Bridges without girders

[roadman65]

I was noticing that many western US States use bridges that are completely reinforced concrete with NO beams beneath them.  Missouri, Kansas, Idaho, and especially California use these bridge types.  Why is that so and why are there not any along the east coast?

If you want a great example go to Wallace, ID and look on streetview and see how the I-90 viaduct is built underneath from the local streets.  The bridge is supported on two piers and the underside is flat.

[J N Winkler]

I was noticing that many western US States use bridges that are completely reinforced concrete with NO beams beneath them.  Missouri, Kansas, Idaho, and especially California use these bridge types.  Why is that so and why are there not any along the east coast?

If you want a great example go to Wallace, ID and look on streetview and see how the I-90 viaduct is built underneath from the local streets.  The bridge is supported on two piers and the underside is flat.

There are several factors at work here.  But, first of all, it has to be noted that no one bridge type dominates absolutely in any region.  There are plenty of steel girder bridges in the Western states and concrete segmental bridges in the Eastern states.  Midwestern states like Missouri and Kansas actually have a pretty even mix of steel and concrete bridges (for example, nearly all of the original overbridges on the Kansas Turnpike were of steel girder construction, and the old Kansas River bridges near Lawrence were steel deck trusses).

Eastern states tend to use steel for bridges (not just girders, but also deck trusses and so on) because they had abundant steel manufacturing capacity and got started on their freeways earlier.  Prestressed concrete bridges were a late development that did not reach the US until after World War II and took some time to diffuse.  The extensive use of prestressed concrete in segmental bridge construction arrived even later, and many state DOTs already had long-span deck truss bridges before that form of construction took off in Europe in the 1960's with bridges like the M2 Medway viaduct in England.

California is in a sense the exception that proves the rule because it did get an early start on its freeway system (by 1947, it was reported as having more freeway mileage than any one of the Eastern states).  Its steel manufacturing capacity was somewhat limited compared to the Eastern states, so it used concrete extensively for bridges.  However, initially these bridges were reinforced concrete with a closed deck section; prestressed concrete came later, and a number of early California freeway bridges used steel not just for deck supports but also for piers, particularly in Northern California which at the time had a shipbuilding industry (which used a considerable amount of steel).

Concrete bridges are becoming more common in the Eastern states as Interstates are reconstructed--for example, the Allegheny River Bridge deck truss on the Pennsylvania Turnpike was recently replaced with a segmental concrete bridge.  On the other hand, some snowy states which have used concrete superstructures with closed sections in the past have moved toward steel girders, partly because corrosion is easier to see and remedy with the latter.  The original Marquette Interchange made extensive use of concrete box girder bridges but the new one uses steel plate girder bridges for precisely this reason.

[roadman65]

Thanks, it really helps a lot.  I was just always wondering why it seemed to be that way, but with California being the first state to use freeways and many of them pre-dated the interstates, it does makes sense why I saw many when out there and in pictures of the collapsed I-5 bridge after a major earthquake decades ago.

[roadman65]

I just seen what you meant.   I just came back from New Jersey and saw that both NJ 35 across the Raritan River and NJ 36 across the Shrewsbury River both have bridges with prefabbed concrete structures.   It does appear Caltrans way is on the east coast for bridging.

[cpzilliacus]

Concrete bridges are becoming more common in the Eastern states as Interstates are reconstructed--for example, the Allegheny River Bridge deck truss on the Pennsylvania Turnpike was recently replaced with a segmental concrete bridge.  On the other hand, some snowy states which have used concrete superstructures with closed sections in the past have moved toward steel girders, partly because corrosion is easier to see and remedy with the latter.

Perhaps the most famous and iconic segmental concrete bridge in the East is the Blue Ridge Parkway's Linn Cove Viaduct, Google Street View (north end) here in Avery County, North Carolina.  Like the Pennsylvania Turnpike bridge over the Allegheny River (and the one over the Susquehanna too), these bridges were designed by Figg Engineering of Tallahassee, Florida.

