In the early 1970's when the ("new") John Hancock Tower was just completed, an engineering flaw caused it to lose many glass panes (4 foot by 11 foot lights, weighing 500 pounds each!), resulting in drastic changes in stack pressure. Even when the window was not lost but the seal only compromised, this caused issues with operation of the revolving and swing doors. The original revolving doors and balanced doors were not compatible with the stack pressure of this 60 story high rise tower. By modifying the height and making various custom alterations to both the revolving doors and the balanced doors we eliminated the stack pressure issue so that the operation of these doors was reliable and consistent. As seen in the above photo, the tower main entrance.