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According to the United States 2014 National Bridge Inventory the number of concrete bridges far outstrips the number of steel bridges.

Reinforced Concrete: 253,336

Prestressed Concrete: 148,333

TOTAL CONCRETE: 401,669

versus

TOTAL STEEL: 181,095

Why would concrete bridges outnumber steel bridges?


Basis for Categorizing By Material Type

The concrete and steel totals are based on the "Main Structure Type" inventory code from the NBI Recording and Coding Guide. This is a three digit code defined as shown below.

NBI material code

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Motivation for Question

In the state where I work we often design steel and concrete "alternates" for each bridge. However, we consistently see the concrete alternate (usually prestressed) selected instead of the steel. I'm trying to understand why this would be since steel seems to offer quite a few benefits...quick construction in the field, predictable material properties, often lighter than concrete girders, damage is usually readily inspectable, etc. etc. I've always chalked it up to some sort of inertial regional preference (when I was in Chicago we saw tons of steel, but in California and Nebraska it's mostly been concrete) and thought I'd see if there were some more substantive reasons.

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    $\begingroup$ I'm going to be snarky and say "because concrete doesn't rust through," but that's a half-answer at best :-) $\endgroup$ – grfrazee Jan 19 '16 at 20:05
  • $\begingroup$ Are you sure you're interpreting these data correctly? Based on a cursory sample of years from the link you provided, it looks to me like the concrete/steel ratio has been increasing since the '90s. In any case, 23 years is a relatively short period of time in the context of large public works projects and bridges vary considerably. Some categories of bridge may be excluded from this inventory. How do you know that concrete bridges are, or have been, more prevalent? If this is a US-specific question, that should be explicit in the title. $\endgroup$ – Air Jan 19 '16 at 22:11
  • $\begingroup$ Also, what do they classify composite construction as? A lot of the interstate overpasses being rebuilt where I live (Chicagoland area) are steel beam built with a concrete deck. I suppose this could be steel and concrete. $\endgroup$ – grfrazee Jan 20 '16 at 15:29
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    $\begingroup$ I've updated the question to reflect US-specificity of the data and to document the NBI explanation for the material code. In the case of girders composite with the deck I believe the structure type code reflects the girder material since the deck has a separate code (Item 107). @air - thanks for the input. I'm going to keep digging into the data looking for regional and historic patterns. I've updated the question to reflect the fact that the limitations of the NBI data means all I can really state is that concrete bridges currently outnumber steel bridges in the total US inventory. $\endgroup$ – CableStay Jan 20 '16 at 17:19
  • $\begingroup$ My dad was the plant manager for a precast concrete operation that did a lot of bridge decks in the '60s - '80s. The only way steel could compete on price was when the Japanese and Chinese were dumping steel on the US market. China was also ramping up cement purchases worldwide at the time, further disrupting the concrete business. The short answer is cost if the concrete fabrication ability existed anywhere within about 300 miles of the bridge site. You need to consider the cost structures in place when those bridges were built. Today's cost structure might lead to different decisions. $\endgroup$ – Phil Sweet Dec 8 '17 at 15:25
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I live in Brazil, so the relevance of my answer to your US experience is somewhat questionable. This is exacerbated by the fact that Brazil is almost 100% concrete, with very few steel or wooden structures (including buildings). That being said, some of these concepts are location-agnostic and others may apply to the US as well.

Firstly, as mentioned by others in the comments, concrete requires basically no maintenance for the duration of its service life. Steel, on the other hand, rusts and therefore must be maintained (or, well, should be, anyway...).

In Brazil at least, concrete is far cheaper than steel, which in infrastructure projects is usually the defining factor.

Related to the above, (non-prestressed) concrete requires less-skilled workers. Concrete is basically just "put up the formwork, place the reinforcement and pour the concrete." Steel requires expert welders and careful torquing of screws. In Brazil the lack of steel-skilled workers is such that a welder easily earns more than an engineer. Weld-checkers swim in cash.

Bridges are often built far from urban centers, meaning your choice of constructor may be limited, reducing the quality of your workers. This relates to the above. Your supply lines may also be problematic.

Concrete is more flexible, allowing for the easy treatment of curved beams, including box-girders which excel at withstanding torsion. It also trivially allows for variable-cross-section elements.

For long spans, prestressed concrete is usually the most efficient choice.

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    $\begingroup$ Weld-checkers swim in cash. Apparently I need to switch jobs and move to Brazil! $\endgroup$ – grfrazee Jan 20 '16 at 15:25
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Initial Cost

When span lengths allow, concrete spans are cheaper than steel spans. This is partially because of material costs and partial because of labor (fabrication) costs.

There are a few situations where concrete is not feasible. These situations could be:

  1. Very long span lengths.
  2. Skewed spans.
  3. Shallow structure depths.

In these situations, steel might be used even if it is more expensive.

Also, concrete spans are usually composed of typical cross sections. Once the forms for these precast, prestressed beams are made, the labor costs are greatly reduced. Every steel girder has the same amount of labor whether it has been previously done or not.

Long-term Maintenance

Concrete spans have reduced maintenance as compared to steel spans. Steel spans typically require periodic painting and cleaning. This isn't a requirement for concrete spans. Also, concrete does not require the same level of inspection as steel spans do. There is no need to check for fatigue cracks in concrete spans.

Added Together

When both the initial cost and and long-term costs are added together, concrete is typically cheaper. This is why there are more concrete bridges.

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    $\begingroup$ Where I live, Canada, steel is also chosen when building over existing, busy highways, as the work can be done at night, with the highway reopened the next day. $\endgroup$ – Dave huh Jun 7 '16 at 20:40
  • $\begingroup$ @Davehuh That can also be done with CPCI girders, next time I drive under 3-4 of the rapid bridge replacements projects in Ottawa I will have to check out if there are steel or concrete. $\endgroup$ – Forward Ed Jun 27 '16 at 20:45
  • $\begingroup$ @hazzey Some other factors affecting choice of steel versus concrete would be the proximity/availability of steel/concrete to the bridge site. Also profile and clearance requirements, The road profile may be fixed and you require a certain clearance below. If I recall correctly steel can provide the same support requirement as concrete using a shallower depth of sections. And finally if you are using beam construction, you can curve steel, where Pre stressed concrete beams do not work. $\endgroup$ – Forward Ed Jun 27 '16 at 20:50
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Steel will have lesser life span than concrete used in a bridge in water. Therefore concrete is a preferred choice.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    $\begingroup$ Any citations, or further explanation for why? $\endgroup$ – wwarriner Jan 19 '16 at 20:23

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