California overpasses

[UptownRoadGeek]

I've noticed recently from pictures and videos that bridges and overpasses in the L.A. area tend to be constructed from poured or formed concrete vs. the steel girder and/or pre-cast concrete construction used in other places. This also the only place I've seen such extensive scaffolding and woodwork used in standard overpass construction. Is there a specific reason for this or is it just a design standard?

[myosh_tino]

I've noticed recently from pictures and videos that bridges and overpasses in the L.A. area tend to be constructed from poured or formed concrete vs. the steel girder and/or pre-cast concrete construction used in other places. This also the only place I've seen such extensive scaffolding and woodwork used in standard overpass construction. Is there a specific reason for this or is it just a design standard?

It's probably a seismic safety issue (i.e. the ability to withstand strong earthquakes) which is a big deal in California.  They don't call this area "Earthquake Country" for nothing.  

There are some overpasses that are built with steel girders (I-5 through the central valley, the I-280/I-880/CA-17 interchange in San Jose, the MacArthur Maze in Oakland and most of the elevated downtown freeways through San Francisco) but all new construction uses the poured concrete method for overpass construction.

FWIW, all of the steel girder type overpasses in the San Francisco Bay Area were retrofitted to withstand a major quake in the years following the Loma Prieta quake of 1989 (magnitude 6.9).

[J N Winkler]

It's probably a seismic safety issue (i.e. the ability to withstand strong earthquakes) which is a big deal in California.  They don't call this area "Earthquake Country" for nothing.   

There are some overpasses that are built with steel girders (I-5 through the central valley, the I-280/I-880/CA-17 interchange in San Jose, the MacArthur Maze in Oakland and most of the elevated downtown freeways through San Francisco) but all new construction uses the poured concrete method for overpass construction.

The reason I have been told is that California has little to no local steelmaking capacity, so steel plate girders do not compete with prestressed concrete on cost.  I am not sure how influential seismic considerations are, because it is possible to build both steel plate girder and prestressed concrete bridges which resist seismic loads--it is just that estimating the loads is not always straightforward (as Caltrans found out in 1989 when the Cypress Viaduct collapsed and it was subsequently shown that its design, although considered conservative for the loads estimated in 1958, did not take liquefaction into account).

[UptownRoadGeek]

Thanks.  I initially though that it would be to seismic activity, but then I though about how my area lacks solid ground.  The ground around here is mainly silt and extremely mobile soil, yet there are steel and precast bridges and overpass all over the place.  I may be wrong, but I figured that if the steel girders can handle this constand expanding, shifting, and contracting; then they should be able to with stand an earthquake.  I also don't know what the ground in SoCal is like either so....

Thanks for the responses. Seems like it could be either answer.

[J N Winkler]

There is some overlap in designs to accommodate swampy ground and seismic loads, but the two problems are different.  If the swampy ground is not earthquake-prone, the main concern usually is to leave enough room for settlement, e.g. by adding hinge points or providing high-capacity expansion joints.  In earthquake-prone areas it is necessary to design to maximum lateral accelerations (which depend largely on proximity to the epicenter) and maximum amplitudes of movement (which depend to an extent on ground conditions).  Some soils are more likely to liquefy than others--i.e., to lose their cohesion under shaking loads and, with it, their ability to resist the lateral movement of embedded or buried elements like bridge foundations.

Steel is strong in both tension and compression, but concrete is strong in compression only.  A lot of the seismic retrofit work Caltrans has had to do in the wake of Loma Prieta and Northridge consists of steel jackets for bridge piers.  These jackets are necessary to help the piers resist torsion (twisting movement), which puts part of the concrete under tension and causes it to break into its constituent aggregate, with a catastrophic loss of structural strength.

A different mechanism of failure, but also involving tension, caused the Cypress Viaduct to come down.  The two levels were not "pinned" adequately to oppose lateral movement, which turned out to be much higher than Caltrans expected in the 1950's based on a contemporary understanding of soil behavior, which did not include liquefaction.  As a result, during the earthquake the top level of the Cypress "rocked" in relation to the base.  Each time it hit its maximum amplitude of movement on one side, part of the piers holding up the top level were under tension, while the opposing parts were under compression.  Then, at the maximum amplitude on the other side, the parts that were under tension previously were under compression, and vice versa.  After a few cycles so much of the piers had turned into aggregate that they could no longer support the dead load and the top deck came down, crushing 42 people to death.

[myosh_tino]

Not sure if JN Winkler pointed this out or not but another factor in the collapse of the freeway was the poor design of the columns holding up the upper deck of the freeway.  The columns were tapered from the upper deck to the lower which, in my mind, created a weak spot if the structure were shaken horizontally.  During the quake a number of these columns simply sheared off at the narrow base causing the upper deck to collapse onto the lower deck.  Here is a quick sketch of what a cross section of the old Cypress Freeway looked like before and after the quake...



IMO, double deck freeways in earthquake prone areas are a bad idea.

[roadfro]

Another consideration in choosing cast-in-place versus steel girder construction, beyond earthquake and soil strength, is constructability. Engineers must consider the construction method and material costs for the bridge depending on other conditions at the project site.

Cast-in-place (i.e. poured form) bridges require the construction of temporary scaffolding in order to pour the structural elements that support the bridge deck. This method takes longer, but my understanding is that it is generally a bit less complex in construction. However, creating the forms may necessitate high-profile vehicle detours and could introduce additional construction risks if the bridge is being constructed over a roadway already open to travel.

