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On a summer afternoon in 2009, a semitruck carrying apples over the Columbia River near Wenatchee abruptly veered left and smashed through an aging bridge.

The cab broke loose and plunged into the river, killing the driver and her husband. The accident mangled trusses on the 50-year-old Highway 97 Beebe Bridge and left one deformed beam holding half the bridge’s weight. The deck sagged ominously.

Still, the bridge stayed aloft.

Whether a span will withstand punishment, as in the Wenatchee area, or collapse as happened Thursday over the Skagit River, depends upon the designs, forces and even an element of chance.

Thursday’s collapse on Interstate 5 was triggered when an overheight truck struck one or more overhead crossbeams, causing an entire truss to split. Three people survived falling in their vehicles to the river, and a major travel route will be severed for weeks or months.

“These kind of accidents have happened in the past,” said De­Wayne Wilson, bridge management engineer for the state Department of Transportation (DOT). Whether a bridge stays standing “all depends on the details.”

There were 21 bridge strikes involving trucks last year, and 24 during 2011.

While Washington’s bridges are generally in good shape, there is no shortage of bridges vulnerable to failure.

Of the state’s more than 7,700 bridges, inspectors believe 95 percent are in good health.

But that still leaves hundreds of aging bridges that could collapse in a bad confluence of events.

Several bridges on Interstates 5 and 90 are considered “fracture-critical,” meaning that if a single major part is broken, the entire span could fall. Others are “structurally deficient,” meaning they have some kind of defect that needs attention.

Twelve bridges that carry an average of 10,000 or more vehicles a day fall into both categories.

One sits just 19 miles from the Skagit, where the southbound lanes of I-5 cross the Stillaguamish River on a steel-truss structure built in 1933. Another is the East Fork Lewis River bridge in Woodland, Cowlitz County.

The picturesque Nisqually River bridge on I-5, between Tacoma and Olympia, is also fracture-critical, lacking the redundancies modern bridges rely on to stay erect in severe crashes.

Compared with other states, Washington ranks sixth-best for having the lowest percentage of bridges needing repair.

But the bridges here are aging fast.

More than one-third are at least 50 years old. The Federal Highway Administration calculated in 2011 that Washington has a $28.6 billion backlog of work, ranging from $6.3 billion in safety repairs to $15.1 billion in efficiency improvements.

The Seattle section of the American Society of Civil Engineers recently gave the state a mediocre grade for bridges — C-minus — in part because over the next 20 years the problem is likely to get worse.

There are 143 state-maintained bridges listed as structurally deficient. Of the 66 of those bridges most in need of repair, the newest was built in 1989 and the rest before 1970.

Forty-one were built in 1955 — the year the Skagit River bridge opened — or earlier.

“I think they’re trying, and I think everyone is aware of the problem, but it’s the prioritizing,” said Tom Whiteman, a consultant with KPFF Engineering in Seattle. “There’s a lot of important needs out there. You’d like to fix them all, but that’s not possible.”

“Particularly bad luck”

On Friday, Gov. Jay Inslee and state Transportation Secretary Lynn Peterson sought to reassure the public that highway bridges are safe.

“This was particularly bad luck of where it was hit and how it was hit,” Peterson said. “It’s basically not a structural deficiency with the bridge itself.”

Inslee quipped that one solution was for truckers to avoid hitting state bridges.

Jeffrey Berman, associate professor of civil and environmental engineering at the University of Washington, said truss bridges like the Skagit River bridge have served the state well and can continue to do so for many years, if maintained properly.

“It really is kind of a freak accident,” he said of Thursday’s crash.

Whiteman, the consultant, said, “Trusses are good bridges. They’re all over the world. They’re all over the country. They’ve done a good job. But they are aging.”

National Transportation Safety Board (NTSB) chairwoman Deborah Hersman said the steel-truss Skagit bridge is “showing its age,” in the sense that traffic has become much busier and loads heavier than imagined when the bridge was in the 1950s.

Without criticizing that particular bridge, she said there’s a great need to repair and replace spans around the country.

“We do have a lot of infrastructure that’s been around for a long time. We have to invest in it. They’re like our house. We have to maintain our house,” she said.

In recent years, the state transportation department has favored concrete-box girder bridges, in which massive, lengthwise beams support the road deck from below. There aren’t overhead structures.

If the Skagit bridge were new, clearance wouldn’t have been a risk factor.

But some aging bridges are at a point where the transportation department apparently depends on a trucker to aim the steering wheel toward the left or court disaster.

