noun
headroom; vertical clearance; maximum height
Technical term used in civil engineering, architecture, and transportation to describe the vertical space available under a bridge, beam, or overhead structure. Often measured from the road or water surface to the lowest point of the structure above.
この橋の桁下は4.5メートルです。
The vertical clearance of this bridge is 4.5 meters.
大型トラックが通れるように、桁下を高く設計した。
The headroom was designed high so that large trucks can pass.
高さ制限 refers to a height restriction or limit, often a regulatory maximum, while 桁下 specifically denotes the actual vertical clearance under a structure.
Compound of 桁 (けた, 'beam' or 'girder') and 下 (した, 'under' or 'below'), literally meaning 'under the beam'.