Eruvin 9b – Long Lechayayim
(excerpted from The Contemporary Eruv)
The Magen Avraham (363:28) states that the walls that surround three sides of an alley (a “mavoi”) cannot be considered the lechayayim for a tzuras ha'pesach on the courtyard's fourth side. The Magen Avraham does not explain the rationale for his ruling (see Nesivos Shabbos , note 39). A possible interpretation of the Magen Avraham's position is that lechayayim cannot exceed a certain maximum shiur (measurement). See the Chazon Ish, ibid. 70:16. The Chazon Ish there raises the possibility that in certain cases the maximum shiur is three or four tefachim. Under normal circumstances, however, it seems that the maximum shiur is four amos. See Nesivos Shabbos, ibid., note 40, in the name of the Makor Chaim. Structures that long are no longer viewed as the “door posts” (lechayayim) of a “door frame” (tzuras ha'pesach) - they are walls. Elevated train line embankments or overpasses that are more than four amos long would therefore not be halachically suitable to serve as lechayayim. The Chelkas Ya'akov 1:166.4 presents arguments that would allow the use of “long” lechayayim such as embankments that support elevated train lines. Some rabbinic authorities follow the Chelkas Ya’akov’s ruling in this area. The Chelkas Ya’akov’s reasoning and proofs, however, are controversial and difficult to follow, and seem not to be in accordance with the opinions of the Poskim mentioned previously in this paragraph. See also Nesivos Shabbos and note 39. However, Rabbi Yosef Menachem Mendel Rapoport noted that the Teshuvos Maharsham, Orach Chaim 1:207 holds that a lechi may be broader than four amos (and, for that reason, he allows a tzuras ha’pesach to cross over a house). The Chazon Ish, Yoreh Deah 172:1, also questions the Makor Chaim’s stringency in this area (see below, page 91, note 184).