As the dramatic collapses of the World Trade Center on Sept. 11, 2001, and the Interstate 35W bridge in Minneapolis on Aug. 1, 2007, showed, when steel weakens, the results can be sudden and catastrophic. There are several forces that can compromise the strength of this metal, leaving it highly vulnerable.
In the collapse of the I-35W bridge in Minneapolis, excessive load was a culprit in the gradual demise of the structure. Inspections four years before the collapse had shown weakening in the U-10 connections, where beams came together. The gusset plates holding these connections together had started to bend in four places. An additional layer of concrete in 1977 and new concrete barriers in 1998 had added a considerable amount of weight, and on the day of the collapse, the state department of transportation had placed thousands of pounds of gravel and sand on the bridge as part of a construction project.
In the investigation of the collapse of the World Trade Center towers in 2001, the failure of the structural steel has been attributed in part to the fires ignited by jet fuel that spread throughout several floors. While estimates vary, the jet fuel fires could have created temperatures as high as 2,700 degrees Fahrenheit. Research has shown that steel begins to weaken at approximately 570 degrees Fahrenheit, and that the critical temperature for failure is approximately 1,020 degrees Fahrenheit. This can vary widely with the load on top of that steel. The powerful impact of the airplanes as well as the intensity of the fire contributed to the collapse of the World Trade Center towers.
As the iron atoms in steel begin to oxidate, rust begins to appear on the surface. Over time, as more and more of the atoms go through this process, the steel begins to weaken. Oxidation can happen simply through exposure to damp air, although the more water that is present, the more quickly oxidation takes place.
While oxidation is a form of corrosion, corrosion can be caused by factors other than moisture. Sewage lines can corrode very quickly because of the presence of sulfuric and carbonic acids in the flow; the acids eat away cement and mortar, and gradually gnaw at the steel. The sun's UV rays can also weaken polymers, causing another form of corrosion.
Corrosion is actually an electrochemical process. If any electrolyte rests on bare metal, electrons can run to areas with lower energy. Frequently, droplets of water act as the electrolyte, but acidic materials can accelerate the process.
- Tony Kennedy; Minneapolis Star-Tribune; Old photos show flaws in steel of I-35W bridge
- National Institute of Standards and Technology: Investigation of the Sept. 11 World Trade Center Disaster
- The A to Z of Building: Fire Resistance of Steel-Framed Buildings
- Sewer Line System Rehabilitation: Cured-In-Place Manhole Liner
- Rustblock: The Corrosion Process Explained
- Vikas Agrawal: If the World Trade Centers were designed to withstand the impact of a jetliner then why did they collapse?
- Photo Credit Ablestock.com/AbleStock.com/Getty Images