Innovation Adds to Bridge Safety


The Arthur Ravenel Jr. Bridge.
Photo courtesy of South Carolina Department of Transportation.

 

Bridge technology is in a state of constant change as engineers test new materials, new design and construction methods and inspection techniques to ensure the continued safety of the nation’s bridges.

Much of the experimentation takes place under the federal Innovative Bridge Research and Construction and Deployment programs. These $142 million programs have funded projects in nearly every state. These projects advance the state of the art by:

  • Developing new, cost-effective and innovative materials;

  • Reducing maintenance and life-cycle costs;

  • Creating faster, safer construction techniques;

  • Developing bridges to withstand natural disasters such as earthquakes and floods; and

  • Developing “non-destructive” testing materials to “see” inside beams and piers without dismantling them.

Long-Term Research Aims to Increase Understanding of Wear

Another ambitious 20-year federally funded research effort is the Long-Term Bridge Performance Program (LTBP). The objective of the LTBP program is to collect, document, and make available high-quality quantitative performance data on a representative sample of bridges nationwide. Data will be collected through detailed inspections and evaluations, supplemented by a limited number of continuously monitored structures and forensic autopsies on decommissioned bridges. In the latter years of the program, the collected data will be analyzed to develop improved knowledge about bridge performance and degradation, better design methods and performance predictive models, and advanced management decision-making tools.

Specifically, the anticipation is that the LTBP program will provide a better understanding of bridge deterioration due to corrosion, fatigue, weather and exposure, and loads. The program also will provide information about the effectiveness of current maintenance and improvement strategies, and should lead to improving the operational performance of bridges with the potential to reduce congestion, delay, and crashes.

 

States also pool their own resources through the National Cooperative Highway Research Program to address a wide range of needed research.

From Alaska to Florida, the states are experimenting in highly controlled efforts to find ways to make bridges stronger and safer.

  • In Alaska, the DOT is testing a new method of “seismic retrofit” on the substructure of the 1,250-foot-long Kodiak Harbor Channel Bridge.

  • In Florida, the DOT is experimenting with several strategies to combat rusting and corrosion of the “rebar” or the strengthening structural steel that runs through concrete bridge decks and piers. They are using stainless steel-clad rebar, and fiber-reinforced polymer composites in the pilings and bridge decks to prevent future corrosion damage.

  • In Iowa, the DOT is testing fiber-reinforced polymers to replace deteriorated concrete decks as well as to build entire new bridges. In addition, it is using high-performance concrete and steel to build new structures that will be carefully monitored for their cost, strength and durability.

  • On the campus of the University of California at San Diego, the California Department of Transportation plans to construct a 450-foot-long cable-stayed bridge using carbon-fiber-reinforced-polymer composites. It will include two lanes for motor vehicles as well as two bike lanes, a walkway and utility tunnels.


Kodiak Harbor Channel Bridge seismic retrofit.
Photo courtesy of Alaska Department of Transportation and Public Facilities.

  • In Maine, the DOT is wrapping fiber-reinforced polymer composites around old, un-reinforced concrete abutments on the Androscoggin River Bridge in the town of Mexico.

A new array of materials is now available for bridge building such as high-strength steel, high-performance concrete, rust-proof components and fiber-reinforced polymer composites. New technology includes electronic gauges to monitor the bridges in real time for stresses, strains, the “scouring” of water at their bases and for the weight of passing trucks.


Kings Stormwater Channel Bridge on Route 86 in Riverside County is constructed of non-traditional and composite materials.
Photo courtesy of http://hpwren.ucsd.edu


The advancement of bridge building is a combination of caution and innovation. One good example is the Kings Stormwater Channel bridge on California State Route 86. The bridge on the highly traveled NAFTA truck corridor is innovative in that it uses carbon-fiber-reinforced epoxy tubes filled with concrete instead of traditional concrete and steel piers. It also has a carbon-fiber deck, which is lighter and faster to construct than a concrete deck. The University of California at San Diego researchers tested full-scale models of the bridge in the laboratory before the California DOT authorized construction. Today, it is wired with nearly 100 gauges, which are monitored by researchers to meticulously track its performance.

Coping with the Assaults of Nature

Mammoth efforts are underway to address the most common causes of bridge failures­—floods, earthquakes and the damaging of bridges by collisions with barges and trucks.

Mammoth efforts are underway to address the most common causes of bridge failures­—floods, earthquakes and the damaging of bridges by collisions with barges and trucks.

A study of bridge failures in the United States between 1989 and 2000 found that floods and the “scouring” of support piers and abutments in large storms accounted for 53 percent of failures, while collisions caused another 12 percent.

Earthquakes

The FHWA Resource Center Structures team reports that the 6.4 magnitude San Fernando Earthquake in California in 1971 prompted increased attention to seismic design and detailing, triggering today’s modern seismic design codes. This earthquake collapsed or damaged 60 bridges and two hospitals in southern California. Since the development of new seismic design methods, billions of dollars have been spent on retrofitting older bridges, and new bridges built after 1971 include integral seismic enhancements.


