April 14, 2012, after the annual Wolverine football spring game. Seating and surface The stadium's original capacity was 72,000, but Fielding Yost made certain to install footings that could allow for expansion over 100,000 seats. Initially, all seating consisted of wooden bleachers. These were replaced with permanent metal seating in 1949 by Fritz Crisler, athletic director at the time. From 1927 to 1968, the stadium's field was natural grass. This was replaced with 3M TartanTurf in 1969 to give players better traction. However, this surface was thought to be unforgiving on players' joints, and the stadium returned to natural grass in 1991. This too became problematic, as the field's below-surface location near the water table made it difficult for grass to permanently take root. The field was converted to FieldTurf, an artificial surface designed to give grass-like playing characteristics, in 2003. In 2010, it was upgraded with a newer version of FieldTurf called Duraspine. Attendance records Michigan Stadium holds the NCAA single-season average home attendance record, set in 2012 at 112,252 fans per game. On September 7, 2013, Michigan Stadium drew a crowd of 115,109 to see Michigan defeat Notre Dame 41–30, which at the time represented a post-1948 NCAA collegiate football attendance record. Previously, and prior to NCAA record keeping for attendance, a 1927 Notre Dame–USC game at Soldier Field in Chicago drew an estimated 117,000–123,000. Both of these records fell in 2016 when Tennessee and Virginia Tech drew 156,990 for a game held at Bristol Motor Speedway, a NASCAR track with a capacity of over 150,000. "The Big House" also holds the record for the largest attendance for an NCAA Division II football game, one involving Slippery Rock University of Pennsylvania. Back in 1959, stadium announcer Steve Filipiak thought it would be amusing to include Slippery Rock with the other football scores he read to the crowd, due to the school's unusual name. Soon, it was a tradition, and Slippery Rock became so popular with U of M fans that on September 29, 1979, "The Rock" played in-state rival Shippensburg at Michigan Stadium, in front of 61,143 fans (Shippensburg won, 45-14). Slippery Rock made repeat trips to Ann Arbor in 1981 and 2014. With an attendance of 104,173, "The Big Chill at the Big House" set the record attendance for a hockey game. The record was broken on January 1, 2014 for the NHL's 2014 Winter Classic, where