Eruvin, Chapter 1, Mishnah 4. In the previous Mishnah we dealt with the beam - the beam laid across the width of the alleyway above its entrance - and with the requirement that it be a tefach wide, so that a half brick can be laid upon it.
How our Mishnah qualifies the requirement:
Our Mishnah adds a qualification to this requirement. As the previous Mishnah stated, the beam must be wide enough to hold a half brick, which is a tefach and a half wide, and from here we learn that a width of one tefach suffices for the beam itself.
Two conditions are required in the beam:
"Rechavah" - wide, a tefach, so that it can hold the half brick.
"Beri'ah" - sound, strong and solid, able to bear the weight of the half brick.
The term "bari" is used in our language to describe the health of people, but a beam that is bari means a solid beam, made of material strong enough to support a half brick. Therefore, if the beam is rickety it is invalid, even though it is wide enough. Width alone is not sufficient; strength is required as well.
The opinion of Rabbi Yehudah:
Rabbi Yehudah disagrees and says: as long as the beam is "rechavah, af al pi she'ein beri'ah" - wide, even though it is not sound - that is enough for it to serve as a beam.
In summary: in this Mishnah we learned that two conditions are required in the beam - a width of a tefach so that it can hold a half brick, and soundness, meaning strength sufficient to bear the weight of the half brick; Rabbi Yehudah, however, validates a wide beam even when it is not sound.