The AIB’s roof structure is a defining element of its aesthetic appeal. As part of his concept for the National Museum, architect Adolf Cluss designed a system of lightweight, exposed wrought iron roof trusses, which were both economical and attractive. The trusses were built with minimal material, and were designed without ornamentation or decoration; their beauty is in their simplicity. Utilization of the trusses also enabled long spans, which facilitated the open design that Cluss envisioned. Finally, wrought iron was seen as fireproof construction material at the time; minimizing fire risk was a major concern in 19th century architecture, as it is today.
The lightness of the truss system provides a graceful balance to the robust masonry system of the museum’s original exhibit hall walls – particularly in the primary halls, where the trusses span over 60 feet. The trusses in the Rotunda, combined with the plentiful natural light shining through the clerestory windows below the dome, make it the focal point of the building and one of the highlights of Washington, D.C.’s National Mall.