The Infrastructure Report Card: What “Deficient” Actually Means

by briancpotter

damaged-bridge

ASCE’s 2013 infrastructure report card was recently released, and can be found here. Once again, the results are dismal. Our infrastructure received a “D+” overall.

Unlike almost everything else that engineers do, the infrastructure report card garners a fair bit of notice. In any discussion of government funding priorities, the state of our infrastructure is frequently brought up, and the infrastructure report card along with it. Because it has such high visibility, there’s also been some skepticism about the accuracy of the ratings. It’s been suggested that the ASCE might be exaggerating the extent of the problem – “juking the stats”, as they say – since a worse infrastructure means more work for engineers.

Because the report card covers a broad swath of infrastructure projects, we’ll just look at one particularly noticeable portion – the bridge section. The salient stat here is the percentage of structurally deficient bridges. The easiest way to pad these numbers would be the inclusion criteria – what qualifies as a “structurally deficient ” bridge. So what does it take to qualify? Are the stats on the level?

Short answer: Yes. A bridge has to experience damage to the main structural members, frequent flooding, or an extremely low load capacity before it qualifies as “structurally deficient”.

Long answer:

The ASCE doesn’t send out an army of engineers to inspect every bridge in the country to produce it’s report card. Instead, it collates and organizes data and reports produced by a variety of other organizations. For the bridge section, this comes primarily from the Federal Highway Administration (FHWA).

Bridges are mandated by the FHWA’s “National Bridge Inspection Standards” to be inspected, at minimum, every 2 years. Inspection involves, among other things, giving a bridge a rating in various “condition” and “appraisal” categories. Condition ratings compare the current state of the bridge to when it was first built, and include things like the deck, the foundation, and the superstructure. Appraisal ratings compare the bridge to a hypothetical new bridge that would be built to current standards, and include things like vertical clearance, approach length, waterway height. Each category is assigned a numerical grade on a scale of 0-9. 9 Indicates the bridge is superior to current construction standards, where 0 typically means the item is so deficient that it caused the bridge to fail.

For a bridge to be considered “structurally deficient”, it must score a 4 or less in four possible condition categories, or a 2 or less in two possible appraisal categories.

Here’s the condition categories:

Deck – The spanning portion of the bridge that makes up the roadway.

Superstructure – The main structural elements of the bridge. This would include things like the trusses, suspension cables, towers, and anything else that supports the load of the bridge.

Substructure – These are structural elements below the bridge, things like piers, abutments, footings, and piles. They transfer the load from the bridge into the ground.

Culvert/Retaining walls – These are non-bridge structures that nonetheless fall under the purview of the bridge inspector. A culvert is simply a large pipe* that runs beneath a roadway to allow water to pass underneath it, and a retaining wall is a wall that holds back soil along a roadway. (The FHWA page indicates retaining walls are covered, but they aren’t included in the coding standards or in the national bridge inventory database).

A 4 or less in any of these indicates “advanced section loss, deterioration, spalling or scour”. In other words, the primary structural members must be physically damaged or reduced in load capacity.

And here’s the appraisal categories:

Structural Condition – This is indicated by a load level which the current structure could support indefinitely. For current bridges, this load is 32.4 metric tons. A 2 or less here means that a bridge can support less than 10.8, 12.6, or 16.2 metric tons** (depending on the traffic), and that the only fix is replacing the bridge.

Waterway Adequacy – This section indicates, essentially, how often the bridge will be underwater due to flooding. A rating of 2 or less here means that (again, depending on the traffic), that the bridge is flooded frequently enough to produce “significant traffic delays”.

Nothing in any of these categories raises an eyebrow or seems out of the ordinary – it’s all what you’d expect to hear about a bridge that is structurally deficient. The one possible point of contention is the inclusion of culverts. Obviously, 1000 structurally deficient bridges would be much different than 10 structurally deficient bridges and 990 structurally deficient pipes underneath roadways. However, this doesn’t seem to be the case. For instance, in Georgia, there are 902 structurally deficient bridges, of which 88 are culverts. Michigan has 1274 deficient bridges, of which 116 are culverts. And it’s not as if culvert failure can’t be a very serious problem in it’s own right.

So, at the level of the grading scale at least, there doesn’t seem to be anything to indicate that the stats are being juked. Barring some massive conspiracy by the individual bridge inspectors, our bridges really are that bad.

*To qualify for inspection, a culvert must ‘span’ at least 20 ft.

**A load this size essentially prevents any large trucks from using the bridge.

References:

2013 Infrastructure Report Card

FHWA – Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges

FHWA – Structural Deficiency Requirements

Bridge Inspector’s Manual

Wikipedia – Culvert

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