Iowa has the fourth highest percentage of structurally deficient bridges in the nation, with more than one-fifth of the state’s bridges in need of immediate repair or replacement because of deterioration, according to a new report released on Feb. 20 by TRIP, a national nonprofit transportation research group based in Washington, D.C.
“Iowa’s bridges are aging and overburdened at a time when the state is unable to fund all needed bridge repairs. Additional funding for needed improvements could prevent Iowa’s already deteriorated bridges from worsening,” said William M. Wilkins, TRIP’s executive director.
The report, titled “Iowa’s Deficient Bridges: Strategies to improve and maintain the condition of Iowa’s bridges,” examines the current condition of Iowa’s bridges and future conditions under current transportation funding levels.
According to the report, 21% (5,153) of Iowa’s bridges are structurally deficient, showing significant deterioration of decks or other major components. An additional 6% (1,455) of the state’s bridges are rated as functionally obsolete because they no longer meet current design and safety standards.
Iowa is falling behind in maintaining its state-maintained bridges, according to the report. The current rate of repairs and funding is inadequate to keep pace with bridge deterioration, which could lead to an even higher percentage of structurally deficient bridges in the future if repairs are not made.
Under current funding, the Iowa Department of Transportation (IDOT) is able to significantly repair or replace an average of 20 bridges annually. However, the number of bridges annually repaired or replaced will drop to 15 in 2008. On average, IDOT estimates that between 20 and 30 state-maintained bridges become structurally deficient each year.
The report lists 100 state-maintained bridges in Iowa (carrying at least 2,500 vehicles per day) that have the lowest rating based on condition, design and traffic volume. Individual components of the bridge are rated on a scale from 0-9 and those scores figure into the overall rating. Typically, the deck of a bridge, the superstructure and the substructure are rated based on their physical condition.
A significant number of state maintained Iowa bridges are close to becoming structurally deficient if repairs are not made. According to IDOT calculations, 622 state-maintained bridges have a deck, superstructure or substructure rating of five. This rating is within one point of a structurally deficient classification.
Deficient bridges can harm a region’s economic development by reducing access, particularly for large commercial vehicles, which are critical to business productivity. Bridges that are closed or posted for a lower weight can impede the flow of goods from farm to market and from business to consumer.
The report suggests several strategies for improving bridge conditions in Iowa. These strategies include increasing bridge investment statewide to allow for an expanded program of bridge repairs, expanding the current bridge maintenance program to slow the rate of deterioration and using high-performance materials when building or reconstructing bridges.
The report also found:
- A bridge is structurally deficient if there is significant deterioration of the bridge deck, supports or other major components. Bridges that are structurally deficient are often restricted to carrying lower weight vehicles or are closed if they are found to be unsafe.
- The classification of a bridge as "structurally deficient" does not mean it is unsafe. Iowa’s bridge safety inspection program ensures that each bridge is safe for vehicles weighing less than the posted weight limit. If the inspection determines a bridge to be unsafe for cars, the bridge is closed until repaired or replaced.
- Functionally obsolete bridges are not automatically rated as structurally deficient, nor are they inherently unsafe. Functionally obsolete bridges do not have adequate lane widths, shoulder widths or vertical clearances to serve current traffic demand.
