 Supported by 4,675 steel-reinforced concrete columns, this type of construction not only served the Donnelly well, it also provided the perfect infrastructure for future tenants.To further the building's support structure, reinforcing bars, normally laid perpendicular, were laid at various angles enabling the floors to bear loads of at least 250 pounds per square foot.Current major tenants of the building include the Chicago Mercantile Exchange, Telx, Equinix, Steadfast Networks and CenturyLink.