Chicago Willis Tower viewing platform cracks

A glass viewing platform high up the Willis Tower in Chicago, USA has cracked while four men were standing on it.

The pane was part of a glass observation ledge 103 storeys up the skyscraper.

A building spokesman said the damage was to the protective coating on the glass and that nobody was in danger.

The cracks could have been the result of a sharp item being dropped on the pane.

Footage courtesy of ABC News