Steel girder bridges can be constructed in a bit shorter time, as forms are not needed and there is no wait time for concrete curing. However, the steel girders must be hoisted into a precise placement. A minor deviation in girder size or connection can result in significant setbacks during construction. Also, when constructed over a roadway open to public travel, this road must be closed whenever girders are being set.

[roadfro]

Bridges in Nevada vary greatly, with many examples of both cast-in-place (i.e. poured form) concrete bridges and steel girder bridges.

Cast-in-place bridges seem to be more prevalent in older bridge designs. However, there's newer examples as well. Practically every bridge on the Las Vegas Beltway, Carson City Bypass, and I-580 extension south of Reno is cast-in-place concrete. Also, every SPUI in Nevada where the signal is on the bridge over the freeway is of concrete construction.

Steel girder bridges seem to have been more prevalent on newer bridges for a while. Girder bridges are used almost exclusively for flyover ramps at interchanges.

There's also a few examples of bridges built with pre-cast concrete segments. All the flyover ramps at the Spaghetti Bowl in Las Vegas were constructed this way.

[Bickendan]

Not sure if JN Winkler pointed this out or not but another factor in the collapse of the freeway was the poor design of the columns holding up the upper deck of the freeway.  The columns were tapered from the upper deck to the lower which, in my mind, created a weak spot if the structure were shaken horizontally.  During the quake a number of these columns simply sheared off at the narrow base causing the upper deck to collapse onto the lower deck.  Here is a quick sketch of what a cross section of the old Cypress Freeway looked like before and after the quake...

IMO, double deck freeways in earthquake prone areas are a bad idea.

... Tell me the Alaskan doesn't use this design.

[Large quoted image removed. --roadfro]

[J N Winkler]

... Tell me the Alaskan doesn't use this design.

It basically does.  The various elements are probably detailed differently--for starters, I don't think the Alaskan has tapered piers connecting the first and second stories--but in terms of its ability to accommodate seismic loads, it has the same weaknesses and, like the Cypress Viaduct, is located in "made" ground which is more than usually susceptible to liquefaction.  This is why it must be either seismically retrofitted or torn down.  Fortunately, the former is underway with bids due to be opened today (!) on a major contract.

http://www.bxwa.com/bxwa_toc/pub/1573/toc.html

--scroll down to "Alaskan Way Viaduct - Replacement S Holgate St. to S King St. - Stage 2, King County, WA."

[Bickendan]

 
It dodged a bullet in the Nisquilly quake.

[hm insulators]

There are some overpasses that are built with steel girders (I-5 through the central valley, the I-280/I-880/CA-17 interchange in San Jose, the MacArthur Maze in Oakland and most of the elevated downtown freeways through San Francisco) but all new construction uses the poured concrete method for overpass construction.

FWIW, all of the steel girder type overpasses in the San Francisco Bay Area were retrofitted to withstand a major quake in the years following the Loma Prieta quake of 1989 (magnitude 6.9).

If I remember correctly, in Los Angeles, the I-5 bridge over California 110 is steel girder.

[Fixed quoting. --roadfro]

[The Premier]

Not sure if JN Winkler pointed this out or not but another factor in the collapse of the freeway was the poor design of the columns holding up the upper deck of the freeway.  The columns were tapered from the upper deck to the lower which, in my mind, created a weak spot if the structure were shaken horizontally.  During the quake a number of these columns simply sheared off at the narrow base causing the upper deck to collapse onto the lower deck.  Here is a quick sketch of what a cross section of the old Cypress Freeway looked like before and after the quake...

IMO, double deck freeways in earthquake prone areas are a bad idea.

Don't forget the other freeways that has same style in San Francisco. At least one of those freeways (the Embarcado Freeway) sustained minor damage before it was demolished due to cost prohibitions.

[Large quoted image removed. --roadfro]

[J N Winkler]

I am not sure there is any significant mileage of double-decker freeway left on land in the Bay Area.  The Embarcadero Freeway was demolished and part of the Century Freeway was torn down while the rest was made single-level.  The only double-decker stretches that come to mind are the Alemany Interchange, the Bay Bridge western approach, and the Yerba Buena Island tunnel.  Most of the Bay crossings that are already double-decked are remaining so (with appropriate seismic retrofitting) with the exception of the Bay Bridge eastern span, whose replacement will be on a single level.

[myosh_tino]

I am not sure there is any significant mileage of double-decker freeway left on land in the Bay Area.  The Embarcadero Freeway was demolished and part of the Century Freeway was torn down while the rest was made single-level.  The only double-decker stretches that come to mind are the Alemany Interchange, the Bay Bridge western approach, and the Yerba Buena Island tunnel.  Most of the Bay crossings that are already double-decked are remaining so (with appropriate seismic retrofitting) with the exception of the Bay Bridge eastern span, whose replacement will be on a single level.

Actually it was the Central Freeway (U.S. 101) that had it's double-deck section demolished after the '89 Quake.  I-105 in Los Angeles is the Century Freeway.

Also, Interstate 280 (a.k.a. The 280 Extension) north of U.S. 101 is double-decked for a short distance.

[TheStranger]

The Richmond-San Rafael Bridge (I-580) is also double-deck as well...