Video: Officials weigh concern over bridges 8/14/07
Bridge load limits are a balancing act
It's hard to overload a bridge.
Most are built to hold lines of 80,000-pound trucks in side-by-side lanes, not to mention the mathematical cushion factored in to weight limits.
But the nation's aging bridges were not built with today's traffic volumes and overloaded tractor-tailers in mind. Half of all bridges were built before 1964. And the aging infrastructure is rapidly deteriorating.
The collapse of a highway bridge in Minneapolis Aug. 1 is expected to spur changes in bridge inspection standards that are seen as the best line of defense against ever-increasing traffic weights and volumes.
"The only real control is inspection, because you can't change the number of trucks," said Michael Chajes, chairman of the Civil and Environmental Engineering Department at the University of Delaware.
Immediately after the collapse of the I-35W bridge, federal officials called for all similarly designed structures to be inspected, or for the latest inspection reports to be reviewed.
Since then, states also have been advised to monitor load limits on bridges, especially during construction projects.
At the time the Minneapolis bridge failed -- it had passed regular inspections but was considered structurally deficient -- about 100 tons of gravel, as well as heavy construction equipment, were weighing it down.
Experts have speculated that the heavy loads could have placed too much strain on the bridge's gusset plates, which act as connectors for a bridge's interlocking steel parts.
Traffic and weight do decrease a bridge's design life, Chajes said, but with regular inspections and maintenance, there shouldn't be a problem.
Fatigue cracks are a function of the weight and the number of trucks, but such cracks are extremely slow-growing, and should easily be caught within the typical two-year inspection cycle, he said.
"The bridges in this state and the country in general are safe," Chajes said. "We just have to make sure we properly inspect them."
National standards require bridges to be built to withstand 80,000-pound trucks. Any trucks heavier than 80,000 pounds are required to seek an overweight permit, and any loads heavier than 120,000 pounds require permits to be reviewed by the state's bridge inspectors.
Each day Delaware inspectors review between five and 10 requests for such "super loads," and it's rare for a permit to be denied, said Darrel Cole, Delaware Department of Transportation spokesman.
In fiscal year 2007 the state granted 1,660 super load permits for trucks weighing more than 120,000 pounds. While data from previous years was not available, officials and inspectors say each year that number grows.
Such loads require police escorts, and no other trucks are allowed to be on the bridge at the same time, Cole said. Passenger vehicles are not restricted.
If a bridge can no longer hold the national standard, a weight limit is posted, he said.
Both the Rising Sun Lane bridge over the Brandywine and the St. Annes Church Road bridge over railroad tracks near Middletown -- two of four bridges the state considers vulnerable -- have posted limits, of 15 tons and three tons, respectively.
"If someone weighed over 3 tons and went on St. Annes, they could fall in and die," Cole said. "That's why we post them, because it's not safe. If trucks are dumb enough to go over the roadway, well, Godspeed to them."
In September and October, inspection crews will examine the four bridges the state considers most vulnerable because of their "fracture critical" design and structurally deficient rating. "Fracture critical" means the entire bridge could collapse if just one beam or pin fails. Structurally deficient bridges score a 4 or lower on the federal 0-9 inspection scale.
The James Street and Walnut Street bridges over the Christina River also are considered vulnerable.
Each of those bridges is safe, Cole emphasized. "If it were unsafe, we'd close it," he said.
'Continuing to fall behind'
One in every eight bridges in the United States, or 12.5 percent, is structurally deficient, according to the U.S. Department of Transportation.
The agency estimates that $495 billion is needed to address all the current deficiencies.
Delaware's bridges fare better than the rest of the nation's, with only 2.7 percent, or 38 of 1,426 bridges, categorized as structurally deficient.
"Delaware does a real good job of taking care of safety issues first, so the number of structurally deficient bridges is very low," said John Grieshaber of the Delaware Section of the American Society of Civil Engineers, which in June gave the state's bridges a "B" on its infrastructure report card.
Delaware struggles with "functionally obsolete" bridges -- outdated bridges not up to standards, such as those with narrow lanes or low guardrails -- that often receive weight restrictions, Grieshaber said.
In 2004 there were 98 functionally obsolete bridges. By 2006 that number had jumped to 131. "Because of a lack of funding, we're continuing to fall behind," Grieshaber said.
Before the General Assembly approved higher tolls and vehicle fees, the state was about $1.5 billion shy for anticipated road needs over the next six years. About $30 million a year goes toward bridge maintenance.
Last week the Delaware Department of Transportation started inspecting the Augustine Cut-off bridge over the Brandywine, and so far tests have shown the span is structurally sound, Cole said.
That bridge has a steel deck truss design -- one of only three in Delaware with the same design as the structure that collapsed in Minneapolis two weeks ago. The inspection, which includes ultrasonic testing to see inside the steel, is scheduled to wrap up next week.
Also next week the department will begin its regular six-month inspection of the Indian River Inlet Bridge.
Like the Minneapolis bridge, the 250-foot northbound and southbound approaches to both the Reedy Point and Summit bridges, which are maintained by the Army Corps of Engineers, also are steel deck trusses, so a team is reviewing the spans' inspection reports from last summer, said Cameron Chasten, a structural engineer with the Army Corps of Engineers' Philadelphia District, which includes Delaware.
So far no red flags have been found that would trigger another inspection before next summer, he said.
Weight test
The reality, Chajes said, is that most bridges are stronger than we think.
For example, a few years ago the Delaware River & Bay Authority allowed Chajes and other researchers to experiment with a bridge that was scheduled for demolition.
They placed 2 million pounds of ground-up asphalt on the bridge, the pile reaching eight feet high and eventually spreading out across the entire span.
Nothing happened.
The bridge showed no signs of weakening or cracking, Chajes said. And the length of time the additional weight sat on the bridge made no difference, he said, because a bridge either can sustain the weight or not.
"That bridge might have been five times stronger than what we conservatively thought it would be," Chajes said. "Bridges just don't tend to get overloaded."
Contact Summer Harlow at 324-2794 or sharlow@delawareonline.com.
