Engineers Test Effects of Fire on Steel Structures. Building fires may reach temperatures of 1,000 degrees Celsius, or more than 1,800 degrees Fahrenheit, said Amit Varma, a Purdue associate professor of civil engineering who is leading the work. "At that temperature, exposed steel would take about 25 minutes to lose about 60 percent of its strength and stiffness," he said. "As you keep increasing the temperature of the steel, it becomes softer and weaker."

http://www.sciencedaily.com/releases/2010/11/101116122857.htm