A support for beams or joists that runs at right angles to the beams or joists. If the top of the girder is within 100 mm (4 in. ) of the ceiling, the girder is a factor in determining the number of detectors and is to be considered a beam. If the top of the girder is more than 100 mm (4 in. ) from the ceiling, the girder is not a factor in detector location.