Eruvin 6b: Doors (excerpted from The Contemporary Eruv)
Halacha distinguishes between the enclosure necessary to convert a carmelis into a reshus ha'yachid and the enclosure necessary to convert a reshus ha'rabbim into a reshus ha'yachid. The Gemara relates that Yerushalayim would have been considered a reshus ha'rabbim had its doors not been closed at night. The Poskim disagree over whether these doors had to have been closed me'd'oraysa or me'd'rabbanan. Some sources maintain that me'd'oraysa a tzuras ha'pesach would have sufficed. Others maintain that in reshus ha'rabbim delasos are required me'd'oraysa.
This requirement is limited, in any event, to reshus ha'rabbim. An enclosure intended to surround an area that fall into the category of carmelis does not require doors to allow carrying therein. The rectification of tzuras ha'pesach is sufficient in such cases. Clearly, therefore, it is highly advantageous to have an area defined halachically as a carmelis as opposed to a reshus ha'rabbim. Besides the lesser expense involved in building a tzuras ha'pesach as opposed to doors, another issue we may thus avoid is the problem involved in the use of doors that cannot be closed at night. Such doors are not completely comparable to the doors of Yerushalayim - which were closed at night. Thus, physically or legally eliminating true reshuyos ha'rabbim from an eruv averts the complex halachic problem of the extent to which the doors must be suitable for closing.
According to most Poskim, a reshus ha'rabbim must be enclosed by delasos (doors) because otherwise, asu rabbim u'mivatlei mechitzta ‑ the passage of the masses through the wall (were it to be just a tzuras ha'pesach) would render it invalid. We will discuss this concept in Section 6 below. The question is whether this concept negates a wall on a d'oraysa or d=rabbanan level. See also Rabbi Chaim Gedalya Tzimbalist's Avodas Avoda (Tel Aviv, 1973 - this work is outstanding in its explanation of the various approaches in Messeches Eruvin -) on the Avodas HaKodesh 3:1, note 2 and Tosefes Biur siman 1.
See Nesivos Shabbos 23:1, note 2.
We must note that this is not the only source in the Gemara for the requirement of doors for a reshus ha'rabbim. We will learn of Poskim who hold that even had it not had doors, Yerushalayim would not necessarily have been a reshus ha'rabbim. Even these Poskim, however, derive from other sources that a reshus ha'rabbim d'oraysa requires delasos. See Nesivos Shabbos, ibid., note 1.
See Nesivos Shabbos, ibid., notes 9‑10.
Caption to Peirush Chai #59
See Eruvin 94b for more detail on pi tikra. In the Tikvas Zecharia, Rabbi Rosenfeld of