Salesforce Transit Center: San Fransisco's $2.2 Billion Cracks

jimminy_cricket writes: It was supposed to be the safest building in the world. Then it cracked.

Structural steel is exceptionally strong, but given certain conditionsâ"low temperatures, defects incurred during fabrication, heavy-load stressâ"it remains vulnerable to cracking. Two types of cracks occur in steel: ductile fractures, which occur after the steel has yielded and deformed, and brittle fractures, which generally happen before the steel yields. Ductile fractures develop over time, as the steel stretches during use, explains Michael Engelhardt, Ph.D., a professor of civil engineering at the University of Texas at Austin and chair of the peer-âreview committee overseeing the STC's response to the cracked-beam crisis.

"Engineers can predict ductile fracture and make adjustments during design, such as redistributing the load among various parts of the structure," Engelhardt says. "Brittle fractures, by contrast, happen suddenly and release a great deal of energy. They're concerning. They arenâ(TM)t supposed to happen."