The roof of the Norick Arena is shaped as an elliptic-paraboloid. The plan view of the roof is an ellipse with major and minor axes of about 400 ft. and 320 ft., respectively. The cross sections of the roof on planes parallel to the planes containing the major and minor axes of the ellipse are also parabolas. The deepest parabolas intersect at the center of the roof where the difference in elevation with respect to its perimeter was designed to be 17 ft. 3 in. from examination of the structural
drawings. Figure 4 shows an overall plan view of the roof structural system and the reinforced concrete columns supporting it, and the section of the roof along the major axis of the ellipse. A concrete ring beam connects the columns and the structural roof. The PT tendons are anchored into the ring beam. Although the dimensions of the columns vary somewhat, the predominant dimensions are 18 in. by 58.5 in. with the strong axis parallel to the axis of the ring beam and a height of 62 ft. The ring beam is approximately 108 in. wide by an average 36 in. in depth.
The roof structural system consists of an arrangement of essentially “T-beams” spaced 10 ft. on center located on planes parallel to the planes containing both the major and minor axes of the elliptic – paraboloid. These beams consist of 9 ft. 7 in. square precast concrete panels of 3 in. thickness. The equivalent section used in the analysis is shown in Figure 6. The surface of the roof is covered by insulating and roofing material protecting it from weather. There were two roofing systems that had been applied over the 40-year history of the arena. The outermost layer of roofing insulation is a closed-cell foam and has a variable thickness ranging from about 4 to 6 in. It is somewhat thicker at the center of the roof. This layer prevented a direct visual evaluation of the current structural condition of the concrete surface of the roof other than at limited exposed areas of the probe openings.
The material properties considered in the analysis were, f’: , concrete compressive strength equal to 4000 psi. The prestressing steel wires that comprise the post-tensioning tendons had an ultimate strength, fpu, equal to 240 ksi.