Can a Sepharadi rely on an eruv made out of tzurot hapetah to carry on Shabbat?
According to many poskim, today’s streets are considered a karmelit. Thus, to permit carrying in the streets on Shabbat, it is sufficient to surround the area with structures resembling doorways (tzurot hapetah) which thereby form a kind of halachic wall around the area. The basic form of a doorway is comprised of two doorposts with a lintel atop them. A lintel can be formed by laying a board across the top of two posts, or by running a string above the posts which is how many eruvim are constructed.
But according to the Shulhan Aruch, if the street is 16 amot wide it is considered a public domain and therefore tzurat hapetah will not be sufficient. Instead there would have to be a proper wall surrounding the city or the neighborhood.
Despite the above, there are 3 reasons why Sepharadim can be lenient.
The Principle of Double Doubt
The first doubt is an opinion that states it is only a public domain when 600,000 people walk there on a daily basis. Many maintain that since the camp of the Israelites in the desert consisted of 600,000 men, all of whom needed to walk to the Mishkan in order to help build it and to hear the Torah taught by our teacher Moshe, the Kohanim, and the Levi’im, it therefore follows that a reshut harabim (public domain) is defined as a road or marketplace that is at least sixteen amot wide which 600,000 people pass daily. According to this opinion, if fewer people traverse it daily it is considered a karmelit, and once we consider a place a karmelit then having tzurot hapetah surrounding the city or the neighborhood would be sufficient to make it a private domain. This is the opinion of Behag, Rashi, Smag, Rosh, and many others.
Therefore, in practice, only megacities such as New York City and Mexico City have streets that are traversed by 600,000 people every day. Even most big cities do not have that many people passing through daily, and so they would be considered a karmelit.
The second doubt is the opinion that a tzurat hapetah is acceptable and equal to a solid wall surrounding the city or neighborhood. That is how many poskim learn from Maimonides, and others learn it from the words of Rosh.
Most Streets Have No Outlet
Many hold that the way our streets are constructed effectively makes them enclosed from three sides, which would make them a private domain.
The example given (see the illustration in Or Letzion, first volume, page 76) is a street running North-South with no outlet at its Southern end. Since it has rows of houses on both sides of the street (and typically there are more structures than gaps between houses) this street is therefore enclosed on three sides: the two rows of houses and the dead end. This street is then considered a private domain.
It then follows that any cross streets that run East-West through the above street would also be enclosed on three sides: the two rows of houses on their sides, and the North-South street which is a private domain. (In fact, the North-South street splits the East-West streets into two sections each enclosed on three sides.) And that makes the East-West cross streets also considered private domains, even though they don’t have a dead end.
Our Streets Aren’t Designed Like the Old Streets Which Were Considered Public Domains
The Aruch Hashulhan says that the streets in those days were different than our streets because they used to have only one main avenue that the entire city would have to use, as opposed to today where there are multiple streets and access roads for every neighborhood. As such, our streets today don't have a name of a public domain.
Therefore, the conclusion is that all agree one is allowed to carry on Shabbat in an abnormal way when relying on an eruv using tzurat hapetah. And even when carrying in a normal way, a Sepharadic Jew has a basis on which to rely