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Bridges | Inspections | Terms | Sensors | Deck truss bridges in Wisconsin

Bridges

Wisconsin has about 13,600 bridges spanning state and local roadways. Of these, about 4,900 are along the state highway system (numbered state and federal highways) and are the responsibility of the Wisconsin Department of Transportation (WisDOT). The state highway system carries some 60 percent of the state’s overall traffic load. Approximately 8,700 bridges are located along the local roadway system (county and town roads and municipal streets) and are the responsibility of local governments. WisDOT generally defines a bridge as any structure spanning 20 feet or more that carries motor vehicle traffic. The overall number of bridges fluctuates from year to year as new bridges are added to the system as part of construction projects, while some older bridges may be permanently removed.

All bridges in Wisconsin are designed and constructed with one primary thought in mind – public safety. Additional considerations are longevity and contributions to economic growth. State and local bridges are inspected at least once every two years. WisDOT is responsible for inspections of all bridges along the state highway system. Municipalities handle inspections for bridges along the local roadway system. WisDOT and local governments closely follow federal guidelines in their bridge inspection and maintenance procedures.

Along with inspecting and maintaining its own bridges, WisDOT works closely with cities, villages and towns to rehabilitate and replace aging bridges. For example, WisDOT oversees the Local Bridge Improvement Assistance program that helps rehabilitate and replace, on a cost-shared basis, the most seriously deficient bridges along the local highway system. A Lift Bridge Aids program reimburses several Wisconsin cities for costs associated with the operation of lift bridges. And each year, WisDOT returns about one-third of all state-collected transportation revenues to local governments in the form of General Transportation Aids (GTA) to help municipalities build and maintain local roads and bridges. In 2007, GTA payments to local governments will total about $400 million.

Inspections

All of Wisconsin’s 13,600 bridges are inspected at least once every two years and sometimes more frequently depending on a bridge’s age, traffic load and any known deficiencies or load restrictions. Inspection dates and reports for all Wisconsin bridges can be found on the Highway Structures Information System portion of the WisDOT Web site.

There are different types of bridge inspections from routine to in-depth depending on a bridge’s individual characteristics and needs. WisDOT’s trained bridge inspectors follow Federal Highway Administration (FHWA) standards and guidelines. Some smaller bridges can be inspected on foot, while others require use of a special "reach-all" vehicle with a jointed arm and bucket that provides inspectors an up-close look at the underside of a bridge. Depending on the size of a bridge, weather conditions and other factors, a bridge inspection can take from one hour to more than a week. During bridge inspections, certified inspectors survey:

• the superstructure or beams that support the deck looking for cracks, rust, or any problems with bolts or rivets.
• the substructure units (which support the superstructure).
• bridge approaches and the deck or surface of the bridge.
• on bridges over large bodies of water, inspections require divers to check supporting piers.

Following a thorough review of the deck, superstructure and substructure, bridges are assigned a "sufficiency rating" number between 1 and 100. The rating takes into account some 75 factors reviewed during an inspection and also considers a bridge’s age, length and width, and the average amount of traffic the bridge handles. WisDOT uses the sufficiency ratings to help prioritize bridge improvements. Under WisDOT's Local Bridge Improvement Assistance program, municipalities are eligible for rehabilitation funding on bridges with sufficiency ratings less than 80, and replacement funding on bridges with sufficiency ratings less than 50. Each year, all states including Wisconsin are required to submit a report to the FHWA that reviews the condition of its bridges.

Terms

Bridge: Generally defined by WisDOT as any structure spanning 20 feet or more that carries motor vehicle traffic.

Deck: The pavement surface of a bridge on which vehicles travel.

Deck truss bridge: On a deck truss bridge, the superstructure typically consists of two or more parallel trusses that are the main load-carrying members of the bridge and the roadway is placed on top of the main members. WisDOT no longer builds deck truss bridges on the state system as they have been replaced with more modern designs. There are currently 14 deck truss bridges along the Wisconsin highway system similar in design to the I-35 bridge that collapsed in Minnesota. WisDOT took immediate steps to inspect these bridges following the Minnesota tragedy. The inspections found no significant problems and reaffirmed the bridges are safe for public travel.

Fracture critical: A fracture-critical bridge typically has a steel superstructure with load (tension) carrying members arranged in a manner in which if one fails, the bridge could partially or totally collapse. Examples of fracture critical bridges are two girder bridges or most truss bridges. A fracture critical designation does not mean a bridge is unsafe. Today, virtually all new bridges built along Wisconsin’s state highway system are redundant or constructed in such a fashion that should one bridge component fail, other elements will pick up the load to avoid a collapse.

Functionally obsolete: Engineering term frequently used to describe older bridges that no longer meet modern geometric standards. For example, it could refer to a bridge with narrow lanes or shoulders. A bridge classified as functionally obsolete does not mean the bridge is unsafe for public travel.

Gusset plates: Metal plates used to connect bridge components and transfer weight between the components. Gusset plates are typically bolted or riveted together.

Redundancy: Constructing a bridge in such a way that if one element should fail, other components will pick up the load to avoid a collapse. Today, virtually all new bridges WisDOT builds along the state highway system are redundant.

Structurally deficient: Engineering term referring to a bridge with one or more elements that will require attention. The classification does not mean the bridge is unsafe for travel. For example, it could refer to a combination of elements on a bridge such as potholes on a bridge deck or rust on metal trusses. These have little to no impact on a bridge’s overall safe function. Depending on the extent of the structural deficiency, the bridge may be load-posted until improvements are completed.

Sufficiency rating: A computed numerical value between zero and 100 used to help determine a bridge’s priority for rehabilitation or replacement and eligibility for state or federal funding. The rating considers structural factors noted during a bridge inspection, a bridge’s geometry and the amount of traffic the bridge handles. A bridge with a sufficiency rating of 80 or less is eligible for bridge rehabilitation funding. A bridge with a sufficiency rating of 50 or less is eligible for replacement funding.

Sensors

Different types of sensors may be placed on bridges to measure motion or the amount of strain being placed on bridge components. WisDOT is overseeing installation of strain gauges on 14 deck truss bridges similar in design to the I-35W bridge that collapsed in Minneapolis on August 1, 2007. On a deck truss bridge, the superstructure typically consists of two or three parallel trusses that are the main load-carrying members of the bridge and the roadway is placed on top of the main members. 

A strain gauge is typically attached to each metal bridge truss. Wires from the gauge lead to a data collection unit that records strain data 24 hours a day, seven days a week. The data collection units can transmit data or the information can be manually downloaded at the bridge site.

Strain gauges have been successfully used before to collect bridge data. Data gathered via bridge sensors can serve as a valuable assessment tool to help determine the amount of strain a bridge is handling.

Deck truss bridges in Wisconsin

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Last modified: May 7, 2009