Seismic retrofit bridge: a viaduct in Washington State.
Photo courtesy of Washington Department of Transportation.

Depending on the seismic zone in which the structure is located, the retrofit could be as simple as securing girders to the substructure with cable restrainers or extending the area upon which beams sit to give more support when they shake in an earthquake.

The California DOT pursued a comprehensive seismic retrofit program following the 1971 San Fernando Earthquake. As a result, during the 1994 Northridge Earthquake in southern California, which had a magnitude of 6.7, almost all seismically retrofitted structures were undamaged or only sustained minor damage that was quickly repaired.


Seismic Retrofitting Freeway Structures
Photo courtesy of Caltrans.

Similarly Washington State DOT completed its seismic retrofit program prior to the 2001 Nisqually Earthquake, which had a magnitude of 6.7. Again, many retrofitted structures were undamaged or sustained only minor damage that was quickly repaired.

Floods

The Federal Highway Administration and its state partners have developed extensive strategies to identify bridges that could be damaged by flooding. The most common problem during floods is that piers and abutments of bridges are undermined when intense flows wash away the earthworks surrounding them. Engineers in recent years have developed software and evaluation methods to predict such potential events. They also have developed enhanced training for inspectors, issued new standards and have enhanced modern designs to prevent scouring.


Rendering of the Skyway Bridge in Tampa Bay, Florida.
Rendering courtesy of FIGG Engineering.

Engineers are using many counter-measures that tend to fall into three categories: 1) they alter the stream flow to direct the strongest currents around the critical structure components; 2) they either armor the bridge with rock or other material to withstand the current; or 3) they redesign the structure to strengthen the bridge components within the streambed.

Collisions

Finally, bridges built today are more protected from collisions than in the past. Barriers are built around piers in navigable channels to ensure that ships cannot reach them. The Sunshine Skyway in Tampa, Florida, replaced an earlier steel structure that collapsed when struck by a barge. The new bridge’s piers are surrounded by “dolphins,” or the large, low barriers that protect its piers. They are designed to withstand the impact of a ship and keep the bridge safe.

 

New AASHTO Bridge Publications On the Way

Advancements in bridge safety continue to evolve. AASHTO is releasing two new publications this year as approved by the Subcommittee on Bridges and Structures at their 2008 annual meeting: a new edition of the AASHTO Vessel Collision Design of Highway Bridges Guide Specifications and a new Guide Specifications for Bridges Vulnerable to Coastal Storms, which deals with hurricane forces and flooding.

Summary

The safety, longevity and economy of U.S. bridges are being constantly improved by innovations in design, materials and technology. Using such advances, a new generation of safe and long-lasting bridges can be built, given the resources.

 

Bridges Move People:

Tennessee: Demonbreun Street Bridge—Bringing a Community Together
by Gayle Fuson; Courtesy of The Tennessean


Demonbreun Street Bridge.
Photo courtesy of Tennessee Department of Transportation.

In a sense, I’m a bit sorry to see work on the Demonbreun Street bridge over the Gulch come to an end.

I’m going to lose contact with a lot of friends whom I’ve gotten to know over the last three years—and I’ll miss out on my Thursday morning Krispy Kreme doughnut fix.

When the old Demonbreun bridge was condemned, I thought I faced my worst nightmare. I’m the chief financial officer at Bohan Advertising/Marketing, and the old bridge was connected to our building at 12th Avenue and Demonbreun.

Yes, the bridge and the building were one—bolted together and separable only with considerable effort, lots of noise, and perhaps some actual danger. I figured productivity would go out the window faster than the construction dust would come in.

As everything turned out, my worst nightmare never happened.

The people building the bridge—engineers, contractors, government officials—needed a place to meet for regular discussion, and my company had a conference room just the right size.
Every Thursday morning for months on end, we had a passel of visitors come in for a confab. I provided the doughnuts and coffee.

As we opened our doors to them, they opened themselves to us.

We in the building got a firsthand education in how a bridge is built. The construction team got to talk regularly with people who were interested in what they did for a living.

This isn’t to say everything was rosy. Employees were uprooted from their offices. There were days when the jack-hammering wouldn’t stop. There were water leaks of mysterious origin.
We took everything in stride and often with a sense of humor. We had our work to do, regardless of outside distractions and inconveniences. And the bridge builders had their job to do, which was to build “our” bridge.

Yes, we feel it’s ours—sort of a family project built with all our friends at Metro Public Works, Ray Bell Construction, Gresham & Smith, and the Tennessee Department of Transportation, consummate professionals every one.

Can coffee and doughnuts build a bridge? Not really, but they can be the bond between two disparate groups of people and teach them communication, compassion, humor, and understanding.

 

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