Preserving the Past, Building for the Future
Blaine Bridge and the Veterans’ Glass City Skyway (inset).
Photos courtesy of Ohio Department of Transportation.
Bridge engineers often face a dual challenge—preserving historic bridges while designing bridges for the future.
Two bridges in Ohio offer good examples of these challenges.
In 2005, a $1 million enhancement project provided by the Ohio DOT repaired the 1828 stone arch Blaine Bridge, one of a handful of stone “S” bridges remaining in Ohio from the original National Road, which was built from Cumberland, Maryland to the then-western frontier in Illinois. They were called “S” bridges because they actually curve to bring the road to a crossing perpendicular to the stream below.
An Evolution of Technology
The history of American technology can be traced through its bridges. Bridges evolved from wood and stone to iron, steel and then concrete. Steel trusses represented the rising dominance of the American steel industry. Beginning in the 1870s and continuing through the 1930s, steel was the most common bridge building material. The truss was one of the most common types of structures. Departments of transportation have inventoried these historic bridges, documented their pasts and try to preserve them for the future when they are not on high-volume roads where their use would pose a risk to the public.
In 2007, citizens of Toledo jogged and strolled across the newly opened Veterans Glass City Skyway. Their new $234 million cable-stayed bridge represented the largest and most complex project ever undertaken by the Ohio DOT. It was a concrete segmental bridge, which meant it was built from pre-cast concrete sections, which were hoisted up and tied together with internal cables. Its tower rises 380 feet above the Maumee River and is lit with colored, low-energy LED lighting, which can change color to celebrate the seasons.
These two projects represent the twin spectrums facing state bridge engineers today. They work to save and preserve historic bridges. At the same time, they are building a new generation of what will become the historic bridges of the future.
And occasionally, bridge engineers use elements from today’s new technology to preserve the bridges of the past.
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Engineers at the Iowa Department of Transportation worked with researchers from Iowa State University to study the use of remote sensing to protect the “Bridges of Madison County.” They installed flame detection devices, infrared cameras and fiber-optic strain gauges on one of the covered bridges. These will monitor the bridges for fire or other potentially damaging events and immediately notify law enforcement of suspicious activity. Similar technology protects covered bridges in Illinois.
The Chesapeake & Ohio Canal flows under the stone masonry arch of the Wisconsin Avenue Bridge in the busy
Georgetown area of Washington, DC. The towpath beside the canal serves as a footpath and recreation resource.
The original wrought-iron railing dates back to 1831.
Photo courtesy of FHWA, Public Roads Magazine,
March/April, Vol. 68, No. 5.
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In the historic Georgetown section of Washington, DC, the Federal Highway Administration worked with other federal agencies to preserve the 1831 Wisconsin Avenue Bridge. The stone arch still carries daily traffic but was deteriorating under the load. An innovative solution of inserting stainless steel rebar into the structure without altering its appearance provided a safe and historically compatible solution.
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The Pennsylvania DOT is helping to preserve the 1913 Chester Spring Road stone arch in Chester County. The greater Philadelphia area has the country’s largest collection of stone arches. PennDot has developed a management plan to preserve and protect these arches whenever possible.
Similarly, states from Maine to California are preserving their covered bridges, such as the Hoffman Bridge in Oregon. Approximately 900 covered bridges remain in the United States, while at one time there were more than 14,000, according to the FHWA Covered Bridge Manual. Annually the Federal Highway Administration’s Covered Bridge Program provides about $9 million for preservation and research regarding the protection of the remaining covered bridges. In addition, the State DOTs regularly invest their own enhancement funds into the preservation of historic bridges, as do local officials.
Veterans’ Glass City Skyway in Toledo, Ohio.
Photo courtesy of Ohio Department of Transportation
Cooper River Bridge.
Photo courtesy of Rob Thompson, South Carolina Department of Transportation.
Technology, New Materials Inspire a New Generation of Bridges
Today, bridge designers have new materials and technology with which to build a new generation of structures. The Toledo bridge is like many modern structures that use high-performance concrete and stainless steel to build stronger, longer spans for much less cost than older materials would have allowed. Its tall pylon is elegant but strong using 10,000 pound-per-square inch high-performance concrete. The tower has a modern stainless steel “cradle” atop it which routes more than 1,500 miles of special steel cable. In these cable-stayed bridges, the massive anchorages of conventional suspension bridges are avoided because the large deck spans balance on either side of the tower, saving costs.
Rendering of the Mike O’Callaghan-Pat Tillman Memorial Bridge that will be built between Nevada and
Arizona near the Hoover Dam.
Photo courtesy of Nevada Department of Transportation
Completed Tacoma Narrows Bridge.
Photo courtesy of Washington Department of Transportation.
The Cooper River Bridge in Charleston, SC, is another example of strength, safety and aesthetics made possible by modern materials and technology. The diamond towers rise 575 feet into the air thanks to high-performance concrete. The towers anchor 128 steel cables, each of which can hold 500 tons. The Cooper River Bridge is the longest cable-stayed bridge in America and has replaced the old, narrow truss seen in the background.
Another example is in the arid canyons surrounding the Hoover Dam between Arizona and Nevada. When it was constructed in the 1930s, a roadway was built on top of the dam, which has since become a traffic bottleneck. The new Hoover Dam Bypass is made possible by the elegant Colorado River Bridge. The first 1,060-foot concrete arch is the centerpiece of a 2,000-foot-long bridge, which will span the Black Canyon about 1,600 feet south of the dam. When completed in 2010, the bridge and bypass will save significant travel time and expedite trade throughout the region.
The Natchez Trace Bridge in Williamson County, Tennessee, used a first-of-its kind concrete arch design to reduce piers and other impacts across the scenic mountain valley. The use of the new design allowed the construction impacts to be minimized while also creating a new landmark that complements its beautiful surroundings.
The new bridges allow a degree of safety unknown in the past. Even in seismically active areas such as northern California, new, massive, iconic structures can be constructed with new techniques and materials to withstand earthquakes as never before. The East Span of the new San Francisco-Oakland Bay Bridge Project represents a state-of-the art marriage of safety, technology and aesthetics. The new bridge is designed to include bearings and shafts that can absorb the movement of an earthquake while protecting the massive new bridge. The bearings and shafts are designed to be replaced after an earthquake. These sacrificial components will absorb the shock and movement of the quake while leaving the bridge undamaged.
Workers on site of I-94 business loop construction of Memorial Bridge North Dakota.
Photo courtesy of Mike Kopp, North Dakota Department of Transportation.
The East Span also represents the world’s longest Self-Anchored Suspension bridge. Traditional suspension bridges generally have two cables anchored into massive concrete structures called anchorages set into the ground. The two anchorages usually hold both ends of the cables, much as two sets of people playing tug-of-war stretch a rope between them. From the taut cable, the bridge deck can be suspended. In the new Self-Anchoring Suspension of the East Span, the anchorages are not needed since the bridge cables are anchored into the bridge deck itself.
Summary
U.S. transportation agencies are simultaneously protecting the historic bridges of the past while also creating a new generation of signature spans that will serve as icons for decades into the future. These new bridges represent the best of modern technology, materials, engineering and construction techniques. They illustrate how it is possible to build safe, efficient structures while at the same time complementing their surroundings. These investments will serve their communities for decades to come and become the new symbols for their communities.
