identify this movie - Janitor opens door to find rest of building obliterated

That sounds like a scene from The Day After Tomorrow (2004). Someone on a forum describes it like this:

there was a janitor cleaning the floors. [...] The janitor notices a lot of light coming from underneath the door so he goes and opens it. Now, the amount of light coming from underneath the door made it look like there was a spotlight pointed at it from the other side and when the janitor opens it that half of the building is gone.

EDIT: The janitor opens the door at the beginning of this YouTube clip.

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