The Yerushalmi cites a unique opinion that a tzuras ha’pesach for the purpose of an eruv consists not only of the customary two doorposts (lechayayim) and the wire or beam than runs from the one to the other (kaneh al gabeihem). Rather, according to one opinion in the Yerushalmi, the tzuras ha’pesach also requires “tzir u’ben tzir” (there is a girsa “ken tzir”). This means that there must be a provision for a hinge – both on bottom (their hinges were pegs that came out of the door and turned in holes of the frame, much the way a Lego door or window works) and the top.
This would eliminate the use of wires for a kaneh al gabeihem (it would seem), as you cannot bore a hole in one that would hold the tzir.
Further down the amud R’ Acha rules that a tzuras ha’pesach must be drawn four amos back into the area it encloses, and the mefarshim have difficulty explaining his rationale.
The Ohe Somayach and R’ Chaim Kanievski explain this halacha on the basis of the tzir u’ben tzir requirement. They note that the Gemara in Bav Basra 12 says that the inhabitants of a mavoi (alley) may not place a door within four amos of the reshus ho’rabbim that abuts the mavoi, as this will impact on traffic patterns.
Thus, since the Yerushalmi cites an opinion that a tzuras ha’pesach must be a completely formed doorway – minus only the actual door – it must conform to the parameters that it would possess were a door actually in place – and, therefore, can be no closer than four amos to the adjacent reshus ho’rabbim.