[J N Winkler]

I have slowly been working my way through California Highways and Public Works from about 1934 onwards, and I have now reached 1951.  In that year there are several articles dealing with California's first prestressed concrete bridge, which was a pedestrian crossing over the Arroyo Seco.  This was also the first experiment with prestressed concrete construction in the western USA.

The longest and most detailed of these articles, in the March-April 1951 issue, describes the prestressing technique and compares the cost and material utilization with reinforced concrete bridges, which were then a popular type of construction for freeway crossings.  For the loads intended to be carried by the Arroyo Seco pedestrian bridge, CHPW says that prestressing allowed about one-third of the steel and about two-thirds of the concrete to be used for about the same construction cost.  Also, since prestressing keeps the concrete in compression whether it is under load or not, it allows structural elements to be fabricated elsewhere and hoisted into place without the need to build falsework in river channels, which in Los Angeles were (and still are) subject to seasonal restrictions on anything that can block flood surges.

In reinforced concrete, the concrete must first exceed its tensile strength limit and crack before an appreciable proportion of the load can be carried by the steel reinforcement.  This cracking is by design and controlled within narrow limits, but--as the CHPW article notes--has the tendency to prompt uninformed members of the general public to fear that the structure is about to fail.  In contrast, properly designed prestressed concrete does not crack under load and so makes the structure easier to "sell" to the public as safe.

[swbrotha100]

I would love to see more bridges built like that if possible. I think they're more aesthetically pleasing.

[agentsteel53]

is there a way to easily tell a prestressed vs. a reinforced concrete bridge?  I look at California bridges a lot, and would be very interested in being able to tell the difference. 

[J N Winkler]

I don't think there is an easy way to tell short of examining construction plans or knowing the construction date of the bridge.  The amounts of concrete used for either bridge type are fairly close to each other for the same design loads, and it is only the concrete that is visible, not the steel reinforcement or prestressing cable.  Both types of bridge tend to be partly hollow in section also, which further conceals differences in concrete usage.

In California I think the most reliable way to tell is by looking for differences in styling.  Older, reinforced concrete bridges tend to have prominent abutment wingwalls (which are deprecated in current design because of their tendency to limit forward visibility, but were favored with reinforced concrete construction since they limited the free spans that had to be supported by the reinforcement).  They also tend to have haunching of deck slabs or girders in the near vicinity of bents, which in turn tend to match the width of the deck and to be skewed if the bridge is on a curve, or crosses an engineered feature such as a roadway or railroad track which is itself on a curved or skew alignment.  (Round piers which widen to a wye or tee section at the top are a later refinement, designed to improve visibility for traffic the bridge crosses over.)

The stereotypical examples of reinforced concrete bridge architecture can be found on the lengths of the Arroyo Seco Parkway and Hollywood Freeway which were built before the early 1950's, using reinforced concrete construction exclusively, and are mostly still in their original configurations.  (The Hollywood Freeway from the Four Level all the way to Barham Boulevard at least is late-1940's construction at the latest; the entire length of the Arroyo Seco had opened by 1940, with the exception of the southbound lanes which bypass the Figueroa Street tunnels through Elysian Park and were built during World War II.)  However, architecture is not a completely reliable guide.  The Four Level Interchange structure, for example, looks like it uses prestressed concrete, but in fact it uses reinforced concrete voided-slab construction throughout, as I have personally verified by looking at the construction plans.

[kphoger]

is there a way to easily tell a prestressed vs. a reinforced concrete bridge?  I look at California bridges a lot, and would be very interested in being able to tell the difference. 

My bridge engineer friend says:

I think most bridge engineers can.  If they are concrete "I-girders" then most likely prestressed.  Also, if span length is greater than around 60 feet then prestressed.  There are exceptions but those are good general guidelines.
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[J N Winkler]

My bridge engineer friend says:

I think most bridge engineers can.  If they are concrete "I-girders" then most likely prestressed.  Also, if span length is greater than around 60 feet then prestressed.  There are exceptions but those are good general guidelines.

Wide spacing of piers is a useful guideline for California, but the presence of prestressed girders is less so, because cast-in-place construction (with closed box sections) predominates so strongly.  In practice I think most if not all concrete bridges of recent construction use prestressed concrete.