Toward the center of the Skagit River bridge, clearance is 17½ feet.

But William Scott, the driver of the truck that struck the span, told federal investigators that while he was off to the right — where the clearance is 14½ feet — hauling his wide load, another truck was in the left lane. The height of the load on Scott’s truck was estimated at 15¾ feet.

“He said he saw this truck approaching and passing him as he was traversing the bridge,” said Hersman, of the NTSB, during a news conference Saturday.

Scott also told investigators that he heard no warning from a pilot-car driver in front of him about the impending disaster.

The impact shattered the one span instantly, as shown on security video from an RV business nearby. In all, the load appeared to have damaged or nicked 10 overhead cross members of the bridge. A second span of the bridge is damaged and will require repair.

Vulnerability varies

Crash damage can vary dramatically, depending on the characteristics of a bridge.

In 1984, a logging truck wiped out the Skokomish River bridge across State Highway 106 near Shelton, a structure considered fracture critical.

When a truck hit a bridge near Easton, Kittitas County, on Interstate 90 a few years ago, “it wiped out a whole span,” Wilson said. But that bridge didn’t collapse, because it was a more modern concrete girder structure.

“We’ve had three in the last year or so where over-height trucks have run into a pre-stressed concrete girder,” he said. All were repaired quickly.

But the whole category of “fracture-critical” can be vague — it’s intended not to foretell crash risks, but to require preservation and maintenance so that parts won’t fail, says Charles Roeder, UW professor of civil and environmental engineering.

Even the modern concrete bridges are “fracture critical” in their own way, he said, if a crash or failure affects one of the few girders below deck.

Simple corrosion can destroy a bridge, as occurred in 2007 in the Minnesota I-35W bridge collapse, which killed 13 people. Steel components below deck failed at gusset plates, which secure truss beams to each other.

Construction attorney Barry LePatner, author of “Too Big to Fall,” estimates there are 8,000 bridges nationally that are both structurally deficient and fracture-critical.

“Every time you hear that, you know they’re playing a game of Russian roulette with the traveling public,” LePatner said. He suggests electronic monitoring devices that give constant data be installed on every such bridge.

The UW’s Berman, who did corrosion research for the NTSB, said the Washington DOT is “one of the most progressive” for vigilance in bridge maintenance.

There is no clear trend on whether the state’s backlog of repairs is getting better, or worse.

Between 2007 and 2011, the number of those bridges in need of repairs dropped from 400 to 391. That figure includes bridges maintained by cities or counties.

But the percentage of bridge deck areas in need of help grew by almost 50 percent during that time.

State records show only one other collapse since 2000, when a lightning-caused brush fire destroyed a tiny Yakima County bridge four years ago.

The 1980 eruption of Mount St. Helens smothered five bridges, floating bridges have sunk twice, and in the most famous failure of all, the 1940 Tacoma Narrows Bridge known as “Galloping Gertie” twisted and crumbled into Puget Sound weeks after opening.

A matter of money

On Friday, the transportation department signed a contract with Atkinson Construction to remove the Skagit River wreckage and build a new span suitable for permanent use.

DOT hasn’t decided yet whether to install a temporary fix, such as a military-style “Bailey bridge,” to carry traffic while the permanent bridge is being built around it.

Meanwhile, the collapse of the Skagit River bridge could play a role in the state Legislature’s debate about whether to pursue a $8.5 billion transportation package that would be financed largely with a hike in the gas tax.

Among the most fiercely disputed parts of the transportation proposal is whether the state should spend $450 million, along with money from Oregon and the federal government, for a $3.5 billion bridge on I-5 across the Columbia River between Vancouver, Wash., and Portland.

Like the Skagit bridge, the old Columbia River bridge is a truss design, with one span built in 1917 and another in 1958. The bridge is considered fracture-critical.

In a 2011 report, the bridge was rated in fair condition, according the National Bridge Inventory. And, over the decades, the bridge has withstood numerous accidents involving vehicles that hit the spans, said Patrick Cooney, a spokesman for the Oregon DOT.

“They (the spans) get hit frequently, and we have never seen a catastrophic collapse,” Cooney said.

Chris Higgins, an Oregon State University engineering professor who studies truss bridges, said a much likelier threat to the existing bridge would be a major hit by a ship rather than a vehicle accident.

Hal Bernton, Steve Miletich, and The Associated Press contributed to this report. Mike Lindblom: 206-515-5631 or On Twitter @mikelindblom