A House is Considered as if it is Filled — Eruvin 76b
תלמוד בבלי מסכת עירובין דף עו/ב
בעא מיניה רבי אבא מרב נחמן לול הפתוח מן בית לעלייה צריך סולם קבוע להתירו או אין צריך סולם קבוע להתירו כי אמרינן ביתא כמאן דמלי דמי הני מילי מן הצד אבל באמצע לא או דילמא לא שנא אמר ליה אינו צריך סבור מינה סולם קבוע הוא דאינו צריך הא סולם עראי צריך איתמר אמר רב יוסף בר מניומי אמר רב נחמן אחד סולם קבוע ואחד סולם עראי אינו צריך
The case here is one of a second floor apartment built atop a first floor apartment. The apartments belong to different people, but there is an opening between the two — a kind of trapdoor in the ceiling of the lower apartment that is the floor of the upper apartment. The Gemara invokes the principle of ביתא כמאן דמלי דמי — a house is considered as if it is filled, and rules that it is not necessary to have a ladder in place in order to consider the two apartments as accessible to each other in order for them to participate in a joint eruv.
R' Moshe Feinstein (Igros Moshe, Orach Chaim 1:139, end of anaf 2) posits that the principle that a house is considered as if it is filled— only applies when the "house" is enclosed by both walls and a roof. As evidence of this, he cites Rashi to Shabbos 97a, who writes that the reason that a house is considered as if it is filled is because the house is dark and its air is thick. Thus, it is only in the case of a closed house that no ladder is necessary, because only then does the principle of a house is considered as if it is filled allow us to treat the case as one in which there is access to the upper apartment by the virtual climbing of the virtual ladder fashioned from the virtual objects that "fill" the house.
Accordingly, writes Reb Moshe, a novel ruling would apply to a case in which the second floor apartment was built over a first floor that consisted of pillars, but no walls. In this case, the virtual ladder is non-existent, since the principle of a house is considered as if it is filled is not applicable. Accordingly, in this case, a permanent ladder providing real access must be in place for the two owners to participate in a joint eruv.