Inspecting and Managing the Nation's Bridges


The Rio Grande Gorge Bridge.
Photo courtesy of New Mexico Department of Transportation.

 

To ensure the safety of the nation’s bridges, every state conducts a thorough and continual bridge inspection and rehabilitation program. Federal regulations require that, with some exceptions, bridges over 20 feet in length be inspected every 24 months by trained and qualified bridge inspectors. In addition to following the federal inspection standards, states often develop more detailed programs appropriate to unique circumstances.

The Washington State Bridge Inspection Manual, for example, has 374 pages of precise instructions about how to inspect the state’s bridges and how to properly record the thousands of pieces of data about each component of its nearly 3,000 state bridges.

Bridge inspections are exacting and detailed. Inspectors with hardhats, safety harnesses and a variety of tools climb, crawl, touch, ping on steel with hammers or even wade through streams to inspect their bridges. Some use ropes and rigging like mountain climbers to scale towers. Often they dangle from long booms in bucket trucks or “snoopers” to inspect members. Some even dive underwater in SCUBA gear and use sonic devices to test the soundness of underwater piers and abutments.

Inspection records track conditions over time and allow engineers to judge the rate at which structures are deteriorating—or to gauge how well improvements have performed.

Data also feeds computerized forecasting systems that can help extrapolate decades into the future how the entire system of bridges in a state will perform. These computer systems can conduct “what if” scenarios that allow the engineers to estimate the health of their bridge inventories under differing funding levels and treatment approaches.

Technology Sees Beyond Human Eyes

Advances in technology are enhancing the ability to assess bridge conditions.

  • New Mexico, Pennsylvania, and other states use ultrasonic testing to “see” inside of the steel pins that often hold together components of older steel bridges. The National Highway Institute within the Federal Highway Administration offers a four-day course in which engineers are updated on the latest technology used to inspect the approximately 200,000 steel bridges in the United States. The use of ultrasonic devices and other state-of-the art technologies allows them to assess components of bridges that are not visible, or are embedded deep within the steel structure.


An engineer tests a bridge pin using ultrasonic technology.
Photo courtesy of Federal Highway Administration.

  • States have experimented with infrared wavelength scanners that span the bridge deck and use thermal imaging to detect defects deep within the concrete. For more than a decade, states have used Ground Penetrating Radar on their bridge decks. A vehicle drives over the bridge and emits short pulses of radar images that “bounce” back to sensors. These sensors can interpret the signal to determine if rust is corroding the steel rebar within the bridge or if gaps or voids have developed within the concrete.

  • The Iowa DOT has used a variety of electronic devices to test bridge conditions and to provide an analytical comparison to the field observations of its own engineers. It has used strain gauges, “accelerometers” which measure vibrations, and displacement transducers to measure the “flex” or deflection of the bridges under truck loadings. It also utilizes a Scour Watch system which uses real-time rainfall and stream-flow data. This data is automatically used to measure and predict stream flow and to compare that flow against the safe amounts that its bridges can accommodate. If the volume of water reaches certain levels, the system automatically warns the engineers who then visit the bridge for an immediate assessment. Any potential danger can result in immediate closure of the bridge and inspection after the flood waters recede.

  • The Florida Department of Transportation deploys a scour monitoring device that uses temperature sensors in a pile driven adjacent to a pier. Any changes in the temperature can automatically alert them to the potential that the bridge foundation is threatened by exposure through scouring.


Photo courtesy of David Gonzalez,
Minnesota Department of Transportation.

  • New Mexico has deployed three fiber-optic strain gauges on concrete beams to test the technology.

  • In Washington State the DOT is measuring miniscule changes in bridge height to detect any settlement on bridges, gathered through Global Position System data that is bounced off satellites.

Through these methods, bridge inspectors are using the results of their personal training and experience as well as technology-enhanced detection to inspect the nation’s bridges. Any unusual findings will result in closing a bridge for emergency repairs. Much more typically, the findings are fed into management systems which then order routine maintenance to address any potential deficiencies that are found.

