Question about determining Required Egress Based on Section 1005 in IBC
Let's say that you have library that's multistory, and the top story accommodates 1200 people.
So 1200 x .2 = 240. Let's say you have four stairwells. Hence, 240 / 4 = 60.
This means that egress components (other than stairways) need to accommodate 60 inches (based on section 1005.3.2.)
Does this mean a priori that you need a double leaf door at the bottom of each stair, because a single leaf can maximum be 48 inches?

Yes sir, it does. That is good that you know that as well.
One thing to keep in mind with stairs is that occupant load at the point of egress is not cumulative. What that means is that, let's say you have a 4story building, and each story is served by the 4 fire stairs that you gave as an example. The 1st floor exit door at each stair does NOT need to accommodate 60" + 60" + 60" + 60" = 240", and that calculation I just did is assuming that each story needs 60" of width. The 1st floor exit door would only need to accommodate 60". Now, if you had an upper floor that for some reason had a greater occupant load than the other stories  say that the 3rd floor needs 72"  you would need to base your 1st floor exit door on that worst case scenario.
Hope that helps!

Lulia,
You are correct. I was thinking the same thing when I read the original post, and you beat me to it.
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

That is correct. See related IBC Section 1005.3.1, Stairway, and Section 1005.3.2, Other egress components, at this link:
https://codes.iccsafe.org/content/IBC2015/chapter10meansofegress
Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

I know this is a very old post, so not expecting a response, but I don’t understand why you would absolutely need a double swing 48 inch door. If we assume the library is fully sprinklered than the door capacity  other egress component!  would be 1200 x 0.15 = 180/4 = 45 inches. You could have a custom fire door 45 inches wide. Actually, common in some of the buildings I have been on design teams for.
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