Stories tagged I-35W bridge


The new I-35W bridge: now bigger, stronger, and greener.
The new I-35W bridge: now bigger, stronger, and greener.Courtesy anjouwu
Ever stand on a sidewalk and wonder about the concrete beneath your feet? Where did it come from, and how did this hard grey material get to be pretty much everywhere? Though you may not think about it at all, concrete is used more than any other building material in the world. In fact, concrete is so ubiquitous that the production of concrete contributes 5% of the world's human-caused carbon dioxide emissions to the atmosphere.

Add it all up and it starts to look like concrete is more than just the stuff of sidewalks and building blocks. Concrete is a V.I.P. (which is how I like to refer to Very Important Polluters).

While concrete is a huge contributor of CO2, it also has loads of potential to be an innovative and important "green" material that helps us to build stronger and more environmentally friendly roads, bridges and buildings. This really great article from the New York Times science section explains the basics of concrete chemistry, and how new concrete mixes are being developed that are not just stronger and better for buildings, but that also can scrub carbon from the air.

Here in the Twin Cities we have our own example of cutting-edge concrete in the I-35W bridge, which was built to replace the bridge that collapsed in 2007, killing 13 people. You might not realize it as you pass over this bridge, but it's made of many different mixes of concrete, each developed to do a particular job.

Some of the concrete in the I-35W bridge was mixed and cured (that's what they call the hardening process) to be strong and stable, others to resist the road salts and other effects of weather and climate in Minnesota. The wavy concrete sculptures on the bridge even scrub pollutants from the air, In fact, they stay white because of a chemical process that uses the sun to help break down staining pollutants. Who knew concrete could be so fascinating?!

More Than You Ever Wanted to Know About Concrete

Science Buzz Posts on the I-35W Bridge Collapse

Investigations into the 2007 collapse of the I-35W bridge are wrapping up and being reported. Here's an interesting report on what role the sun and its energy may have played in the timing of the bridge's collapse on that fateful Aug. 1 day. Oh, and my apologies to Elton John for swiping his lyrics for the headline of this post.

A friend of mine has a holiday visitor from Lithuania who's a highway engineer and very interested in the I-35W bridge collapse. Looking for ways for him to learn more about last summer's bridge collapse, I just found this at the official bridge building website: View the construction progress every Saturday at 11 AM for a free Sidewalk Superintendent Talk led by project representatives who will describe the design/build process and answer questions. Everyone is welcome! Meet at the corner of Washington and 19th Avenues (in front of Grandma's Saloon).


Vibration problem?: A combination of vibrations coming from various area sources could be factors in the fall of the I-35W bridge in Minneapolis on Aug. 1. ( photo by Diversey)
Vibration problem?: A combination of vibrations coming from various area sources could be factors in the fall of the I-35W bridge in Minneapolis on Aug. 1. ( photo by Diversey)
tangled roadwayI’m not a structural engineer and don’t even play one on TV, but I’m curious to see what kind of dots get connected on this scientific phenomenon and the I-35W bridge collapse.

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.

According to this article in the New York Times ("Engineers see Dangers in Aging Infrastructure"), disasters like yesterday's bridge collapse may become more common if we don't find better ways to detect potential problems and invest in maintaining our aging roads, bridges, and other public works...

Later this afternoon, the US Army Corps of Engineers is going to lower the water level of the river by two feet in the area of the bridge collapse.

After the initial investigation is done, the Corps will open the roller gates at the Ford dam (i.e. Lock&Dam #1), dropping the level of the pool and giving emergency workers and investigators better access to the wreckage.

The silver lining of our recent drought is that the Mississippi is flowing at only 15% of normal. That means the river is shallower than it would otherwise be, and the debris in the channel isn't creating a massive dam.