Curved glass is used in the lower barrel of the rotunda, with simple curvature required in one direction only. Compound curved glass to provide true curvature to the dome surface was considered but ultimately rejected for budgetary reasons. Instead, the dome surface is faceted with flat panel shapes hung from cable trusses. The rotunda’s enclosure is unusual in its use of an exterior structural system to support the glass. Stainless steel cable trusses span horizontally between structural steel columns in the rotunda and the arched beams that form the dome. The sole compression elements in the system are the stainless steel spreaders that, along with approximately 2.5 miles of stainless steel cable, comprise the trusses. Spider-type fittings attach to the inside end of the spreaders, providing the point fixings that secure the glass. A detailed installation method statement was developed to facilitate assembly and installation of these cable truss systems and required pre-tensioning forces up to 22,000 lbs. Hydraulic equipment was used to achieve these pre-stress forces.

Arguably the most challenging design aspect of this project was accommodating the large inelastic building drifts required by the new California Building Code in areas of high seismic activity. Conventional point-fixed systems can be inadequate in these applications, which lead Enclos to develop a new system involving a custom spider design capable of providing for large in-plane movement. The system allows for up to 3˝ of in-plane movement at each fixing. A custom spider component was required to accommodate this movement. Enclos developed a custom spider that is investment cast of a special heat-treated stainless steel alloy with mechanical properties well beyond the 316 stainless alloy typical to conventional spider fittings. An extensive testing and mockup program were undertaken for the spiders and dome structure, in addition to seismic, water and air infiltration.

At 100´, the rotunda’s diameter surpasses that of the U.S. Capitol building and is virtually all glass. Through the structure’s 12 bays — each 26´ wide spanning between AESS structural steel arched beams, rising to a height of 108´ — the project is able to reflect the transparency and openness of the democratic government in which the United States prides itself.