I-55 overpass closes to fit atomic cannon

’50s policy calls for span to be raised

When the Arkansas Department of Highway and Transportation Department closes an overpass on Interstate 55 in Mississippi County for a month beginning today, a contractor will be raising it high enough to fit an atomic cannon underneath it.

The atomic cannon was once the U.S. Army's largest artillery gun and the first capable of firing a nuclear device. Officially known as the M65 and dubbed "Atomic Annie," the towed artillery piece no longer is part of the nation's arsenal, having been in service from 1953 to 1963.

But the policy spawned in the Cold War lives on more than 60 years later, a span so long that state highway officials were unfamiliar with the policy.

The department said it is closing the Arkansas 150 bridge north of Blytheville in order to raise it 17 inches to meet a minimum vertical clearance of 16 feet between the bottom of the overpass and the top of the pavement. Agency officials cite a long-standing policy to maintain 16-foot vertical clearance on overpasses crossing interstates, which was put in place back in the Atomic Annie days.

A review of the Highway Department's bridge inventory last week found the Arkansas 150 bridge was one of 32 bridges over Arkansas interstates that have a minimum vertical clearance of less than 16 feet. U.S. 67/167 was included in the review because even though it isn't an interstate, it is built to interstate standards, said Danny Straessle, a department spokesman.

One source attributed the 16-foot standard indirectly to "Atomic Annie," which most sources say had a height of 12.2 feet, although it could have been higher if it was in its towing configuration.

"This particular specification was created to allow for military apparatus (such as the huge atomic cannon) to pass cleanly under an overpass," according to the interstate-guide.com, an online resource to interstates, which doesn't provide a source for its assertion. "Although military equipment of this size is not commonly transported on the Interstate Highway System (and the atomic cannon is no longer in use), the standard remains."

Official sources, such as the Federal Highway Administration, say the standard is a result of military needs, though those needs aren't specified. As recently as 2009, agency documents reflect that the military still wants the 16-foot clearance.

"The military continues to have a need for the 16-ft (4.9m) clearance," according to an April 15, 2009, memorandum from Dwight Horne, director of the Federal Highway Administration's program administration office, that is available on the agency's website. "While the size of future equipment that may use the Interstate System is unknown, [the military] needs to ensure options remain for the routing of military equipment."

An agency spokesman confirmed last week that remains the case.

"The requirement for 16 [feet of] clearance is for the safe rapid transit of military assets during deployment events," according to a statement the spokesman provided.

Though the justification for the interstate system was for its civilian uses -- such as economic development, safety and congestion relief -- the Department of Defense supported the system enough for Congress to add "and Defense" to the system's official name, National System of Interstate and Defense Highways, according to the Federal Highway Administration.

The vertical clearance minimum standard originally was 14 feet because that met civilian needs, according to the agency. The height of a typical tractor-trailer rig, for example, is 13 feet 7 inches.

But by 1960, the military had persuaded the agency to increase the standard to 16 feet and has periodically revisited the issue over the intervening decades with no changes. But neither the military nor the Federal Highway Administration is in a hurry to see the overpasses that don't meet the standard rebuilt.

"If the work necessary to obtain the clearance is a logical addition and the cost is not excessive, it should be incorporated into the overall project," the civilian highway agency said in a 1978 notice.

That is what Arkansas highway officials are doing, according to Rick Ellis, head of the bridge division for the state Highway and Transportation Department.

The work to raise the Arkansas 150 overpass is part of a $25.3 million project to rebuild a section of I-55 from just north of Blytheville to the Missouri border.

"We take the opportunity to better our system when we can," he said. "We want to take advantage of any efficiencies."

Typically, overpasses are raised by using a series of small hydraulic jacks placed between the bridge cap -- a horizontal member resting on the bridge column -- and the beams, said Highway Department spokesman Randy Ort.

Once the jacks are in place, they are raised slowly at the same time to the desired height, he said. At that point, steel blocks are placed in the extra space.

Ort said the Arkansas 150 overpass is being raised to a greater degree than similar projects. The project will require rebuilding the bridge approaches as well, he said. That may involve adding asphalt or even adding to the base structure on which the asphalt is poured.

As part of the department's $1.2 billion interstate repair program, adopted by voters in 2011, Ellis said, all contractors awarded jobs are required to review overpass clearances on their projects and, if they don't meet the vertical standard, figure out the best way economically to bring the overpasses to within the standard.

"While we're in there, we might as well raise the bridges," he said.

One other bridge has been raised on another 1-55 project awarded a contract under the program, Ellis said.

An earlier interstate repair program begun in 1999 resulted in dozens of bridges being raised, Ort said. Most of the bridges didn't require the extensive work that the Arkansas 150 overpass required, he said. As a result, the work typically was done in one night.

Department guidelines for new construction require interstate overpasses to exceed the standard, Ellis said.

They are built to have a vertical clearance of 16 feet, 6 inches because bridges typically last two or three times longer -- 50 to 75 years -- than the pavement below the overpasses. Pavements last 20 to 25 years.

The extra space in the vertical clearance allows at least one good overlay of 6 inches to keep the overpass within the minimum vertical standard.

Metro on 03/02/2015

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