feet above the stage floor is a grid of steel that is designed to carry the loads of all the cable and rope systems with the counterbalances that are used on the stage. The catwalk level, above the acoustically transparent ceiling, is necessary to support lighting for certain types of events. Roof trusses support the ceiling and catwalks and also a concrete floor and concrete roof to create sound isolation. Concrete mass is acoustically important even at the roof level. The beams spanning truss-to-truss to support the concrete slabs were purposely aligned with the front to back stage line so the sound would not be trapped and bounce around the inside of the hall. The designed structural characteristics of this imaginary facility are regularly applied to most performing venues. but now let's look at two specific buildings. Ladder columns -- over 60 feet tall -- during erection. © Walter P Moore. Ladder columns at wave roof. © Walter P Moore. CASE STUDY 1: Cobb Energy Performing Arts Centre Walter P Moore recently served as the structural engineers on Cobb Energy Performing Arts Centre, a multi-use venue in Atlanta, Georgia. The distinctive architectural features of this venue provided complex design and construction challenges. The exterior of the building has three unique roof waves. The roof curves both horizontally and vertically, making every line of the wave different. These roof and wall waves are supported on straight structural steel trusses and curved girts. The curves required close coordination with the architect and steel contractor to limit the number of different structural frames and to economize on the steel detailing efforts. Giving the Cobb Energy Centre its dramatic look at an economical price was a goal that we helped achieve. The entrance and lobby area are a major architectural design element, with the high ceiling and full-height glass walls that give the patrons views of both downtown and midtown. The one-of-a-kind, exposed structural system demanded engineering innovation for it to become a reality. This signature front entrance is clad with a floor-to-ceiling glass curtain wall supported by exposed pipe ladder columns. Walter P Moore's design for the architecturally exposed structural steel frame support for this wall was chosen to reduce the steel tonnage for the support, while maintaining the desired open Construction of the signature front entrance lobby-a floor-to-ceiling glass curtain wall supported by exposed steel ladder columns. © Walter P Moore. F A L L 2009 theatre design & technology 59