Answer:
The trapezoidal sheet is modeled as an orthotropic surface. In the default setting, the arches of the trapezoidal sheet are in the x directions, which is the same direction of the IPE 120 beams.
If you set the orthotropy direction to 90 degrees - that is, perpendicular to the beams - the moments will increase. If the sheet metal arches are perpendicular to the beams, the stiffness in the second direction is very small, so the load is transferred first between the IPE 120 beams.
