Wiss, Janney, Elstner Associates, Inc.

Investigators say they expect that by the end of next week they will have a way to take the 2-in. wobble out of the 37-ft-wide exit-ramp tower at three-year-old Lincoln Financial Field in Philadelphia. There is nothing structurally unsound about the 85-ft-tall steel frame that juts out about 120 ft from the seating stands, says Kimball J. Beasley, a principal of Wiss, Janney, Elstner Associates Inc., an investigator for Philadelphia Eagles. The "million-dollar question" is whether the wobble was the result of a design issue or something else. The movement was most likely caused by the lining up of the resonant frequency of the structure in the transverse direction to the driving frequency of lockstep walking, says Beasley. The stadium was engineered by the Los Angeles office of Arup, which also confirmed its structural integrity and code compliance. Retrofit options to stiffen the tower could include cross bracing.