Gilbert worked with structural engi

Gilbert worked with structural engineer Gunvald Aus on the innovative foundation and steel frame. Sixty-nine pneumatic caissons ranging from 6.5 and 18.75 inches in diameter were driven down to bedrock 100 to 120 feet below grade to support the towering mass. Closely spaced steel beams, called a grillage, were installed atop each caisson to transfer building loads. Because construction on the caissons began before Woolworth acquired additional property and enlarged the design, additional caissons and enormous transfer girders spanning between the original 38 concrete piers were required.