A former chairman of the National Transportation Safety Board, who just happened to be in the Twin Cities the day of the bridge collapse, raised some interesting questions. Jim Burnett was interviewed by the Star Tribune today and said that bridge collapses usually have two main causes: vibration and fatigue cracking.
Earlier inspections of the bridge have noted that it showed signs of fatigue cracking, but not to the degree that officials felt the bridge needed immediate attention. But I haven’t heard a lot of comment in press accounts, yet, about vibration. Burnett pointed out that two primary causes of vibration were going on or near the bridge at the time of the collapse: construction work on the bridge’s road deck and also vibrations from a train passing underneath it.
“Vibration is one of things that cause cracking to propagate," he said to the newspaper. "They will be looking at that."
He was on the scene of the collapse at 5 a.m. today (Aug. 2) before sharing his preliminary thoughts on the collapse. But his analysis got me thinking to the role vibrations have played in other bridge failures.
Remember the Hyatt Regency hotel walkway failures in Kansas City in 1981? About 2,000 people were there for a dance contest. The hotel featured several walkways suspended at the second, third and fourth levels over the hotel’s atrium. When people started dancing on the walkways, the vibration of the feet moving together caused them to collapse, killing 114 people and injuring more than 200 others.UPDATE: See the comments below for clarification on the Kansas Hyatt disaster.
Digging around a little more, I found these postings by engineers at the Ask a Scientist website.
Here’s some observations from engineer Vance Calder: An army troop marching at full cadence is ordered to stop marching and walk across a bridge at each soldier’s own individual gait. The "in time" stepping produces vibration, the multitude of soldiers adding to the vibration. When trying to think of waves, think of waves in water. Opposing waves can cancel each other. But like the troops moving in the same direction, the waves can add to each other and gain extra strength.
More points from engineer James Prxewoznik include: Vibration, in general, is bad for materials. It can lead to fatigue of parts and eventual failure. There are two types of vibration: free vibration and forced vibration. Free vibration occurs through actions of forces inherent with its design. Forced vibrations come from external forces outside the design of an object. If those two forces coincide, the vibration oscillations can magnify causing an object to come apart.
There might have been a lot of vibration at play on the 35W bridge: jack-hammering and other construction work, the train passing underneath, and don’t forget, it was an extremely windy day in Minneapolis on Aug. 1. A lot more needs to be checked out, of course. But investigators will likely be looking at how many different forms of vibration combined on the bridge at the time of its collapse.
A new book, Cape Wind: Money, Celebrity, Class, Politics, and the Battle for Our Energy Future on Nantucket Sound, tells the story of efforts to build wind turbines off the coast of Cape Cod to provide clean, renewable energy for the state of Massachusetts. However, some of the wealthy people who live in the area – including some renowned environmentalists – object to the project located so close to their own homes.
This article from the Cape Cod Times describes some of the legal maneuvering that has thus far blocked the project. One objection is that wind turbines kill migrating birds. The reporter did some research and came up with the following statistics:
Human-caused bird deaths
• Domestic cats: Hundreds of millions a year
• Striking high-tension lines: 130 million - 1 billion a year
• Striking buildings: 97 million to 976 million a year
• Cars: 80 million a year
• Toxic chemicals: 72 million
• Striking communications towers: 4 to 50 million a year
• Wind turbines: 20,000 to 37,000
Source: National Research Council
Clearly, turbines are not a major threat to birds, while the clean energy they provide would be a major boost to the environment. So why are some environmentalists opposed? The authors of the book say it’s because the turbines, several miles off the coast, would still be visible from their beach-front property. (It is also interesting to note that some of the anti-turbine legislation has been proposed by congressmen from states that just happen to produce a lot of coal.)
For an overview of the issue, read this article from The Boston Phoenix.
Even as the fires were crackling through the branches of pines, birch and spruce, the start of new tree growth was already popping. The heat from the burning trees pops open the cones on those trees, releasing seeds that have been waiting to get loose for years. Millions of those seeds were dumped on the forest floor and within less than a month, a fraction of those seeds to germinate, pop through the soil and start growing into little seedlings. Give them another 30 or 40 years, and hikers going through the area where this year’s fires have wiped out most trees, those seedlings will have grown tall enough that hikers won’t even know the area had been devastated by fires in 2007.
In fact, fire is a natural part of the forest’s regeneration system. Most forest trees need to be exposed to fire every 50 to 100 years to invigorate new growth. As we found out in Yellowstone National Park nearly 20 years ago, suppressing forest fires too long can actually be detrimental to forests. Extreme efforts to prevent forest fires there led to a huge consumption of trees when fire finally broke out.
University of Minnesota-Duluth biology professor John Pastor was quoted this week in the Star Tribune saying: “When people canoe through the Boundary Waters Canoe Area (BWCA) and see all these beautiful forests, every one of those forests started with a fire.”
As a rule of thumb, timber experts say that any particular chunk of ground in the forest should be touched by intense fire every 50 to 100 years.
But the power of the fire is just the first step in forest regrowth. Weather patterns in the affected area over the nest year will play a big role in how the new forests develop. A summer of drought could kill the newly released seeds and short-circuit any new growth. That could give new species of trees a chance to grow in the area. Normal rains mixed with the nutrients left on the ground from the fire could be a great booster shot to getting the seeds off to a flying start.
