The purpose of the eruv is to enclose on all sides the area in which one wants to carry, so that it becomes a private domain, a reshut hayachid. If the area in question is a karmelit, a space that qualifies neither as a public domain nor as a private domain, gaps in the eruv structure may be bridged by means of a constructive or symbolic doorway called tzurat hapetach. A tzurat hapetach is made up of two posts, each called a lechi, and a crossbeam or overhead wire called a korah.
Most streets today qualify as a karmelit. The Riverdale neighborhood of New York City qualifies as a karmelit, not a reshut harabim, because it does not meet all the qualifications of a reshut harabim. First, fewer than 600,000 people pass through the streets of Riverdale, including the Henry Hudson Parkway, in one day. Second, the streets of Riverdale are not mefulash, open-ended on two sides, but are surrounded by walls and buildings on all three sides. Third, the Henry Hudson Parkway is covered with roofed bridges in several places.
Even if the Henry Hudson Parkway were a reshut harabim, it has barriers which can be locked at night. This in fact occurred when tall ships passed through on the Bicentennial festivities. Many urban eruvin utilize telephone poles and overhead cables as a tzurat hapetach, even though telephone poles are not meant to serve as doorways. The rationale for this is that the intent of the Jewish population to incorporate them in the eruv as doorways renders them doorways.
In order for the tzurat hapetach to be legally acceptable, the cable must run across the top of the telephone poles, the same way a lintel runs across the top of doorposts. If, as is often the case, the cable is attached to the side of the telephone pole rather than the top of the telephone pole, the eruv is not valid. This problem can, however, be easily rectified by attaching a wooden plank to the telephone pole. By virtue of a halachic principle known as gud asek mechitza, the halacha draws an imaginary line straight up from the top of the attached plank. If this imaginary line hits an overhead cable, the wooden plank rather than the telephone pole is regarded as the doorpost, lechi, thereby validating the tzurat hapetach and the eruv. The application of this device can be witnessed in many neighborhoods.
One of the boundaries of the Riverdale eruv runs north-south, along Fieldston Road, between West 252 Street and West 254 Street. In that stretch of eruv, the boundary of the eruv is bridged by telephone poles, constituting a tzurat hapetach. A cable passes alongside one particular telephone pole in that area rather than over it, thereby threatening to invalidate the eruv. To overcome this problem, the Riverdale Eruv Committee has attached a wooden plank, lechi, to that telephone pole. Were the plank to extend straight up, it would hit the overhead cable. Accordingly, the attached plank, together with the principle of gud asek mechitza, solves the problem and validates the eruv.
When carrying near the outside perimeters of an eruv, care should be taken to walk inside the eruv‘s boundary, not outside it. For example, when carrying between West 252 Street and West 254 Street on Fieldston Road in Riverdale, one should walk on the sidewalk opposite the eruv’s boundary, not on the sidewalk adjacent to the boundary. This will ensure that one does not inadvertently carry outside of the eruv.