main joist of suspended ceiling

例句：

1. 1. The main joist ofthe roof is holding up the ceiling.
   屋顶的主梁支撑着天花板。
2. 2. Themain joist of thehouse is rotting away.
   房子的主要梁正在腐烂。
3. 3. Themain joist of thebridge is being replaced.
   桥梁的主要梁正在被替换。