Bridges do more than connect one side of a road to the other

Mile-long-bridgeYou probably drive over dozens of them every day but, unless you’re a structural engineer, you likely don’t give a lot of thought to the condition of Iowa’s bridges. That’s the way it should be. Safety is one of the Iowa Department of Transportation’s core values. We think about it a lot, so you don’t have to. 

The Iowa DOT’s team of structural engineers, analysts, and technicians spend a lot of time analyzing data about bridges. They use a variety of methods to evaluate and monitor the more than 4,200 bridges on the state highway system that includes roads marked as state and U.S. routes and interstates. That data is used to determine which bridges - critical transportation assets - need to be rehabilitated or replaced to make sure you are able to get where you need to go safely.

TAM.fwIn general, managing transportation assets, such as bridges, is all about providing the right treatment to the right asset at the right time. For bridges, selecting the right treatment starts with examining all of the documentation on the history of each bridge, including original construction and repair plans, inspection reports, and observations made by our field personnel.

When the I-35W bridge collapsed in Minneapolis in 2007, the world was introduced to the terms “functionally obsolete” and “structurally deficient.”  These terms are associated with a bridge when data held in the National Bridge Inventory (NBI) database, with information supplied by each state, meets certain criteria.  The NBI terms might sound a little scary; but when a bridge is said to be functionally obsolete or structurally deficient, it does not mean the bridge is unsafe.

A functionally obsolete bridge simply means the bridge doesn’t meet current design criteria with regard to any or all of the following:

  • width to accommodate wider vehicles
  • height above the roadway to account for taller vehicles passing underneath
  • curves that may be too tight according to current standards
  • how much weight it was designed to carry

For a bridge deemed as structurally deficient, some element of the bridge has been determined to have a defect, but steps, like imposing weight limits, can be taken to minimize any risk to those utilizing the bridge until it can be repaired.

DSCN3644In addition to these two terms, the two words you don’t want to hear, but are a fact of life for Iowa bridges is “fracture critical.” This term means a steel bridge was built in such a way that if one of the steel beams cracks, the whole bridge could become unstable. Although these types of bridges are not commonly built anymore, there are 105 fracture critical bridges on the state highway system and a total of 1,100 on all roads in Iowa.

Gary Novey from the Iowa DOT’s Office of Bridges and Structures, said, “These bridges are economical to build, so for years, they were considered a viable design alternative.”  He went on to say these types of structures are safe, they just require more frequent inspections to assure they remain that way.  

 All Iowa bridges, whether on the state, county or city level, are inspected at least once every 24 months by well-trained bridge inspectors. During an inspection, Iowa DOT inspectors review all areas of the bridge, sometimes using a specially equipped boom-type truck that can lift an inspector under the bridge.  Inspectors need to get up close to the underside of a bridge for a visual inspection. If an inspector sees something that may need further review, especially at a critical location in the structure, ultrasound technology may be used to detect cracks or other damage.

Local jurisdictions inspect county and city bridges, any maintenance or repair plan for those bridges is up to the local jurisdiction. The Iowa DOT inspects and plans for maintenance and repairs for bridges on U.S. or state routes or interstates.


Bridge inspection documentation
Currently, bridge inspectors keep detailed sketches of every crack on the bridge deck. The drawings are scanned and entered into a database. The Iowa DOT will begin testing the use of tablets to collect data electronically later this summer.

Each inspection is documented in the Iowa DOT’s database. The documentation for an inspection includes; photos, sketches, inspector’s notes, condition ratings for specific pieces of the bridge, bridge characteristic data, and recommendations for maintenance. The inspection documents are reviewed by the quality control team in the Office of Bridges and Structures. Once the QC team has reviewed the inspection, they make recommendations for routine maintenance or forward the report to a staff engineer for further review. The staff engineer reviews the more complex issues and makes recommendations for repair, rehabilitation, or replacement.

The main bridge office in Ames maintains a list of priorities and recommendations for bridges on the state system. Iowa DOT experts annually review all the newly recommended projects from the past year to determine which bridges should be recommended for inclusion in the next cycle of construction projects, listed in the agency’s five-year Iowa  Transportation Improvement Program.

Priorities for recommending which bridge projects should be included in the program are set based on the inspections and other data.  For bridges that staff has determined need work, repairs range in priority from a four, which means the structure doesn’t need immediate work, but will need to be fixed in the future, to a priority one, which indicates a more urgent need.

Priority one projects are scheduled first, then priority two, then three, and so on until the funding limit has been reached. When more projects are needed, but there is not enough funding to complete them all, district staff make necessary short-term repairs until the bridge can eventually be scheduled for a more major overhaul.

The Iowa Transportation Commission, the panel appointed by the governor to oversee the Iowa DOT, reviews the staff recommendations that have been added to the five-year program.  Following the Commission review, the final program is approved each June. The program can be amended and projects moved up in the repair or replacement cycle when additional funding is available. This is exactly what happened this year with the additional money from the gas tax increase.

This post gives you an understanding how the Iowa DOT determines which bridges need to be built or rehabilitated, but there’s much more to the transportation asset management story. In the next blog post, we’ll take a look at maintenance needs and how they are addressed.

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