See last link for "communicating stairThere's probably other ways of expessing this but this is basically what it means. I think the form of the stairs may change from place to place but all have in common the wish to have people interact and linger or even sit on the steps, likened to sitting on the Spanish Steps or stadium steps.
Some examples I found:
https://immeuble-shift.com/le-coeur-de-vie/
Pour répondre à la dématérialisation croissante du travail, des configurations inédites ont été imaginées dès la conception du projet. Elles proposent des lieux partagés pour décloisonner les idées et créer de l’intelligence collective.
A chaque étage, 500m² sont aménageables en zones de discussion et d’échange. Parce que l’architecture favorise le hasard des rencontres, SHIFT relie tous les étages par un escalier de convivialité, facilitant la circulation des gens et des idées.
http://www.crochon-brullmann.com/index3.php#restructuration-...

https://beta.theglobeandmail.com/life/home-and-garden/archit...
“the stairs are much more than stairs,” says Michael Moxam, vice-president of Stantec and one of the project’s lead designers. “They encourage a vertical flow from the city and the campus upward into the building, and they’re a visible, strong alternative to the elevator....The stairs snake upward through a series of open lobbies with seating, inviting students to linger. Interdisciplinary thinking is an explicit goal of the architecture. ...These resemble the Spanish Steps in Rome; Kuwabara calls them “stadium seating.”
http://www.som.com/news/the_new_schools_university_center_to...

"...while the broader, open "communicating stair" allows for travel between floors at a more leisurely pace. With faceted walls clad in glass-fiber-reinforced concrete panels, the high-use stairways are a place for chance meetings between students and faculty, and encourage social interaction and interdisciplinary exchange..."

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Note added at 3 hrs (2017-10-01 01:03:22 GMT)
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or communicating or "OPEN" staircase here, again to encourage collaboration
https://www.fastcompany.com/3004498/how-todesign-office-coll...

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Note added at 147 days (2018-02-25 12:30:13 GMT) Post-grading
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Just came upon another term, in a report on a new Microsoft office with this type of staircase, which might be used, particularly in US/Cdn English where the word is used in sports "BLEACHERS" https://www.collinsdictionary.com/dictionary/english/bleache...