Tzuras HaPesach

The building of most eruvin in major cities nowadays relies on the concept of tzuras hapesach (see Eruvin 11b). A tzuras hapesach is basically a doorway frame (two standing up poles and a pole across the top). The first issue regarding tzuras hapesach is what exactly does a tzuras hapesach have the ability to do? The Mishna Berurah (OC 362:10) brings the Pri Megadim who holds that a tzuras hapesach only works on a d'rabbanan level. Meaning, if an area is a reshus harabbim on a d'orayssa level, a tzuras hapesach cannot enclose the area and make it into a reshus hayachid. Only in an area like a karmelis can a tzuras hapesach work to allow carrying because the tzuras hapesach acts as a heker. This would limit the use of tzuras hapesach to areas that are not reshuyos harabbim d'orayssa.

The Chazon Ish (OC 70:13) disputes the Pri Megadim and holds that tzuras hapesach works to enclose an area even on a d'orayssa level. He bases this on fact that the gemara (Eruvin 110) says that one can use a tzuras hapesach to separate a field and thus avoid problems of kilayim. This is a d'orayssa halacha. Thus, the Chazon Ish proves that tzuras hapesach works on a d'orayssa level.

Others (see Minchas Yisrael Eruvin 110) dispute the Chazon Ish and say that tzuras hapesach by kilayim only works to be mafsik - to separate, but not to makif - to enclose an area. For example see the Chiddushim of Rav Chaim on the Rambam Shabbos 16:16 where Rav Chaim explains that a tzuras hapesach has a "din mechitza", but is not a real mechitza. He explains that if all you need is a mechitzos to surround an area or mechitzos to separate one area from another, so tzuras hapesach works. However, if you need a certain shiur of mechitzos (like 3 walls for reshus hayachid or a tefach wall for sukkah) so tzuras hapesach will not be effective. The psak in general seems to follow the Chazon Ish that tzuras hapesach could work to enclose even a reshus harabim d'orayssa, if not for a different problem which we will now explain.

The problem is that there is another rule of "asi rabim um'vatli mechitza" (see Eruvin 22a). This rule says that if the rabbim pass through a mechitza, they can be mevatel the entire shem mechitza. This could seemingly mess up our entire use of tzuras hapesach for eruvin. However, for this we rely on a Maharam M'rotenberg (see Mordechai Eruvin Perek HaDar Siman 510) who paskens that we don't say asu rabbim u'mevatli mechitza by areas that are not reshus harabbim d'orayssa. It is for this reason that we first must define an area as a karmelis, before deciding that we can establish an eruv in that area.

[For more on this topic see an article by R' Hershel Schachter in The Journal of Halacha and Contemporary Society. Much of the information in this piece was taken from that article. Also, note that the term "Eruv" in this article refers to the building of the tzuras hapesach. This is how the term is used in Yiddish, even though in the gemara it refers to the matzos that are later set aside.]