Do they bring in stones from outside and use them to break the glass?
When I went in October 2010, I noticed at least 2 panes of glass were cracked so those windows were sectioned off.
I remember at Twycross that the Orangs would bring in stones from outside and bash the glass very hard with them.
I would imagine that when natural outdoor Orang enclosures are being prepared care is taken to avoid any stones being left around . On a visit to Dublin Zoo with an IZES group some time ago , their big male orang had managed to find a big stone somehow . The keepers were trying to bribe him to swap the stone for some of his favourite treats before he did any damage .