Pi Tikra (2) — Eruvin 95a
תלמוד בבלי מסכת עירובין דף צה/א
פי תקרה יורד וסותם
From The Contemporary Eruv:
When, in 1952, Rabbi Moshe Feinstein and Rabbi Rapahel Ber Weissmandel discussed the use of an elevated train track as pi tikra (see Daf Digest to Eruvin 90), neither Rabbi Weissmandel nor Reb Moshe considered the possibility that the elevated train line might serve as a tzuras ha'pesach. We may assume that the reason for this omission is related to the logic we developed above, that a tzuras ha'pesach must resemble a traditional door frame. By definition, a tzuras ha'pesach whose lintel is more than four tefachim wide is a roof. We can no longer view it as a door frame.
The Chazon Ish (Orach Chaim, 111:5) brings evidence that a roof cannot serve as tzuras ha'pesach from an observation that above (94b) Rav and Shmuel argue whether the roof of an achsadra (a shelter consisting of a roof supported by poles and no more than two walls) allows one to carry beneath it on Shabbos because of pi tikra yored v'sosem or not. Neither Amora entertains the seemingly obvious possibility of tzuras ha'pesach. Nesivos Shabbos 19:17, end of note 39, discusses the possibility of using an overhang or ridge on a roof as the lintel of a tzuras ha'pesach. He reasons that this is permissible where such a feature - and accompanying lechayayim - are at the edge of the roof. This would apply to an overhang on an overpass or elevated train line. Where the overhang is at the edge of the overpass it may be incorporated in an eruv. Cases where only a lechi is under the extension of a roof are problematic. See Halachos of the Eruv Chap. VII, J and note 20. The edge of the overhang may be considered a pi tikra, effectively cutting off the lechi from the rest of the eruv.