Willis Tower’s glazed floor cracks. How reasonable is the fear of breaking through?

Scary News: The glazed platform of Willis Tower (formerly Sears Tower) in Chicago, 103 stories up, on top of one of the tallest skyscrapers suddenly cracked under the feet of a family.
The news went viral although only the thin protective layer of the glazed platform was affected, not the supporting panels.

But are glazed floors more dangerous than others? Not really: Strict requirements specify the capacity of floors, whether they are made of glass, timber or reinforced concrete.
So why are people scared of breaking through? One reason is that the performance of structural glass is still widely unknown and the characteristics ascribed to the material are still those of traditional glass.
A study we made some years ago focused othis particular issue. We observed the behaviour of users with respect to glazed floors via hidden cameras placed in the atrium of Leopold Museum (Vienna) and found out that most of the visitors avoided stepping on the glass, although many visitors showed interest in the construction of the floor and observed the action in the room below. Those who crossed the glass element without hesitation were mainly employees.


The image illustrates the most frequently used ways of crossing the atrium.

Our results clearly showed the impact of the visual height and the transparency of the material used. With rising height, concerns regarding the stability of glass, i.e. the fear of breaking through and falling, are increasing too. People feel more insecure due to the absence of visual barriers, the transparency of glass and the thus associated greater influence of acrophobia. Furthermore the fear of falling is natural. Gibson and Walk described the phenomenon as the “visual cliff”.
There remains the question: Is the use of high transparency glass constructions such as floors functional in public buildings?







Design and Acceptance of Glass Constructions in Architecture
Authors: Dembski, Fabian; Englhardt, Oliver; Bergmeister, Konrad
IABSE Symposium Report, IABSE Symposium, Budapest 2006, pp. 1-7(7)
International Association for Bridge and Structural Engineering

Link: Abstract (free)
Link: Paper
Link: Leopold Museum

cover picture (royalty-free), source: https://www.flickr.com/photos/daveynin/8028151152/sizes/o/in/photostream/

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