Inspection Data Guides Bridge Decisions

The data collected on bridge conditions is critical to systematic, long-term approaches to managing the bridge inventory. The goal is finding the right balance between fixing immediate problems, conducting preventive maintenance and periodically replacing a reasonable number of old bridges to keep the health of its bridge population stable. This Asset Management approach produces stable, long-term conditions for the entire inventory of bridges for the lowest life-cycle cost.

Computerized or other systematic forecasts develop optimum strategies combining preventive maintenance, reactive maintenance to short-term deficiencies, and the periodic replacement of bridges that are no longer economical to repair.

Nevada typifies how a DOT seeks to optimize its scarce dollars to sustain the highest level of conditions over time. Nevada DOT is fortunate to have had sound maintenance practices and a relatively young bridge inventory because of its more recent “Sun Belt” development. But the DOT is taking a forward look by using its bridge management system to develop a mix of funding strategies to keep this $1.7 billion worth of bridge assets in good condition indefinitely.

It has adopted the following strategies to sustain its bridge conditions:

  • Replace or rehabilitate structurally deficient bridges before they become hazardous or need to be posted for load limits to a point they inconvenience the user;


Nevada Spaghetti Bowl in Henderson
Photo courtesy of Nevada Department of Transportation

  • Replace or rehabilitate functionally obsolete bridges before they become an impediment to users;
  • Seismically retrofit bridges that do not meet earthquake resistance standards; and
  • Apply timely repairs to structures as deficiencies are identified.

This approach has led to the following budgeting and investment recommendations for the state over the next 10 years.

System Corrective Maintenance Rehabilitation Replacement Seismic Retrofit Total
Interstates/Principal Arterials $14.5 $10.0 $0.0   $24.5
Principal Arterials $4.3 $9.8 $0.6   $14.7
Minor Arterials $3.8 $2.4 $0.0   $6.2
Major Collectors $4.3 $2.1 $2.7   $9.1
Minor Collectors/Local $1.1 $1.1 $2.6   $4.8
Statewide       $75.0 $75.0
Total $28.0 $25.4 $5.9 $75.0 $134.3

Nevada has followed the path of many states and laid out a logical, long-term series of options for how it can manage its bridge inventory given various financial scenarios. It illustrates how one state DOT manages the invaluable asset of a state’s bridge inventory, once the state has the necessary resources to pay for preventive maintenance, repair, rehabilitation and replacement as they are needed.

Summary

Inspecting and maintaining the nation’s bridges requires both experience and technology. Managing the nation’s bridges for the future requires both sophisticated forecasting and resources to keep them safe.

 

Bridges Move People

Texas: The Queen Isabella Causeway — An Economic Lifeline

The Queen Isabella Causeway, in Southern Texas “is a true lifeline” says Dan Quandt, Executive Director of the South Padre Island Convention and Visitors Bureau. The 5,000 people who call South Padre Island home depend on it for the delivery of all their food, medicines and supplies and to get their children to school. An average of 25,000 people—residents, tourists and workers—inhabit the island on any given day.

The Queen Isabella Causeway is their primary evacuation route should a hurricane threaten. “When we talk about connections between people, we’re talking about bridges,” Quandt said. South Padre Island lost its only connection with the mainland on September 15, 2001, when a barge collided with a bridge support sending three 80-foot sections of the causeway into the water. Unaware of the collapse several motorists drove their vehicles into the water below. Eight people died, and three were rescued. Thousands were stranded on the island and private charter boats and other water craft took part in the massive evacuation.

The economic impact to the South Padre Island was enormous. Not only were customers and employees cut off from businesses the collapse severed the island’s telephone lines, which ran under the causeway. Phone, fax, internet, and ATM banking services were lost. Island businesses estimated that they lost nearly $87 million in sales during the two months the causeway was closed for repairs.

“The big push right now is to get a second causeway built to the mainland.” Quandt says. “We’re in a hurricane zone and we learned a lesson on what happens if we lose our lifeline to the mainland. A new causeway will reduce evacuation times, decrease traffic congestion, and spur economic development,” Quandt said.

 

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