|The 38th Street Bridge in Tacoma |
The safety of bridge structures in Washington State is ensured through a meticulous inspection system, incorporating state and local agency inspection contributions. The condition of all bridge decks, superstructures and substructures are rated based on these inspections.
USDOT’s Federal Highway Administration (FHWA) requires all state transportation agencies to report state, city, and county Structurally Deficient (SD) and Functionally Obsolete (FO) bridge ratings each year. These ratings are used to help determine federal bridge replacement and rehabilitation funding levels to the states.
With the past collapse of the I-35W bridge in Minnesota much of the national discussion has focused on bridges being classified as structurally deficient and functionally obsolete. Unfortunately, these discussions have led to much confusion as to the actual meaning of the terms.
|The US 101 Simpson Avenue Bridge |
over the Hoquiam River
Aside from tracking structurally deficient and functionally obsolete, WSDOT’s Bridge Program emphasizes the importance of cost effective preservation programs, such as bridge replacement and rehabilitation, seismic retrofit, bridge painting, bridge deck rehabilitation, and bridge foundation scour mitigation.
Structurally deficient means that a bridge requires repair or replacement of a certain component. This may include cracked or spalled concrete, the bridge deck, the support structure, or the entire bridge itself. If the condition is such that it no longer is able to carry its intended traffic loads it may be weight restricted. Being structurally deficient does not imply that the bridge is in danger of collapse or unsafe to the traveling public.
WSDOT has 135 state owned bridges, including one large span culvert, that are classified as structurally deficient as of Feb 21, 2013. Contract work is in progress or complete on 15 bridges.
|The State Route 20 Deception Pass Bridge |
to Whidbey Island
Functional obsolescence is assessed by comparing the existing configuration of each bridge to current standards and demands.
A bridge can be categorized functionally obsolete a number of ways like having substandard lane widths, or narrow shoulders. Another example would be a bridge that doesn’t have enough vertical clearance for large trucks to pass under, causing repeat hits and damage to the bridge.
The sufficiency rating formula is a method of evaluating a bridge’s sufficiency to remain in service, based on a combination of several factors. The result of the formula is a percentage in which 100 percent represents an entirely sufficient bridge and zero percent represents an entirely insufficient or deficient bridge.
The sufficiency rating doesn’t necessarily indicate a bridge’s ability to carry traffic loads. It helps determine which bridges may need repair or replacement, not potential for collapse.
|View from under the SR 20 Deception|
Like all other states, Washington annually submits a report to the Federal Highway Administration (FHWA) containing all of the required information for each of our bridges. FHWA uses this information to determine sufficiency ratings. Many factors are included in the ratings, a few examples include traffic volume, roadway width, structure type, roadway alignment, and the condition of the road deck and structure.
A bridge’s sufficiency rating affects its eligibility for federal funding for maintenance, rehabilitation, or replacement. For bridges to qualify for federal replacement funds, they must have a rating of 50 or below. To qualify for federal rehabilitation funding, a bridge must have a sufficiency rating of 80 or below.
Good, Fair, and Poor Condition
Good: A range from no problems to some minor deterioration of structural elements.
Fair: All primary structural elements are sound but may have deficiencies such as minor section loss, deterioration, cracking, spalling, or scour.
Poor: Advanced deficiencies such as section loss, deterioration, cracking, spalling, scour, or seriously affected primary structural components. Bridges rated in poor condition may be posted with truck weight restrictions. As of June 2011, 5% of WSDOT bridges are in poor condition.
Fracture critical refers to a bridge component that might cause the entire bridge to collapse if that component failed.