The SkyDeck ledge of the Willis Tower cracks under visitors' feetttps://edition.cnn.com/2019/06/12/us/willis-tower-skydeck-cracks-trnd/index.html
Glass floor usually embedded in observation deck of skyscraper is basically manufactured so as to be durable, even if fat visitors jump on the glass several times.
But, it matters when they jump thousands of times.
So, designers of glass floors have to estimate how long they are durable to decide the time to replace glass floors.
In some cases, there would be much more people on the glass than designers expected. It may cause cracks inside the glass.
