shear off 復習

フィリピンで起きたバス衝突事故で、死亡者数が15人にものぼりました。被害者のほとんどは、キャンプに行く途中の大学生とのことでした。

Death toll in Philippine bus crash climbs to 15

The death toll has risen to 15 from a bus crash in the Philippines, officials said Tuesday, and could climb further as many of the nearly 50 others who were hurt in the accident are in serious condition.

Most of the dead and injured were college students on the way to a camping trip on Monday when the brakes on their chartered bus apparently failed on a steep downhill road. The out-of-control bus then smashed into an electric post, shearing off most of its roof.

Berlito Bati Jr., a disaster-response officer in Tanay town in Rizal province east of Manila, where the accident happened, said the dead included the driver and a professor.

He said the latest victim died late Monday. There were 62 people on board, although bus manifest listed only 58, Bati said.

http://abcnews.go.com/International/wireStory/14-killed-philippine-bus-crash-45605144

　記事の中で気になった表現は、"shear off"です。
もともと"shear"には「(羊毛など)を刈る、(羊などの)毛を刈る」などの意味がありますが、"off"が付くとどのような意味になるのでしょうか。
『ジーニアス英和辞典』第4版(大修館)で調べてみると、「こわれる、ばらばらになる、ばらばらにする、こわす」という意味があることが分かりました。 LDOCEでは、"if part of something shears off, or is sheared off, it suddenly becomes separated, especially after being pulled or hit with a lot of force"と定義されていました。

「バスは急な下り坂から落ちた後、電信柱(electric post)にぶつかり、屋根がばらばらになって壊れた」と解釈できます。(Akim)