These pictures were taken inside duct work that measured 10’0” (3m) high x 40’0” (12.1m)  wide. Steel support bracing like this is very typical in large duct work.

Note that the expansion joint between the support columns are about 2’6” (762mm) wide and runs completely around the inside perimeter of the duct work. A Duct Balloon that measures 40’ (12.1m) wide x 10’ (3m) high x 3’ (.9m) deep can be installed in this area and become secured by the duct work supports to prevent any movement.