Other natural benefits can be seen from fires. For instance, the once-rare black-backed woodpecker is now a regular site in the BWCA with the abundance of dead trees from recent smaller fires and the 1999 wind blow down of trees. New shrubs and ground vegetation is appealing to different kinds of wildlife to snack on.
Ever wanted to be a storm spotter? Now's your chance! The National Weather Service (NWS) relies on local SKYWARN storm spotters to confirm, from the ground, what meteorologists are seeing on radar. NWS storm spotters are not tornado chasers like the folks in the movie "Twister." Instead, they report wind gusts, hail size, rainfall, cloud formations, and the like to NWS and local emergency management agencies.
New radar equipment is still not sensitive enough to determine the existence of an actual tornado. It can only predict where severe weather is likely to occur. So the NWS needs trained volunteers to verify actual severe weather.
With peak storm season just around the corner (mid-June here in the Upper Midwest), free, 2.5-hour classes are being offered to train new SkyWarn volunteers.
The 2007 Atlantic hurricane season officially begins June 1 and runs through November 30.
Check back often for the latest predictions, forecasts, and discussion.
An earlier thread discussed the movie An Inconvenient Truth. Now, a British television network has produced a documentary of its own, entitled The Great Global Warming Swindle. (Streaming video; one hour and 13 minutes.) The film interviews many top scientists who disagree with the theory that human-produced CO2 causes global warming. It offers compelling evidence that climate change is driven by the Sun. And it ends with a rather disturbing look at how science and politics have influenced each other, with potentially dire consequences.
Regardless of how you feel about climate change and global warming, it's worth watching to film, to hear another side of the debate.
This summer will mark the 40th anniversary of Loving vs. Virginia, a United States Supreme Court decision which declared state laws against inter-racial marriage were unconstitutional. In 1967, inter-racial marriages were illegal in 27 states, and comprised only 2% of all marriages. Today, 7.5% of marriages are inter-racial. Experts cite two reasons for this increase:
1) thanks largely to immigration, the US is a more diverse country today than it was 40 years ago. People of different races have a much higher chance of meeting and falling in love.
2) There has been a trend toward marrying later in life. An adult in their 30s or 40s is less likely to submit to parental pressure than someone in their early 20s.
All this has led to a great change in attitudes. In 1972, some 39% of Americans still felt there should be laws against inter-racial marriages. 30 years later, that had shrunk to 10%, and was even smaller among young people, meaning this taboo is clearly fading from American life.
This Thursday (Feb 15) at 10 am Reno, Nevada time, a quarter scale, 110 ft., 4 span bridge section will be shaken by forces twice the intensity of the 1994 Northridge earthquake.
The Network for Earthquake Engineering Simulation (NEES) website has provided links to a variety of resources:
Storm chasers know that puffy cumulus clouds often cause sudden rainstorms, while storms associated with stratus clouds form more slowly. Now physicists at England’s Open University have finally found an explanation.
They propose that neighboring water droplets in a stable stratus cloud don’t crash into each other because they’re all moving at about the same speed. But fast-forming, turbulent cumulous clouds contain water droplets moving at many different speeds. They crash into each other and form larger drops. As the turbulence grows, the drops grow quickly and fall as rain within a few minutes.
Sun and rain
Ever noticed the bright, moving lines on the bottom of a stream, bathtub, or swimming pool? They’re called caustics, and they’re caused when ripples on the water’s surface focus sunlight. (Caustics form whenever light rays are bent by a curved surface or object and then projected onto another surface.
Caustics have a characteristic shape. Physicists can graph the phenomenon mathematically, and the graph also describes other phenomena, such as particle motion or the movement of raindrops within a cumulus cloud.
Atmosphere to outer space
The researchers say their finding won’t have any impact on weather forecasting. But particle collisions in turbulent gases must have been involved in planet formation. Perhaps the same theory can be applied?
If you're at the museum on Saturday afternoon (11/18), the MakeIt team can help you play with caustics. Does bending mylar in a different direction produce a new pattern? Does using a different color flashlight or a brighter or dimmer light affect the design?
You can also play with caustics at home.
At the State Fair I observed as several farmers were researching whether a 1.5 million dollar wind turbine would make them money. The biggest factor was how much wind was available where they lived.The break even point was if they had better than 7.5 mph average wind speeds( see map pdf). Apparently several banks and also John Deere are financing projects if the numbers look good. Power companies will give a 20 year contract to buy electricity. The wind generators usally have a life expectancy of 25 years. Most farmers pay back the loan in ten years, then can reap profits of over $100,000 a year for the next 15 years. Sounds tempting, doesn't it?
Iowa Winds LLC hopes to build a 200- to 300-megawatt farm covering about 40,000 acres in Franklin County.
Company officials said the farm could be the nation's largest -- depending on the permits and the county's power grid infrastructure. If the county approves the project, construction would start next spring and take about a year, said Franklin County Supervisor Michael Nolte. LiveScience
Texas leads the nation with 2,370 megawatts of wind energy installed and California has 2,323 megawatts (American Wind Association). Iowa is in third place with 836 megawatts. Minnesota is fourth with 794 megawatts. The total United States capacity is about 10,000 megawatts. These numbers and rankings are changing. Wind energy output is growing by about 30 percent a year globally.
Want more? Go to the Minnesota Dept. of Commerce wind energy information web page.