Gemara

(Beraisa): If five sat on a bench, and a sixth sat on it, and it broke, he is liable.

(Rav Papa): The sixth was (fat) like Papa bar Aba.

Question: What is the case?

If it would not have broken without him, obviously, he is liable!

Answer #1: Rather, it would have broken without him.

Objection: If so, he should be exempt!

Answer #2: Rather, without him, it would have broken in two hours. Because he sat on it, it broke in one hour. The others can say 'had you not sat on it, we would have left before it broke!'

Objection: He can say 'without the five of you, I would not have broken it alone!'

Answer #3: Really, it would not have broken without him. The case is, he did not sit on it. He only pressed on the men sitting on it. One might have thought that his force is unlike (damage of) his body. The Beraisa teaches that this is not so.

Rishonim

The Rif brings our Gemara.

Nimukei Yosef (DH Ihu): Whenever it would not have broken without the last, he alone is liable. If it would have broken without him, only the first ones are liable. If they could have risen but did not, also they are responsible, i.e. if it would have broken later due to them. If it would break only due to him, with or without them, he is liable even if they could have risen.

Rambam (Hilchos Chovel u'Mazik 6:15): If five sat on a chair, and the last man sat, and it broke, even though it would have broken without him, he is liable, for he made it break sooner. They can say 'had you not pressed on us, we would have risen before it broke.' If they sat down together and it broke, all are liable.

Lechem Mishneh: Why didn't the Rambam mention that the last man was fat? He holds like R. Tam, that he is liable for merely not letting them rise. The Magid Mishneh explains the Rambam like Rashi. I disagree. Rashi explains that it would not have broken without the last man, for if not, he did nothing. The Rambam holds that also the conclusion holds that without him it would have broken, but later. The Rosh holds like the Rambam.

Rosh (1:10): The Rashbam says that only an obese person is liable if the bench broke under him. R. Tam says that Rav Papa taught about the conclusion. Only a strong person is liable, for he can stop them from getting up. Rav Papa answered his own question. This is difficult. Also, why did Rav Papa need to mention a strong man? From the answer (he pressed all of them on the bench) we know that he forced them, for if they could get up also they are liable!

Poskim

Shulchan Aruch (CM 381:1): If five sat on a chair, and the last man sat on it, and pressed on them and did not allow them to rise, and it broke, even though it would have broke due to them, since he made it break sooner, he is liable. They can say 'had you not pressed on us, we would have risen before it broke.' If they sat down together and it broke, they are all liable.

SMA (4): We do not say that his force is unlike his body, and it is mere Gerama (causing damage).

Question (Drishah DH v'Hinei): The Rosh and Tur are unlike Rashi in two ways. (1) Rashi holds that we conclude that it would not have broken without him, but even so he alone is liable only if he did not allow them to rise. The Rosh and Tur connote that it would have broken later without him, therefore we distinguish between whether or not he let them rise. (2) Rashi explains that he did not sit with them. He stood and pressed on them. They explain that he also sat with them. The Rambam differs with Rashi in these two ways, but does not say that he pressed and did not let them rise, nor that he was fat. Why did Rashi explain that 'it broke while he pressed on them' refers to what we said above (it would not have broken without him), and not to the last thing said (it would have broken later without him)? If he teaches a Chidush, that the law is true even if it would not have broken without him, he should have specified! Also, why did Rashi mention that some texts say 'd'Yazga Mizga (he sat and leaned) on them'?

Answer (Drishah DH v'Nir'eh): Rashi holds that the first text (he pressed on them) connotes that he was standing. If it would have broken without him later, why did the Gemara say obviously, he is liable? Rashi concluded that without him, it would not have broken. Therefore, obviously he alone is liable. Rashi brings another text that says that he sat and leaned on them. I.e. it would have broken without him, but he did not let them rise. The Rosh and Tur adopt the latter text, for the Gemara did not say 'Ela' to show a retraction. The Rambam had Rashi's first text, but explains that it broke the moment he pressed on them to sit with them. It would have broken without him later. We asked that they should have risen. They can say 'had we known that you will sit with, we would have risen.' Rav Papa said that it discusses one like Papa bar Aba, for a normal person would not break it just by pressing on them.

Gra (1): The Shulchan Aruch is like Rashi, who explains that the question 'he can say 'I would not have broken it alone!'' refers also to the first suggestion (without him, it would not have broken). It cannot refer only to the last answer (without him, it would have broken later), for if so, we should have answered only that he is liable for not letting them rise. Why did the Gemara add 'it would not have broken without him'? Rather, this explains the Chidush (even though he pressed without sitting, he is liable, for his force is like his body). The Gemara said that he did not let them rise to answer why he can't say 'you should have risen.'

Gra (1): The Rambam's text omits the question 'he can say 'I would not have broken it alone!'' Rather, it asks 'they can say 'had you not sat on it, we would have left before it broke!'' It answered that it broke while he pressed on them. This is why the Rambam and Shulchan Aruch obligate all of them if they sat down together, but if not, only the last is liable, unless it would have broken without him (then, surely he is exempt). Rashi is difficult. Initially the Gemara said that if it would not have broken without him, obviously, he is liable, and later we ask that he can say 'I would not have broken it alone!'

Rema: Some say that a Stam bench is lent to Stam people. If it broke, they are exempt, like for a borrowed animal that died while working, unless they were heavier than regular people. Then, if they sat down together and it broke, they are all liable. If they sat one after the other, if it would not have broken without the last, he alone is liable. If it would have broken without him, he is exempt. If without him it would have broken, but later, he alone is liable if he did not let them rise. If not, they are all liable.

Gra (1): This is like the Tur, who had our text and explained that 'he can say 'I would not have broken it alone!'' refers only to the last answer (without him, it would have broken later). The above question, which forced Rashi to say that it applies also to the first suggestion, is difficult. The Rambam's Perush is best.

Chazon Ish (Bava Kama 3:11 DH v'Lulei): The Poskim do not argue. If it broke immediately after the last one sat, before the others realized the need to rise, he alone is liable, even if it would have broken later without him. If they had time to rise, all are liable. If when he sat, it already was sure to break, he did nothing. If he prevented them from rising without pressing down, this is Gerama, and he is exempt. If he pressed, he is liable.

Gra (2): The Tosefta (2:8) distinguishes between whether they sat at once, or one after the other.