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Updated Dec 18 2015, by Craig Helmann, PSRC

National Highway System Bridge Conditions Holding Steady

Ensuring that the region’s infrastructure is maintained and preserved in a safe and usable condition is the top priority in the regional transportation plan and comprises nearly half of the investment included in Transportation 2040. Bridge conditions measure the safety, usability, and overall condition of bridges over 20 feet in length within the region.

Safe bridge crossings facilitate the movement of people and goods across the region’s many valleys, lakes, creeks, rivers, and other infrastructure. When bridges are not properly maintained consequences include weight restrictions and wholesale closures, which can have significant impacts on the ability for freight and people to efficiently use the transportation system.

What is the Overall Bridge Condition trend in our Region?

Figure 1 depicts the percentage of bridge deck area classified as in good, fair, and poor condition for the entire region as well as local, and state owned facilities, based on analyses defined by the Federal Highway Administration and Washington State Department of Transportation. While individual bridges may have changed categories based on work performed over the last year, the overall system condition has stayed consistent with slightly over 90% of facilities falling within the “good” to “fair” range, and only 10% of facilities falling within the “poor” category.

Figure 1. Regional Bridge Conditions
Figure 1. Regional Bridge Conditions
Good : Bridges in good condition range from those with no problems to those having some minor deterioration of structural elements. Fair : Primary structural elements are sound; may have minor section loss, deterioration, cracking, spalling, or scour. This is the most cost-effective time to rehabilitate before the underlying structure is damaged. Poor : A bridge in poor condition has advanced deficiencies such as section loss, deterioration, scour, or seriously affected structural components, and may have weight restrictions. A bridge in poor condition is still safe for travel.

The same consistency holds relatively true when examining the trend for both local and state owned facilities independently. Local facilities (as defined by bridge deck area) in good or fair condition have increased approximately by 2% while state owned facilities have declined by approximately by 1%. Looking ahead, percentage of state bridge deck area classified as good or fair is expected to increase considerably over the next few years due to the replacement of the Alaskan Way Viaduct and SR 520 floating bridge, which comprise a large portion of square footage that has been classified as “poor”.

Structurally Deficient Bridges

Structurally deficient bridges are those that rate very poorly in one or more of the critical elements of a bridge (bridge deck, superstructure, substructure, or culvert). A facility can also be designated as such if its load carrying capacity is significantly lower than current design standards or the waterway below frequently overtops the bridge during floods. This differs from functional obsolescence in that structurally deficient bridges are classified based on their physical condition whereas functionally obsolete bridges are identified as such based on key design elements relative to the current use of the facility. For example, lanes too narrow for traffic or a misalignment of the bridge approaches.

Figure 2. Percent of Local and State Bridges Identified as Structurally Deficient
Figure 2. Percent of Local and State Bridges Identified as Structurally Deficient

In 2015, structurally deficient bridge facilities represent just over 9% of the approximately 40.1 million square feet of bridge deck in the region, holding steady from 2014. Taking a closer look, locally-owned structurally deficient facilities represent 12% of the total local bridge deck area whereas state owned facilities classified as structurally deficient represent 9% of total state owned bridge deck area in the region. This is a similar relationship to 2014, where local structurally deficient local bridges made up 13% of the total local bridge deck area and state owned structurally deficient bridges represented 9% of state-owned bridge deck area. The gap between the two years appears to be shrinking. Figure 2 depicts the trends for, and the relationship between, locally and state bridges classified as structurally deficient.

Federal Requirements

MAP-21 implemented minimum condition standards for structurally deficient facilities on the National Highway System, limiting the square footage of facilities classified as such to 10% of all bridge deck area on the NHS within the state.

Figure 3 depicts structurally deficient bridge deck area on the National Highway System within the region. The analysis confirms that bridges on the NHS within the central Puget Sound region fall within the parameters outlined in MAP-21 at 9.9%, up slightly from 9.7% in 2014.

Figure 3. Bridges Classified as Structurally Deficient on the National Highway System
Figure 3. Bridges Classified as Structurally Deficient on the National Highway System

As work on the Alaskan Way Viaduct and SR 520 floating bridges is completed these percentages will drop significantly, as these are also two of the largest structurally deficient bridge facilities in the state.

What are Future Implications?

For the time being, bridge conditions are holding steady; however, as WSDOT and local agencies continue to face budget shortfalls an increasing percentage of available resources will be required to maintain and improve upon these conditions.