Omed [Standing Structure] and Parutz [Open Space] in an Eruv – Eruvin15b (excerpted from The Contemporary Eruv).
Many Poskim rule that the presence of a tzuras ha'pesach allows even breaks that are longer than ten amos. However, according to the Rambam this leniency does not apply to [a scenario] where the enclosure is made up of more open space than built up structure [parutz merubeh al ha’omed].
Tosafos, however, assert that even if an area’s enclosure consists entirely of tzuros ha'pesach made up of four poles planted in the ground at the four corners [of the area] and a lintel atop them that is a sufficient enclosure.
Generally, the Ashkenazic custom is to rely on the Tosafos while the Sephardic custom is to be stringent in accordance with the Rambam.
Every one agrees that even a gap that is less than ten amos wide can also invalidate an enclosure even if it does not take up an entire side - if there is more open space than wall-like structure on that side of the eruv.
Shulchan Aruch, ibid., 362:8-9. The Maharal, Eruvin 11a, and others suggest that we determine omed merubeh on the basis of the entire perimeter. Since the Shulchan Aruch rules there that omed k’parutz is sufficient, if the entire enclosure was at least 50% wall-like structure, then gaps of up to ten amos anywhere along the perimeter would not invalidate the eruv. The Mishna Berura 362:45, however, rules that we must also take each side independently. Thus, the side in which the gap exists must in and of itself be omed k’parutz in order to allow a gap of less than ten amos without any additional rectification. As we have mentioned, there are many Poskim that do not take tzuros ha'pesach into account in making this determination (see Nesivos Shabbos 14:15 and note 33). Thus, to allow breaks of up to ten amos would require actual wall-like structure over at least 50% of the entire perimeter of the eruv, and over the length of each side of the eruv as well. See Nesivos Shabbos 14:7-16 for more details on how to reckon the omed vs. the parutz.
Rabbi Yosef Menachem Mendel Rapoport and Rabbi Avrohom Moshe Seidman, noted, however, that the Maggid Mishne 16:16 (cited in the Beis Yosef, Orach Chaim, towards the end of siman 362, d.h. U’Ma She’Kasav u’Bilvad) seems to hold that each side must be taken independently even according to the Rambam (see also Sha’ar Ha’Tziyun 363:7). It is not clear, however, that this is, indeed, the portent of the Maggid Mishne,