriggers (on the 880/980/24 Interchange in Oakland), bridges on bridges (on the 5/110 Interchange in Los Angeles), and holes in bridge decks (on the 405/90 Interchange in Los Angeles). However, if we get really desperate, we can always use a C-bent like the one I designed for the 105/110 Interchange (shown in the photo).

There's no room for a column under the deck and no room for a second column on the right side of the bridge. So at this location the column is offset several feet from the center of the superstructure. These C-bents have to be designed for the large moments and shears due to the eccentric loads they carry. This is difficult for dead and live load, but it can be extremely problematic for earthquake loads.

Architects typically design a special look for freeways and interchanges. On this interchange the columns have a special shape and the outside of the barrier rails have a texture that we called fish scales (although the architect probably wasn't thinking about fishes).

California Bridges - Route 105/110 Separation (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

California Bridges - Northwest Connector

I've mentioned bridges on bridges but I also wanted to show bridges through bridges. The Northwest Connector is a continuous 13 span RC box girder (except for two precast box girder spans over I-405) on single column bents (and end diaphragm abutments). This bridge is just a little east of Venice, California.

This long connector spans over I-405, State Route 90, Slauson Avenue, and Jefferson Boulevard (see figure above). Because of all the bridges underneath, one of the columns  on the Northwest Connector pierces the deck of a lower structure. Although this idea may seem clever, it looks dangerous to drivers, especially during earthquakes. Unless the opening is large, the two bridges (with different periods) will move in different directions during earthquakes, possibly damaging the column and possibly both bridges.

California Bridges - Northwest Connector OC by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

California Bridges - Elysian Viaduct

The Elysian Park Viaduct is a long steel girder and RC box girder (4 cell) bridge on multicolumn and single column bents and pile foundations. Its part of a network of connectors between I-5 and I-110 in Los Angeles. I've written about how hard it is to find a place to put your columns in urban interchanges. At the Elysian Park Viaduct the engineers had to design a bridge to support one of the single column bents over the Arroyo Seco Flood Control Channel.

I like the somewhat paradoxical concept of a bridge supporting a bridge. When the science writer Douglas Hofsteader has a group of people at his disposal, he likes to have them sit on each other's laps in a ring. There's something odd about the fact that the entire group is self-supporting. Can a bridge superstructure support some of its columns? A special prize for the best self-supporting bridge drawing sent to this blog!

I have a few more bridge on bridge photos to share. I also have several photos of a bridge column going through a hole in one bridge deck to support a higher level bridge. I always wondered what would happen when the two bridges start moving out-of-phase during an earthquake? Can you think of any other odd alignments between bridges in an interchange?

You can spot the Elysian Viaduct on the southwest side of the interchange shown in the Google Earth Map below (Bridge #53-1424). Like many of the bridges in Los Angeles at the time of the Northridge earthquake, the columns on Elysian Viaduct had been encased in steel shells and the bridge had almost no damage.

In Greek mythology, the Elysian Fields are the final resting place for the souls of heroes. Of course, there are Elysian Parks all over the world including one just west of this interchange (and north of Dodger Stadium).

California Bridges - Elysian Viaduct by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

California Bridges - Mulholland Drive Overcrossing

When I first saw this bridge high above the  San Diego Freeway, I thought it was really great. The tall columns and haunched girders make for a dramatic entrance into Los Angeles. The Santa Monica Mountains are covered in a sunburnt chaparral with no sign of human habitation.

Mulholland Drive Overcrossing has a three span, reinforced concrete box girder superstructure on three column bents. It was built in 1959 and it has been designated as a historic structure.

I was less pleased by its appearance after we encased the concrete columns with steel shells in the aftermath of the 1994 Northridge earthquake. However, I was really displeased upon attending a meeting last month (on the widening of I-405 to ten lanes) to discover they are now going to remove this historic landmark! The people of Los Angeles must take an unsentimental attitude towards their bridges.

California Bridges - Mulholland Drive Overcrossing by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

California Bridges - Route 105/110 Interchange

I don't know if these bridges are particularly attractive, but I like this photo as a composition. This is part of a complicated interchange where the Harbor and Century Expressways (along with light rail bridges, city streets, and HOV ramps) come together.

I sometimes feel the Tall Bridge Guy and the Happy Pontist live in a different world where aesthetics still plays a role while my biggest problem is trying to find an unoccupied place in crowded interchanges to put my columns.

These bridges were designed before the Loma Prieta earthquake taught us about problems with outrigger bents. I think I put as much longitudinal steel as possible in the columns, not realizing I was actually putting larger demands on the knee joints and  bent caps if there was a large earthquake.

We'll take a look at a few more of these crowded California expressways during the next few days.

California Bridges - Route 105110 Interchange by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.