OPINIONS: Rebbi Eliezer and the Chachamim argue whether one is permitted to add to a temporary structure on Shabbos. Rebbi Eliezer considers adding to a temporary structure the same as making an Ohel, while the Chachamim do not consider it the same as making an Ohel. The Gemara says that their argument applies to erecting a wall for a Sukah on Shabbos.
May we infer from the Gemara that even erecting a wall is considered making an Ohel and may not be done on Shabbos?
(a) RASHI in Shabbos (125b) says that a wall is certainly not an Ohel. He refers to the Gemara later in Shabbos (138a) which says that one may hang a curtain on Shabbos and there is no concern that he is making an Ohel. According to Rashi there, only making a roof is included in the prohibition against making an Ohel.
How, then, does Rashi understand the Gemara here that implies that erecting a wall is also considered making an Ohel?
Rashi in Shabbos (125b) writes that hanging a curtain on Shabbos is permitted because it is done only for purposes of modesty. It is considered a temporary structure and not one that will be used on an ongoing basis. A roof, in contrast, is made to provide shade and shelter and is considered permanent. According to Rashi's words there, we may suggest that a wall that is made to fulfill a Halachic need, such as the one mentioned in the Gemara here (a wall for a Sukah or a Mechitzah to make a Reshus ha'Yachid), is made with the intention that it remain there to serve its purpose (of forming the Sukah or the new Reshus), and it is not merely for the sake of modesty. Therefore, erecting such a wall is included in the prohibition against making an Ohel.
(b) TOSFOS (DH Pekak) explains that one is always permitted to erect a curtain on Shabbos when it is an addition to a temporary Ohel ("Tosefes l'Ohel Arai"). It is prohibited, though, to set up a new curtain or partition ("Techilas Ohel") that is not an addition to a pre-existing structure. In the cases of the Gemara here, in which erecting the wall would create a new Halachic status ("Mechitzah Materes") by either forming a valid Sukah or a Reshus ha'Yachid, it is not considered an addition to a pre-existing Ohel. Rather, it is considered making a "new Ohel," even though a physical Ohel already existed, and thus it is forbidden.