OPINIONS: The Gemara says that a Mechitzah over which people walk ("Darsi Lah Rabim," or a "Mechitzah Nidreses") is not considered a Mechitzah.

In many modern metropolitan Eruvin, the Mechitzah (wall or Tzuras ha'Pesach) around the city sometimes passes underneath bridges and overpasses. What are the laws of an Eruv constructed from a Mechitzah that passes under a bridge or overpass? Is that Mechitzah usable as part of the wall around a city in a modern metropolitan Eruv?

The NODA B'YEHUDAH (OC 2:42) wrote a responsum to one who asked him about bridges that pass over Mechitzos. The inquirer suggested that the only potential problem is that people walk "through" the Mechitzah by way of the bridge above it, and that should annul the Mechitzah because of the rule, "Asu Rabim u'Mevatlei Mechitzasa" (Eruvin 22b). We rule (in accordance with Tosfos on 22b; see Insights to Eruvin 22:2) that the presence of people walking through a Mechitzah invalidates the Mechitzah only if it is a natural partition, and not a man-made one. Therefore, if the bridge passes over a man-made Mechitzah, then the Mechitzah should remain valid.

The Noda b'Yehudah argues that when a bridge passes over a Mechitzah, the Mechitzah is a "Mechitzah Nidreses," a trampled Mechitzah, as mentioned in the Gemara here. A trampled Mechitzah is different from one through which people walk. The presence of people trampling over a Mechitzah invalidates even a man-made Mechitzah.

The MISHNAH BERURAH (OC 363:118) rules that if the bridge is more than ten Amos wide, it effectively makes a ten-Amah breach in the Mechitzah below, invalidating the Mechitzah. In such a case, a Tzuras ha'Pesach must be built over the bridge to close the breach in the Mechitzah. If the bridge or the road that leads to the bridge crosses directly from one side of the Reshus ha'Yachid (the enclosed city) to the other side of the Reshus ha'Yachid (i.e., it is "Mefulash"), then it will not suffice to erect a Tzuras ha'Pesach to close the breach. In such a case the road (or bridge) that enters the city must have doors or gates that close off the road in order for the Eruv to be effective. (This requirement applies to any Reshus ha'Rabim that is Mefulash and passes directly through a Reshus ha'Yachid, as the Gemara earlier (6b) teaches.)

Therefore, a city that has bridges or roads that pass straight through the city needs gates at the points where the roads enter the city in order to permit one to carry in the city (which is virtually impossible to accomplish in today's modern cities).

It is not clear, though, exactly what is considered Mefulash.

(a) RAV MENACHEM KASHER (Torah Sheleimah 15:178) cites some early sources that imply a road is considered Mefulash only if it passes straight through the city without bending at all. (This is obviously a much more lenient ruling, since it is very uncommon to find a road that "pierces" straight through a city.)

(b) RAV MOSHE FEINSTEIN (Igros Moshe OC 1:140) writes that any road that starts at one side of Reshus ha'Yachid and exits at the opposite side, regardless of how much it turns and bends in between, is considered Mefulash.

QUESTION: The Gemara says that when there are grapevines in a small Chatzer that opens into a large Chatzer, one is permitted to plant wheat in the large Chatzer. The opening into the large Chatzer is considered part of the Mechitzah around the large Chatzer that separates it from the small Chatzer, and thus there is no problem of Kil'ayim.

However, the small Chatzer does not have Mechitzos. When the wheat is planted in the large Chatzer, the grapevines in the small one should become forbidden as Kil'ayim! How can one be permitted to plant wheat in the large Chatzer if doing so will make the grapevines in the small Chatzer forbidden?

(a) RASHI explains that the Gemara is not discussing the prohibition of Kil'ei ha'Kerem (mixtures of grapevines and wheat). Rather, the Gemara is referring to mixtures of other plants (Kil'ei Zera'im), such as vines and beans, which are prohibited only mid'Rabanan. The Rabanan prohibited what grows from such mixtures post facto as a penalty for planting in a manner in which they forbid. When one plants different species of plant together in a permissible way (for example, he plants one in the large Chatzer after he plants the other in the small Chatzer), the produce of both plants is permitted. The Rabanan saw no reason to penalize him in such a case, since he did not perform an act that was prohibited. Therefore, even the plants in the small Chatzer remain permitted.

(b) TOSFOS says that when there is a large and small Chatzer, not only are the contents of the small Chatzer considered to be "together" with the contents of the large one, but the classification that is given to the large Chatzer is given to the small Chatzer as well.

The Torah forbids planting wheat in an area defined as a "Kerem," a vineyard: "You shall not plant Kil'ayim in your Kerem" (Devarim 22:9). We derive from the verse that the prohibition does not depend on whether there actually are grapes there. (In fact, in the case of the Gemara here, there are no grapes within four Amos of the small Chatzer, and the fruit itself does not intermingle with the other species). Rather, the prohibition depends on whether the area is defined as a "Kerem." Accordingly, when there are grapevines in the large Chatzer, the large Chatzer is defined as a "Kerem," and the small Chatzer becomes a "Kerem" as well, and wheat may not be planted in it. On the other hand, if there are grapevines in the small Chatzer, the large Chatzer is not considered a "Kerem," because it is not affected by what is planted in the small Chatzer. Therefore, wheat may be planted in the large Chatzer. (Although planting wheat in the large Chatzer will cause the small Chatzer to acquire the title of a "Sadeh," wheat-field, we do not find that it is prohibited to plant grapes in a Sadeh; it is forbidden only to plant wheat in a Kerem.)