Does the passage of many people through a wall render it invalid? — Eruvin 22a
(excerpted from The Contemporary Eruv)
Among the basic issues considered in the discussions of the Gemara in Messeches Eruvin are: a) asu rabbim u'mivatlei mechitzta - whether the passage of many people through a wall renders it invalid (Eruvin 20a and 22a.). We unequivocally accept that asu rabbim u’mivatlei mechitzta does apply to natural walls. The question is whether this principle extends to artificial barriers as well.
The issue of asu rabbim u'mivatlei mechitzta is argued here by two Amoraim. Rabbi Yochanan holds that the passage of many people through a wall renders it invalid; Rabbi Eliezer disagrees. The Mishna Berura [Bi’ur Halacha 364:1, d.h. Ve'Achar She'asa La, see also Avodas Avoda on the Avodas HaKodesh 2:4, note 46] explains that this disagreement has major halachic consequences. If we were to accept Rabbi Yochanan's approach, it is likely that almost no urban tzuras ha'pesach eruv would be valid. Utility poles must inevitably cross over city streets, and the Mishna Berura posits that city streets fall into the category of asu rabbim. Only if we can rely on the approach of Rabbi Eliezer may such tzuras ha'pesach eruvin be valid - in his view, the masses do not invalidate the enclosure. The Rishonim do not provide a definite unambiguous ruling on this issue. It is, therefore, one of the subjects of the conflict between the Mishkenos Ya'akov - who decides in favor of Rabbi Yochanan's opinion; and the Beis Ephraim - who decides in favor of Rabbi Eliezer's opinion. The Mishna Berura writes that the lack of a clearcut lenient decision in the Rishonim is another reason that a Ba'al Nefesh should not rely on urban tzuras ha'pesach eruvin. The Aruch HaShulchan4 and Chazon Ish,5 however, both accept the ruling of the Beis Ephraim and rule, in practice: lo asu rabbim u'mivatlei mechitzta.
The definition of the traversing rabbim (masses) necessary to invalidate an enclosure according to Rabbi Yochanan is a major, unresolved, issue. The Aruch HaShulchan, ibid., 353:50, seems to hold that the definition of rabbim that are mivatlei mechitzta is the same amount as that which creates a reshus ha'rabbim. This would mean that only a street that already has the problem of reshus ha'rabbim will have the problem of asu rabbim u'mivatlei mechitzta. He seems, however, to contradict this position later (ibid., 354:1). See Nesivos Shabbos 25:11 and notes 42-45. (The Poskim who are inclined to be lenient note that the Rambam, Hilchos Shabbos evidently rules according to Rabbi Eliezer.)