This is what I found
http://kashrut.org/forum/viewpost.asp?mid=1827&highlight= eiruv chatzerot
Message: Hi. I am confused about the following:
[1] is a apartment building hallway actually considered a 'chatzer' for shabbat purposes so that i would need an eiruv chatzerot ? why isnt it a reshut hayachid, its surrounded by 4 walls and is more than 4x4 [especially if the building is owned by a private individual and not a corporation]!
[2] if it does require an eiruv, is this true on yom tov too, or is carrying always allowed on yom tov ?
[3] if my building is in the general comunity 'eiruv' [tzurat hapetach], do i also need an eiruv chatzerot ?
[4] is there a difference in this for ashkenazim and sefaradim ?
[5] practically , what exactly do i need to get the non jews in the building to agree to when they sell me theitr rights ?
thank you
Reply: 1) If there are two or more that share the hallway, it requires an Eiruv Chatzeirot. The why's you can learn in Mesechet Eiruvin.
2) Allowed
3) If the Eiruv in the town is Kosher, the same Rabbi(s) will automatically make an Eiruv Chatzerot, too.
4) No
5) If you only need to use the hallways and shared areas, then just rent from them for a few cents rights in the hallways....
AA