Bridge safety a bit shaky

Sunday, Sept. 16, 2007 | 1:26 a.m.

With last month's fatal bridge collapse in Minneapolis raising nationwide concerns about bridge safety, Nevada officials say the potential threat they need to worry about most deals not with the spans' structural deficiencies, but earthquakes.

The Nevada Transportation Department has compiled a list of 142 state bridges, including 30 in Clark County, that might be eligible for seismic retrofitting to make them better equipped to survive earthquakes.

All Southern Nevada bridges on the list, with one exception, are either on Interstate 15 or U.S. 95 and were built before 1983, when earthquake standards in building codes for bridges were substantially upgraded. Most I-15 bridges on the list are northeast of the Spaghetti Bowl and all but one of the U.S. 95 spans are southeast of the Spaghetti Bowl.

Most of the bridges are in corridors that the state Transportation Department plans to widen or otherwise upgrade to handle more traffic. That means the bridges could be retrofitted when other improvements are made.

The potential obstacle, as is so often the case, is money.

The Transportation Department this year reported that it had a $134.3 million backlog of bridge work, including $75 million in seismic retrofitting.

"Highway maintenance funding in general is lacking and bridge work mirrors that," said Todd Stefonowicz, the department's assistant chief bridge engineer.

In its 2007 State Highway Preservation Report, the department blamed the backlog on "huge highway construction inflation that was not matched by revenue increases from gasoline taxes and vehicle registration fees."

The stretch of U.S. 95 immediately southeast of the Spaghetti Bowl that is known as the downtown viaduct - a hollow structure propped up by 100 diamond-shaped columns running from the railroad tracks to 21st Street - is the county bridge with the highest seismic risk.

The state was so concerned about the bridge, built in phases beginning the late 1960s, that a $450,000 engineering study was commissioned in 2001 to determine the extent to which an earthquake could damage the viaduct and how it could be strengthened.

The study, led by UNR civil engineering professor M. Saiid Saiidi, who specializes in earthquake engineering, involved testing a quarter-scale version of the viaduct on motion equipment that can mimic quakes of various magnitudes. For comparison, the researchers used the 1994 Northridge earthquake in Southern California , which registered a magnitude of 6.7, killed 72 people, injured 9,000 and caused $15.3 billion in damage.

At one-quarter the strength of the Northridge temblor, or about magnitude 4.5, deep diagonal cracks known as shear cracks formed at the ends of beams beneath the road bed. At slightly more than one-third the strength of Northridge, or roughly magnitude 5.0 , vertical flexural cracks appeared in the columns.

But, Saiidi said, the viaduct likely could withstand an earthquake as strong as 6.5 to 7.0 before suffering damage forcing its closure.

"If there's a strong earthquake, the bridge will drop 4 to 5 inches, but it probably wouldn't collapse," Saiidi said. "It's unlikely you'll see what you saw in Minnesota because of the redundancy you have with so many columns."

The bridge that collapsed into the Mississippi River on Aug. 1 in Minneapolis, killing 13 people, was a so-called deck-truss structure. Nevada has only one deck-truss bridge - across a wash in Caliente, 180 miles northeast of Las Vegas - and it is supported by four trusses, two more than the Minneapolis structure, giving it greater stability.

Saiidi's team recommended that the downtown viaduct be seismically retrofitted to make the columns less likely to buckle.

The most common ways to retrofit a bridge column are to surround it with steel casing and grout or wrap it with a fiberglass or carbon material coated with epoxy resin.

Two years after the study was launched but before it was completed, the state Transportation Department began studying widening U.S. 95 southeast from the Spaghetti Bowl to Henderson.

The recommendation , pending an environmental impact statement and funding, would widen that corridor from six to 10 lanes. It would mean at least part of the viaduct would be upgraded and existing columns would be retrofitted at a n estimated cost of at least $10 million.

But construction is not scheduled to begin until at least 2012. In other words, the Transportation Department is taking a chance - admittedly remote - that a big quake won't strike Las Vegas in the next five years.

