University of Illinois Memorial Stadium: Investigation and Rehabilitation

In September 1982, the flexibility of the University of Illinois Memorial Stadium balcony allowed spectator‐generated vibrations and movements that were disturbing to occupants. Both horizontal and vertical deflections were reported by spectators. An investigation was undertaken to assess the safety of the stadium, to determine the cause of the reported movements, and to make recommendations for any necessary corrective work. Inspections revealed corrosion of structural steel and deterioration of concrete, but these were not significant enough to influence the deflections. Measurements of the vertical and horizontal response verified the flexible nature of the steel structure. The investigation also determined that the character of the live load probably generated the disturbing vibrations and deflections. Balcony girders were strengthened and stiffened. A bracing system was installed to reduce the vibrations felt and observed by spectators. This rehabilitation reduced the natural vertical frequency and the maximum peak‐to‐peak displacement of the structure. In addition, the rehabilitation eliminated spectator reports of discomfort due to the deflections. Throughout this investigation and rehabilitation, it was recognized that the dynamic characteristics of structures must be such that magnitudes of deflection are not disturbing to occupants.