bar underframe

例句：

1. 1.The car's bar underframe is damaged.
   这辆车的底架受损了。
2. 2. The engineer used a bar underframe to support the roof.
   工程师使用一个底架来支撑屋顶。
3. 3.The construction worker needs to replace the bar underframe before starting work.
   建筑工人需要更换底架才能开始工作。