A Briefing on Bridges

Last week NPR had a story on our infrastructures in the US. There was a mayor who spoke for mayors across the nation stressing the failure of our government to address our infrastructure. Our government has turned it's back on the very people it is suppose to represent and allows these type of disasters to occur. Why do I say this, because these bridge collaspes are occurring more and more every day. I live in St. Louis, MO and one a part of a bridge in St. Louis just collasped earlier this year. It is ridiculous to say no one is to blame. Of course the one who is the blame are the ones who are responsible for building the bridge and the ones responsible for monitoring the bridges. I have seen all to often our greed to reap rewards of contracts and contracts awarded on shear dollars instead of integrity. Our government has to put billions into our infrastructure just like it is putting billions into the ridiculous war. We need to hold our government accountable and stop hiding behind excuses to get our infrastructures updated. That along with hunger, education, and employment must become a priority in our country. Not ridiculous wars which have only been a total failure but have allowed many white men to become rich from the top to the bottom. Once again the failure to fund our infrastucture must become a priority or we will continue to see these types of failures and the ridiculous answers the guest gave in response to accountability!!about money and not integrity!

Sent by Annsih Serud | 4:16 PM | 8-2-2007

Question: Would there be any way to instrument bridges to warn of dangerous conditions?

Sent by Matthew | 2:54 PM | 8-3-2007

Does anyone here have any current knowledge of structural steel?

We've been hearing on NPR for two days about "fatigue" ; does that really apply? I ask because my last exposure to acedemic information on strength of mterials came about the time that bridge was being designed. At that time we learned that, unlike aluminum, steel did not fatigue as long as you avoided "plastic" deformation. (Ira's example with the spoon involves plastic deformation.) Has this science changed?

Ira worried about a bridge bouncing as he drove on it. A bridge, like most structures, is designed to flex under load. The trick, at least under the design rules of the 1960s, is to limit the flexure to the elastic, as opposed to plastic, deformation range, with a suitable safety factor.

A correlary to Murphey's Law states "Nature always sides with the hidden flaw." A hidden crack in a girder or weld can reduce the effective cross section of the member. Even though a structure is computed to be within the "elastic" range based on the gross cross section, a load well within the design range can push the reduced, effective, cross section into plastic deformation.

Finally, one reason our "infrastructure" isn't replaced frequently is that we are too stretched building new infrastructure to handle a burgeoning population, one-third of the growth of which is from immigration, of which two-thirds is illegal.

Sent by richard cole | 6:45 PM | 8-3-2007