1. 底架：杆件 member | 底架 underframe | 有中梁底架 underframe with center sill

2. 底托架：undercutting chamber 拉底峒室 | underframe 底托架 | underground 地下的

3. 底架,底框：underexposure曝光不足 | underframe底架,底框 | underlay垫物,衬层

4. 车身底板：车身承梁bodybolster | 车身底板underframe | 车身底座underbody

noun

1. the internal supporting structure that gives an artifact its shape

e.g. the building has a steel skeleton

Synonym: skeletonskeletal frameframe