Sets the value of TransomThickness attribute. This attribute represents thickness of the transom (horizontal separator of window panels within a window), measured parallel to the window elevation plane. The transom is part of the lining and the transom depth is assumed to be identical to the lining depth. If the TransomThickness is set to zero (and the TransomOffset set to a positive length), then the window is divided vertically without a physical divider.

IFC4 CHANGE: Data type changed to IfcNonNegativeLengthMeasure.