"Are we rolling the dice with the downtown viaduct?" Saiidi said. "Yes. If there's a significant earthquake in Las Vegas between now and 2012 ... there could be significant damage."

Another civil engineering professor who has studied Clark County bridges and their vulnerability to quakes, Arya Ebrahimpour of Idaho State University in Pocatello, was with a team of researchers that this year released a study identifying six county bridges it says are at greater risk than the downtown viaduct.

Topping its list was the I-15 southbound bridge over Bonanza Road. As with the rankings used by Transportation Department, Ebrahimpour's team - which included UNLV associate engineering professor Barbara Luke - considered both the bridges' vulnerability and traffic volume, but it used different methodology. Four of the six bridges are on the Transportation Department's list of the bridges with the highest seismic risk, the exceptions being U.S. 95 spans over Stewart Avenue and Gibson Road.

Although Nevada is not commonly thought of as an earthquake-prone state, it ranks third behind Alaska and California in seismic activity and is no stranger to big shakers.

From 1915 to 1954, the state recorded four quakes with a magnitude of 7.0 or greater, centered in the Northern Nevada communities of Fallon (twice), Winnemucca and Gabbs. Since the 1850s almost all of Nevada's temblors with a magnitude of at least 5.0 have been centered north of Clark County.

Engineers and researchers, though, caution against overconfidence, because the Las Vegas Valley has eight key faults capable of producing quakes of magnitude 6.5 to 7.0. As geologists are fond of saying, wherever there are mountains , there is bound to be seismic activity close by.

"Just because there hasn't been a big earthquake in Las Vegas in 150 years means nothing," said geophysics professor John Louie, associate director of the Nevada Seismological Laboratory at UNR.

"We don't think Clark County has an exceptional earthquake hazard, but we can't prove it. We're just starting to get a handle on earthquakes in Las Vegas."

Using data from the U.S. Geological Survey, the Nevada Bureau of Mines and Geology reported there is less than a 1 percent chance that a quake measuring at least 7.0 will be centered within 30 miles of Clark County in the next 50 years. That probability rises to less than 5 percent for an earthquake of at least 6.5, 10 percent to 20 percent for one at least 6.0, and 40 percent to 50 percent for a moderate temblor of at least 5.0.

The strongest quakes ever recorded in Clark County were two 5.0 readings that struck near Lake Mead in 1939 and 1952, according to the seismological laboratory.

The Sun reported the day after the 1952 quake that it probably was caused by an unprecedented flow of millions of tons of snowpack runoff into the lake via the Colorado River. Although no injuries or major damage were reported, the Sun reported that "buildings and homes swayed, furniture dislocated and dishes and windows rattled with force."

Shortly after the Minneapolis bridge catastrophe, the Nevada Transportation Department issued a news release that proclaimed, "Nevada Bridges Named Nation's Best," citing a Better Roads magazine survey. The department reported that only 4 percent of the state's bridges - roughly 70 of 1,800 - are considered structurally deficient or functionally obsolete, separate categories from bridges that need seismic retrofitting. That's the lowest percentage in the nation based on federal funding guidelines.

Although the designations do not mean the 70 bridges are unsafe, some have weight restrictions or other limitations on traffic.

Of the bridges, only six are in Clark County and only three are in the valley - the Martin Luther King Boulevard interchange at U.S. 95, Warm Springs Road at I-15 and Pilot Road at the Airport Connector near McCarran International Airport. Martin Luther King and Warm Springs will soon be off the list because of widening of the former and replacement of the latter.

The Transportation Department stated in its 2007 highway report that because Nevada bridges "are relatively young and located in a generally warm, arid climate, they are in good condition compared to bridges in most states."

The agency inspects every state bridge at least once every two years. Because bridges normally last about 50 years, the Transportation Department anticipates the state will face major bridge work beginning in 2010, when the 360 bridges built in the 1960s - the most in any decade - will near the end of their life expectancy.

"I would say there's pretty significant concern that if we don't invest significant funds into our bridge structures, we'll be in a lot of trouble," Stefonowicz said. "The inventory is reaching that age where things will require a lot more work to keep them functioning."

Sun reporter Mary Manning contributed to this report.

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