The transparent viewing deck – called The Ledge – opened in 2009, with the three-inch thick meant to withstand 5-tons of weight.
The Garibay family were visiting the attraction, Chicago’s most iconic structure, when the glass appeared to crack, forcing staff to evacuate them from the ledge.
A spokesperson from the Willis Tower said that, although the incident is unusual, it has happened previously – and is perfectly safe.
Apparently the protective layer over the ledge began to break but this does not compromise the walkway’s safety.
Staff called glaziers and technicians to make the repairs straight away to the glass boxes.
The attraction, which looks down onto the streets of the windy city, is set to reopen soon.